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Impact of frying conditions on hierarchical structures and oil absorption of normal maize starch
Accepted Manuscript Impact of frying conditions on hierarchical structures and oil absorption of normal maize starch Long Chen, Rongrong Ma, Zipei Zhang, David Julian McClements, Lizhong Qiu, Zhengyu Jin, Yaoqi Tian PII:
S0268-005X(18)32022-8
DOI:
https://doi.org/10.1016/j.foodhyd.2019.105231
Article Number: 105231 Reference:
FOOHYD 105231
To appear in:
Food Hydrocolloids
Received Date: 12 October 2018 Revised Date:
15 July 2019
Accepted Date: 17 July 2019
Please cite this article as: Chen, L., Ma, R., Zhang, Z., McClements, D.J., Qiu, L., Jin, Z., Tian, Y., Impact of frying conditions on hierarchical structures and oil absorption of normal maize starch, Food Hydrocolloids (2019), doi: https://doi.org/10.1016/j.foodhyd.2019.105231. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Impact of frying conditions on hierarchical structures and oil absorption of normal maize starch Long Chena,b,c, Rongrong Maa, Zipei Zhangc, David Julian McClementsc, Lizhong
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Qiud, Zhengyu Jina,b,d, Yaoqi Tiana,b,d* State Key Laboratory of Food Science and Technology, Jiangnan University, 1800
Lihu Road, Wuxi 214122, China
School of Food Science and Technology, Jiangnan University, 1800 Lihu Road,
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Wuxi 214122, China
Department of Food Science, University of Massachusetts, Amherst, MA 01003,
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Zhucheng Xingmao Corn Developing Co., Ltd, Weifang 262200, China
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* Corresponding author
Yaoqi Tian, Professor in Food Science
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Email: [email protected] (Y. Tian); [email protected] (L. Chen)
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Graphical abstract:
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Impact of frying conditions on hierarchical structures and oil
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absorption of normal maize starch
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Long Chena,b,c, Rongrong Maa, Zipei Zhangc, David Julian McClementsc, Lizhong
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Qiud, Zhengyu Jina,b,d, Yaoqi Tiana,b,d*
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a
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Lihu Road, Wuxi 214122, China
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b
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Wuxi 214122, China
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c
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State Key Laboratory of Food Science and Technology, Jiangnan University, 1800
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School of Food Science and Technology, Jiangnan University, 1800 Lihu Road,
Department of Food Science, University of Massachusetts, Amherst, MA 01003,
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USA
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Zhucheng Xingmao Corn Developing Co., Ltd, Weifang 262200, China
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* Corresponding author
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Yaoqi Tian
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Professor in Food Science
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Email: [email protected] (Y. Tian); [email protected] (L. Chen)
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Abstract Knowledge about oil absorption during frying is crucial for the design and production of healthier reduced-fat food products. For this reason, we systematically
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investigated the structural changes and oil absorption of normal maize starch (NMS)
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during simulated frying conditions. In particular, the impact of initial moisture
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content, frying temperature, and frying time were examined. The hierarchical
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structures of the fried samples were firstly characterized using scanning electron
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microscope (SEM), X-ray diffraction (XRD), infrared spectroscopy (FTIR), and size
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exclusion chromatography. Furthermore, the impact of frying conditions on oil
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absorption by NMS was investigated using LF-NMR and ATR-FTIR based methods.
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During frying, the granular morphology of the starch granules was lost, their internal
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crystalline structures were disrupted, their double helices were broken down, and
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starch molecules were degraded. These changes were related to the absorption of oil
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by the starch granules during frying. The initial moisture content of the starch samples
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had the most pronounced influence on the amount of oil absorbed during frying. The
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oil content first increased and then decreased with increasing moisture content, being
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0.2396, 0.6602, 0.3614, and 0.2531 g/g starch for 20, 40, 60 and 80% moisture
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content, respectively. The fraction of oil present at the exterior of the samples after
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frying increased with increasing moisture content, frying temperature, and frying
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time. The variation in oil absorption were attributed to changes in the hierarchical
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structures of the fried samples, including granule morphology, crystalline property,
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double helices, and molecular features.
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Keywords: Fried starchy samples; Frying conditions; Hierarchical structures; Oil
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contents; LF-NMR; ATR-FTIR
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Chemical compounds studied in this article
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Starch (PubChem CID: 24836924); Water (PubChem CID: 962); Amylose (PubChem
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CID: 53477771); Amylopectin (PubChem CID: 439207); MnCl2.4H2O (PubChem
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CID: 643989); n-Hexane (PubChem CID: 8058).
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Please note: Edible oil is not a specific chemical, it is the mixture of triglyceride and
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fatty acid. Thus, soybean oil will not have a specific record in PubChem.
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1. Introduction Frying is a common food preparation method used in both industrial food production and household cooking (Contardo, Parada, Leiva, & Bouchon, 2016;
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Mehta & Swinburn, 2001). In addition to cooking, frying also plays an important role
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in the dehydration and sterilization of foods (Durán, Pedreschi, Moyano, & Troncoso,
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2007). The boiling and evaporation of water during frying mainly occur at the
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surfaces of foods, which makes room for hot oil to be absorbed (Dana & Saguy,
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2006). The amount of oil taken up by foods depends on the level of dehydration
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during frying, and may fluctuate between about 8 to 40% by total weight (Mehta &
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Swinburn, 2001; Mellema, 2003). The overconsumption of highly palatable fried
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foods is of concern to many consumers and medical professionals because of their
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potential to promote obesity and related diseases (Guo, Ye, Bellissimo, Singh, &
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Rousseau, 2017; Kurek, Ščetar, & Galić, 2017). Despite this, fried foods are still
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popular in both developed and developing countries. Consequently, it is important for
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food scientists to understand the factors that impact the fat content of fried foods so
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that high quality versions with better health profiles can be developed.
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Frying involves a combination of heat and mass transfer processes that are
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impacted by frying conditions and food properties (Mehta & Swinburn, 2001;
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Naghavi, Dehghannya, & Ghanbarzadeh, 2018). Numerous studies have been carried
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out to establish the effects of these parameters on oil absorption and food quality so as
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role in oil uptake, which has mainly been attributed to a combination of condensation
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and capillary mechanisms (Mellema, 2003). Typically, a higher initial moisture
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content leads to a higher oil uptake (Chen et al., 2018b; Mehta & Swinburn, 2001).
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The frying temperature is another important factor affecting oil uptake with higher
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temperatures typically leading to a lower oil content (Abd Rahman, Abdul Razak,
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Lokmanalhakim, Taip, & Mustapa Kamal, 2017; Moyano & Pedreschi, 2006).
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However, some studies have reported an increase in oil absorption with increasing
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frying temperature (Gamble, Rice, & Selman, 1987; Garayo & Moreira, 2002). The
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level and distribution of oil absorbed by a food product are also affected by the
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duration of frying. Increasing the frying time has been reported to increase the oil
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content of potato chips (Cruz et al., 2018). This may partly be because the progressive
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evaporation of water during frying leads to an increase in the calculated oil content
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when reported on a wet-weight basis. Conversely, frying time and temperature have
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been reported to have little impact on the oil content of corn chips (Yuksel et al.,
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2017). One of the main reasons for the conflicting findings reported in different
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studies is that real foods are often used as test materials, which have different
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compositions and structures. Another in-negligible reason for these contrary
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conclusions between studies is that the results are often reported using different
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concentration units, i.e., wet- or dry-weight basis. Typically, the oil content is higher
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when the results are reported on a dry-weight basis rather than a wet-weight basis. It
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This is because the total weight of samples (oil+solids+water) will vary with the
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temperature and treatment duration (caused by the evaporation of water during frying)
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when calculated on a wet basis, confusing the comparison and analysis of related data
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in different studies.
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Many different analytical techniques have been used to determine the level, migration, and distribution of oil in foods, including the Soxhlet extraction,
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differential scanning calorimetry (Aguilera & Gloria, 1997), confocal microscopy
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(Zhu, Zou, Shi, Zhao, & Huang, 2017), Raman spectroscopy (Dong, Wu, Chen, &
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Liu, 2017), and infrared microscopy (Bouchon, Hollins, Pearson, Pyle, & Tobin,
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2001). These techniques provide valuable information about the properties of oil
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during frying but they are time-consuming, required sophisticated sample processing,
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and/or have high susceptibility to interference from other food components.
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Conversely, low field nuclear magnetic resonance (LF-NMR) is a rapid, non-invasive,
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and low-cost technology that can be used to detect the distribution and mobility of the
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protons of certain small molecules in complex food systems (Li et al., 2015).
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LF-NMR has been successfully used for the characterization of water in starchy gel
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(Chen, Tian, Tong, Zhang, & Jin, 2017b), the measurement of oil in botanical seed
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(Niu, Li, Chen, & Xu, 2014), and the real-time monitoring of the dynamic change in
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the lipid concentration in the cells during microalgal fermentation processes (Wang et
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al., 2016). In our previous work, a LF-NMR based method has been established for
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(Chen et al., 2017a). However, LF-NMR could only analyze the overall oil content of
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the samples, rather than the distribution of oil between the internal and external
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portions. Therefore, an ATR-FTIR based method has been used to determine the level
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of oil on the external surfaces of fried starch granules, which is based on the limited
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penetration depth of the infrared beam from the ATR accessory (Chen et al., 2018b).
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Starch is the main component in many fried foods, including potato chips, wheat dough, corn paste, instant noodles, and coating flour. Little information is, however,
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currently available about the impact of frying conditions on the structural changes and
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oil absorption of starch. This lack of knowledge is hindering the effective control of
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oil absorption by starch-based foods during frying. For this reason, a starch-oil-water
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model system with a well-defined composition and structure has been developed to
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facilitate the performance and interpretation of frying studies (Chen et al., 2018a;
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Chen et al., 2018b).
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In the present work, the hierarchical structures of the fried starch samples,
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including granule morphology, crystallinity, double helix formation, and molecular
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properties, were characterized using scanning electron microscope (SEM), X-ray
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diffraction (XRD), infrared spectroscopy (FTIR), and size exclusion chromatography,
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respectively. We postulated that integration of the information obtained from these
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different analytical methods would lead to a more comprehensive understanding of
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mobility and total oil content of the fried starch samples were determined using a
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LF-NMR method, whereas the external oil content was determined using an
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ATR-FTIR method. The information obtained from these methods is useful for
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understanding the influence of frying conditions on oil absorption.
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Overall, this study should therefore provide valuable insights into the major
factors impacting oil uptake during the frying of starchy foods, which may be useful
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for developing healthier fried foods with lower fat contents.
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2. Materials and methods
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2.1. Materials
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Co., Ltd. (Suzhou, China). It contained 27.3% amylose, 13.1% moisture content,
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0.95% free lipids, and 0.1% proteins. Soybean oil produced by Yi Hai Kerry Co., Ltd.
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(Shanghai, China) was purchased from the local supermarket. MnCl2.4H2O and
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n-hexane of chromatographical purity were supplied by Sinopharm Chemical Reagent
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Co., Ltd. (Shanghai, China). Petroleum ether with a boiling range of 30 to 60°C was
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purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other
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chemicals and reagents were of analytical grade unless otherwise stated.
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2.2. Preparation of fried starchy samples under different conditions The heat-moisture treatments of starch in oil phase under different conditions
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were performed as described in our previous studies (Chen et al., 2019a, b, c; Chen et
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al., 2018a) with some modifications. Initially, model samples with different moisture
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contents were prepared as follows. The NMS was first hydrated to water contents of
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20%, 40%, 60%, and 80%, in sealed plastic bags for 10 h to achieve moisture
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equilibrium. Then, the hydrated NMS (5 g starch, dry basis) was directly dispersed
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into 100 mL of soybean oil at 20°C using a magnetic stirrer (C-MAG HS 7, IKA Co.,
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Germany). Subsequently, the starch-oil-water mixtures were treated at 180°C in an oil
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bath for 20 min to mimic frying. Afterward, the hot samples were rapidly removed
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from the hot oil by vacuum filtration. The total and external oil contents of the fresh
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fried samples were rapidly analyzed using LF-NMR and ATR-FTIR methods,
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respectively. Then, the fried samples were defatted by Soxhlet extraction using
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petroleum ether as an organic solvent, dried in a vacuum drying oven at 40°C for 12
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h, milled to pass through a 100-mesh sieve, and preserved in vacuum bags prior to
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further structural analysis.
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Samples fried at different temperatures were prepared as follows. The NMS was
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first hydrated to 40% moisture content in a sealed plastic bag for 10 h, then dispersed
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into 100 mL of soybean oil, and then heated at 120, 150, 180, and 210°C for 20 min
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using an oil bath. Subsequently, the fresh fried samples were promptly isolated from
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the hot oil using vacuum filtration. The post-treatment process of fried samples was
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the same as above.
Samples fried for different durations were prepared as follows. The NMS was
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firstly hydrated to 40% moisture content in a sealed plastic bag for 10 h, dispersed
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into 100 mL of soybean oil, and then heated at 180°C in an oil bath for 5, 10, 20, and
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30 min, respectively. Subsequently, the fresh fried samples were promptly isolated
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from the hot oil using vacuum filtration. The post-treatment process of the fried
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samples was then the same as described earlier.
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2.3. Scanning electron microscope (SEM)
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The morphology of the starch granules after frying was monitored using SEM (Quanta 200, FEI Inc., Hillsboro, OR, United States). The fried starchy samples were
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defatted by Soxhlet extraction, dehydrated by vacuum drying, spread on a specimen
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platform, and then coated with gold palladium. Finally, their microstructures were
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observed at 1200 × resolution at a low voltage of 5.0 kV to avoid the sample damage
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induced by the electron beam.
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2.4. X-ray diffraction analysis (XRD)
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Before measurement, the fresh fried samples were first defatted using Soxhlet
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extraction. The long-range ordered structural properties of defatted starchy samples
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were then determined using a Bruker X-ray diffractometer (D2 PHASER, Bruker
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CuKα radiation, with λ= 1.5406 angstrom (monochromatic). Diffractograms were
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recorded from 4 to 40° (2θ) with a step size of 0.005° to improve the resolution of
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diffraction peaks. It is worth noting that both elastic and inelastic scattering coexist in
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starchy samples because of the fact that both of them are diffraction phenomena. It is
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not, however, possible to separate the signals of elastic and inelastic scattering in the
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X-ray diffraction pattern. Therefore, a calculation of the percent crystalline material
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present is not accurate (Rodriguez-Garcia, Londoño-Restrepo, Ramirez-Gutierrez, &
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Millan-Malo, 2018).
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2.5. Attenuated total reflection - Fourier transform infrared spectroscopy
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(ATR-FTIR)
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The short-range ordered structures of fried starchy samples were studied using a FTIR spectrometer (IS10, ThermoNicolet Inc., Waltham, MA, United States).
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Generally, the structural changes of starch during frying started from the surface,
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therefore, in the present work ATR-FTIR was used to detect the possible short-range
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structural changes on the external region of the starch. The IR spectra of an empty cell
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(air as the control) and defatted sample were collected at a resolution of 4 cm-1 for 32
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scans. Spectra acquired in the range of 1200-800 cm-1 are sensitive to the degree of
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molecular order of starch and so they were used to calculate the short-range ordered
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structure parameters, i.e. the ratios of IR absorbances at 1047 to 1015 cm-1 (R1) and
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spectrum was first smoothed and then deconvoluted at a half-width of 19 cm-1 and a
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resolution enhancement factor of 1.9. Subsequently, the R1 and R2 values of the
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samples were calculated.
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2.6. Size Exclusion Chromatography
Size exclusion chromatography was used to evaluate the molecular structures of
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the fried samples as affected by frying conditions according to our previous work with
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some slight modifications (Sun, Tian, Chen, & Jin, 2017). Briefly, samples (5 mg, dry
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basis) were dissolved in 5 mL 90% DMSO by heating in a boiling water bath with
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continuous stirring for 12 h. Then, the starch molecules were collected by alcohol
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precipitation using an 8-fold volume of ethanol. The precipitate was collected and the
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extra ethanol was removed using the vortex nitrogen sweeping method. The starch
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was then dispersed in the mobile phase (0.1 M acetate buffer containing 0.02% (w/v)
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NaN3 and 50 mM NaNO3), and injected into a high-performance liquid
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chromatography (HPLC) system equipped with a multi-angle laser light-scattering
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(MALLS) detector (Wyatt Technologies, Santa Barbara, CA, USA), a
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refractive-index (RI) detector (Wyatt Technologies, Santa Barbara, CA, USA), and
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three Phenogel columns (guard column, Shodex OHpak SB-806 HQ, Shodex OHpak
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SB-804 HQ) (Showa Denko K.K., Kawasaki, Japan). The flow rate of the mobile
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phase was set at 0.6 mL/min and the temperature in the column was maintained at
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instrument software (Astra version 5.3.4, Wyatt Technologies, Santa Barbara, CA,
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USA).
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2.7. Determination of total water and oil contents of fried samples using
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LF-NMR
Low-field nuclear magnetic resonance (LF-NMR) measurements were conducted
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using a 23 MHz NMR analyzer (NMI20-015V-I, Niumag Co., Ltd., Suzhou, China)
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according to our previous work (Chen et al., 2017a; Chen, Tian, Tong, Zhang, & Jin,
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2017b). Samples (2-3 g) were accurately weighed into 10 mL glass sample bottles that
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were then sealed with three layers of Teflon tape. The sample bottle was then
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carefully transferred into an NMR test tube of 25 mm diameter. Spin-spin relaxation
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time (T2) signals of the samples were then acquired at 32°C using the
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Carr-Purcell-Meiboom-Gill (CPMG) sequence.
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system, 3% MnCl2·4H2O aqueous solution and soybean oil were used as the standard
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materials in the preparation of calibration curves for water and oil, respectively.
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Briefly, the T2 signals from reference substances of known mass were collected and
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inverted to T2 distribution curves. For LF-NMR analysis, the areas under the curves
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corresponding to specific T2 ranges are proportional to the number of protons in
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different environments. This approach was used to determine the amount of water and
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(Chen et al, 2017a). All samples were measured under the same operating conditions
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and all data were normalized by sample weight to ensure the signals were comparable
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in terms of intensities. The fried samples were defatted and then dried to a constant
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weight. To ensure the accuracy of the analysis, the results in present work were
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expressed on a dried-weight basis, i.e. (grams of oil or water) / (grams of defatted dry
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solids). Furthermore, for the purpose of comparison, Soxhlet extraction (AOAC,
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1990) was also used to measure the total oil contents of the fried samples.
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2.8. Determination of external oil contents of fried samples using ATR-FTIR
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Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) analysis was used to determine the level of surface oil on the fried samples due to the short
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penetration depth of infrared light (Chen et al., 2018b). A commercial FTIR
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spectrometer (IS10, ThermoNicolet Inc., America) combined with a 50 µL ATR
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accessory was used to acquire the spectra. Soybean oil was dispersed into n-hexane to
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prepare oil reference substances with varying oil concentrations. Then, the soybean
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oil dispersions or the fried starchy samples were rapidly daubed onto the surface of
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the ATR crystal, and the FTIR spectra were measured from 600-4000 cm-1 at a
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resolution of 4 cm-1. After baseline subtraction and smoothing, the peak area for 1743
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cm-1 was calculated by integrating from 1693 to 1793 cm-1, and the calibration curves
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for the quantitative analysis of surface oil were obtained by linear regression between
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starchy samples were calculated by substituting the peak area values for each fried
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sample in the linear regression equation. Both the fried starchy samples and their
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defatted counterparts were analyzed to distinguish the oil fractions that associated
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strongly or weakly with the starch molecules close to the surface. The fried samples
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were defatted and then dried to a constant weight, and the results were expressed as
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(grams of oil) / (grams of defatted dry solids).
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2.9. Statistical analysis
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Means ± standard deviations of triplicate determinations were used unless otherwise stated. One-way ANOVA (Tukey’s test) was applied to assess the statistical
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significance of data using SPSS 20.0 (SPSS Inc., Chicago, USA). The value of p <
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0.05 was considered to be statistically significant.
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3. Results and discussion
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3.1. Morphological changes of fried starchy samples
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shown in Fig. 1. Compared to native NMS, no obvious changes could be observed in
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the samples initially containing 20% moisture content after frying (Fig. 1A), while
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considerable distortion and deformation of the starch granules could be observed for
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the sample initially containing 40% moisture content (Fig. 1B). The starch granules in
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granules with pinholes and flaws on their surface (Fig. 1A), which was consistent
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with previous studies (Chen et al., 2018a; Dhital, Shelat, Shrestha, & Gidley, 2013).
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The expansion of the starch granules was limited due to the inhibition of swelling and
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gelatinization caused by frying (Chen et al., 2018a). When the sample was treated at
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40% moisture content, the pores and flaws on the granule surface disappeared and
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membranous or fragmented structures were observed (Fig. 1B). It is worth noting that
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the gelatinization of starch in oil is different from the classical gelatinization of starch
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in water. The hydrophilic amylose molecules that leach from the starch granules
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during frying cannot simply diffuse into the oil phase. Instead, they deposit onto the
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surfaces of the starch granules, which would account for the observed disappearance
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of the porous structure on the starch granule surfaces. Additionally,
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erythrocyte-shaped granules with concave cores and thick edges were observed after
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frying (Fig. 1B), supporting the hypothesis that the core of the starch granules was
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less densely packed and less thermally stable than the outer layer of granules
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(Copeland, Blazek, Salman, & Tang, 2009). When the moisture content increased to
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60%, the majority of the starch granules dissociated into small fragments and then
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merged into large clumps (Fig. 1C).
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As the frying temperature was increased, the fraction of granules that had lost
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their integrity increased. When fried at 150℃, most of the starch granules were still
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visible, but pores and flaws on their surfaces disappeared (Fig. 1D). At 180℃, many 17
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some membranous or fragmented structures on their surfaces (Fig. 1E). The
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morphology of the fried starchy samples then only changed a little as the temperature
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was further increased from 180 to 210℃ (Fig. 1F).
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Intact starch granules with membrane-like materials on their surfaces were also
observed in samples fried at different time. However, the treatment time did not have
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a major impact on morphology of the granules (Fig. 1G-I). Even so, there did appear
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to be slightly more disruption and merging of the starch granules for the longest
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treatment (Fig. 1I).
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The crystalline regions in native starch granules are composed of oriented double-helices of amylopectin side chains, while the amorphous regions are made up
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of amylose and the branching points of amylopectin (Tester, Karkalas, & Qi, 2004).
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The internal structure of the starch granules is believed to be largely held together by
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hydrogen bonds between the starch molecules. According to previous studies, the
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gelatinization of starch in a low moisture content environment (such as frying) could
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more reasonably be defined as the “melting” (Liu, Xie, Yu, Chen, & Li, 2009).
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During the initial stage of frying, the presence of water molecules in samples
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promotes the swelling of the amorphous regions, which leads to the disruption of the
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hydrogen bonds holding the starch molecules together. In addition, the swelling of the
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amorphous regions around the crystalline regions could pull apart the crystalline
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ACCEPTED MANUSCRIPT structures by stripping of starch chains from the crystallites (Donovan, 1979). When
333
the frying conditions are intensified, the amount of water present diminishes and the
334
internal temperature increases. Under these conditions, starch molecules become more
335
flexible, the hydrogen bonds between the starch molecules are progressively ruptured,
336
the crystalline regions melt, and eventually the granules become deformed, fractured,
337
and aggregated (Fig. 1).
338
3.2. Long-range crystalline structures of fried starchy samples
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XRD was applied to further investigate the impact of frying parameters on the
340
long-range crystalline structure of the fried samples. The XRD patterns of the defatted
341
samples are presented in Fig. 2. The X-ray diffraction pattern of starch clearly
342
depended on the nature of the frying conditions used, suggesting that there were
343
pronounced changes in the long-range molecular organization of the starch molecules.
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The initial moisture content had an important impact on the crystalline characteristics of the fried starch (Fig. 2A). The native NMS exhibited broad
346
diffraction peaks (Fig. 2), indicating the existence of nanocrystals and lamellar
347
structures in the starch granules (Londoño-Restrepo, Rincón-Londoño,
348
Contreras-Padilla, Millan-Malo, & Rodriguez-Garcia, 2018). The native NMS and
349
sample fried at 20% moisture exhibited features of a A-type pattern, with distinct
350
peaks at 2θ values of 15, 17, 19 and 23° (Popov et al., 2009; Londoño-Restrepo,
351
Rincón-Londoño, Contreras-Padilla, Millan-Malo, & Rodriguez-Garcia, 2018). In
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ACCEPTED MANUSCRIPT addition, the small peaks at 2θ values of 6.5 and 21.5° (Fig. 2) indicated the
353
occurrence of oleic or linoleic acids in the samples, which were the main components
354
of soybean oil. These results suggested that the oil absorbed by starch during frying
355
cannot be completely removed during the defatting process. The diffraction peaks
356
corresponding to the crystalline structures in the native NMS gradually decreased as
357
the initial moisture content of the samples increased. This result showed that the
358
crystalline structure was destroyed during frying. The higher the initial moisture
359
content, the more the crystal structures were destroyed. It is well known that water is
360
a good plasticizer for starch (Paris, Bizot, Emery, Buzaré, & Buléon, 1999), therefore,
361
more water molecules will promote changes in the structure of starch granules during
362
frying, including the expansion of the amorphous area, the disintegration of
363
crystalline zone, and the exudation of starch molecules (especially amylose).
364
The diffraction patterns of starch also indicated that the semi-crystalline
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structures in the samples were gradually destroyed when the frying temperature was
366
increased (Fig. 2B). The diffraction intensity for the peaks progressively weakened
367
when the frying temperature was increased from 120℃ to 210℃. Obviously, the
368
higher the temperature, the more heat was transferred into the starch granules. For this
369
reason, more hydrogen bonds are broken at higher temperatures, leading to the
370
destruction and meltdown of the crystalline regions in the starch.
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ACCEPTED MANUSCRIPT The intensity of the X-ray diffraction peaks gradually decreased with increasing
372
heating time (Fig. 2C), which was indicative of greater destruction and meltdown of
373
the crystalline structures within the starch granules. Obviously, the longer the time,
374
the more heat was transferred into the starch granules, promoting the destruction and
375
meltdown of crystalline structures in starch.
376
3.3. Short-range structures of fried NMS-PUL mixtures
SC
377
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The short-range order of the external region of the starch samples was evaluated using ATR-FTIR. The original and deconvoluted FTIR spectra in the range of
379
1200-800 cm-1 are shown in Fig. 3 and the ratios of absorbances at 1047/1015 cm-1
380
(R1) and 1015/995 cm-1 (R2) are summarized in Table 1.
The FTIR spectrum of starch in the range of 1200 to 800 cm-1 is known to be
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sensitive to changes in short-range order. Specifically, the absorbances at 1047 cm-1
383
and 1015 cm-1 are indicative of the amount of the crystalline and amorphous
384
structures, respectively (Miao, Zhang, Mu, & Jiang, 2010). For this reason,
385
measurement of the R1 value is widely used to quantify the degree of ordered
386
structure in starch.
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As shown in Fig. 3A-C, the bands at 1047, 1015, and 995 cm-1 in the original
388
spectra overlapped with each other, making it difficult to accurately calculate the R1
389
and R2. After deconvolution, the bands that overlapped with each other were
21
ACCEPTED MANUSCRIPT 390
successfully uncoupled (Fig. 3D-F). The bands at 1047, 1015, and 995 cm-1 could be
391
clearly distinguished.
392
As shown in Fig. 3D-F, the intensity of the band at 1047 cm-1 gradually decreased, while the intensity of the band at 1015 cm-1 increased with the intensifying
394
of frying conditions. Fried samples had lower R1 values than that of their native
395
counterpart (Table 1). For instance, the R1 significantly decreased from 0.620 in
396
NMS to 0.543 in NMS-20%, 0.492 in NMS-40%, 0.460 in NMS-60%, and 0.436 in
397
NMS-80%. R1 also significantly decreased with the increasing frying temperature
398
(Table 1), but it did not change appreciably with frying time (Table 1). The observed
399
changes in band intensity and R1 values suggested that the short-range order of the
400
starch molecules at the exterior of the granules gradually became disordered during
401
frying. The short-range order detected by FTIR is mainly related to the level of double
402
helices present, while the long-range order of starch detected by XRD is mainly
403
related to the packing of the double helices. Therefore, the decreased R1 observed in
404
the fried samples indicated that the double helices in the external region of the starch
405
granules gradually uncoiled during frying, especially at higher moisture contents and
406
higher temperature. As discussed earlier, more water was available at higher initial
407
moisture contents to plasticize the starch molecules. Moreover, at higher frying
408
temperatures the starch molecules are more likely to become disordered due to
409
configurational entropy effects. Thus, the hydrogen bonds between the starch
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ACCEPTED MANUSCRIPT 410
molecules that normally hold the double helices together are gradually destroyed,
411
thereby accounting for the observed decreased R1 in Table 1.
412
The absorbance peak at 995 cm-1 is sensitive to water and so the R2 value can be used to evaluate the interaction between starch and water (Van Soest, Tournois, De
414
Wit, & Vliegenthart, 1995). In the present work, the R2 values of fried samples were
415
significantly higher than that of their native counterpart (Table 1). For instance, the
416
R2 values of NMS-120℃, NMS-150℃, NMS-180℃, and NMS-210℃ were 0.739,
417
0.748, 0.740, and 0.749, respectively, which were all higher than the R2 value of NMS
418
(0.680). This result was due to the partial gelatinization of starch during frying, which
419
increased the interaction between starch molecules and water.
420
3.4. Molecular weight and size distribution
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The HPSEC-MALLS-RI chromatograms of native and fried starchy samples
422
treated at different conditions are illustrated in Fig. 4. The molecular weight and size
423
parameters including Mw, Ra, and PDI are also summarized in Table 1. In the refractive index (RI) chromatograms (Fig. 4B), native NMS showed a
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425
typical bimodal molecular weight distribution, with the left peak corresponding to
426
amylopectin and the right shoulder peak corresponding to amylose. The Mw, Ra, and
427
PDI values of NMS were 31.58×106 g/mol, 117.2 nm, and 1.121, respectively (Table
428
1).
23
ACCEPTED MANUSCRIPT 429
As shown in Fig. 4A, C, and E, the peak of fried starchy samples shifted to the right and the signal intensified at high elution volume, suggesting that degradation of
431
starch molecules had occurred during frying. Furthermore, in Fig. 4B, D, and F, the
432
right shoulder peak became more and more pronounced, indicating the formation of a
433
higher fraction of small molecules during frying. The ratio of the left peak to right
434
peak also decreased after frying, implicating that some amylopectin molecules were
435
degraded during frying, which was consistent with our previous work (Chen et al.,
436
2018).
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437
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With the intensifying of frying conditions, starch molecules degraded to a higher extent. For instance, the Mw decreased from 31.58×106 g/mol in NMS to 25.26×106
439
g/mol in NMS-20%, further decreased to 20.53×106 g/mol, 15.32×106 g/mol, and
440
12.08×106 g/mol when the initial moisture contents increased to 40%, 60%, and 80%,
441
respectively (Table 1). The Mw also significantly decreased with increasing frying
442
temperature and duration (Table 1). In addition, the apparent decrease in Ra and the
443
increase in the PDI (Table 1) proved the apparent degradation of starch molecules
444
and the enlarged structural heterogeneity of starch mass.
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Previous studies have shown that thermal treatment, with or without shear, can
446
induce the breakdown of starch molecules, especially for amylopectin (Liu, Halley, &
447
Gilbert, 2010; Van den Einde, Akkermans, Van der Goot, & Boom, 2004; Van Den
448
Einde, Van Der Goot, & Boom, 2003). The degradation of starch molecules during
24
ACCEPTED MANUSCRIPT frying results from two types of reaction, thermal hydrolysis (which occurs during
450
heating of moistened starch) and dry thermal depolymerization (which occurs during
451
heating of dry starch) (Van Den Einde, Van Der Goot, & Boom, 2003). It is
452
reasonable to speculate that the thermal hydrolysis reaction dominated during the
453
initial stage of frying, while the dry thermal depolymerization reaction dominated at
454
the later stage because most of the water disappeared. Regardless of the degradation
455
mode, both degradation processes of starch molecules enhanced when the frying
456
conditions were intensified, inducing a reduction in Mw and Ra, and an increase in
457
PDI (Table 1).
458
3.5. Total water and oil contents of fried starchy samples
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LF-NMR was applied to evaluate the effects of frying parameters on oil absorption in fried starchy samples in the present work. The spin-spin relaxation time
461
(T2) distinguishes proton signals from water and oil and can therefore be used to
462
simultaneously determine these two components in fried starchy systems (Chen et al.,
463
2017a). The T2 relaxation time spectra of fried starchy samples treated using different
464
frying conditions are shown in Fig. 5. The quantitative analysis of the oil and water
465
content calculated from the calculation curves using a method described earlier (Chen
466
et al., 2017a) are shown in Table 2. The oil contents obtained by LF-NMR analysis
467
were slightly higher than that given by Soxhlet extraction (Table S1), but this
468
difference was not significant in most samples except for the samples treated at 80%
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ACCEPTED MANUSCRIPT moisture or 210℃ (Table S1). This difference might result from the shortcoming of
470
Soxhlet extraction as discussed previously (Chen et al., 2017a). In samples treated at
471
intensified frying conditions, a small fraction of fatty acids was tightly combined with
472
starch molecules (Table 2, Fig. 2), and was not easily removed by organic solvent
473
extraction, thus leading to an underestimate of the oil contents in fried samples by
474
Soxhlet extraction.
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The peaks in the T2 relaxation time spectra of fried starchy samples could be roughly divided into two groups with T2 values < 10 ms and T2 values > 20 ms,
477
respectively (Fig. 5). According to our previous work, peaks with T2 values < 10 ms
478
corresponded to bound water, whereas those with T2 values > 20 ms corresponded to
479
oil (Chen et al., 2017a). Thus, the variation in the molecular mobility and distribution
480
of water and oil absorbed in the fried starch samples could easily be investigated. The
481
T2 relaxation time spectra of fried starchy samples changed appreciably with frying
482
parameters (Fig. 5).
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Compared to frying temperature and heating time, the initial moisture content had the most pronounced influence on the T2 distribution of both the bound water and oil
485
in the fried starchy samples (Fig. 5A). Two fractions of bound water could be
486
discerned in the fried samples with different initial moisture contents: (i) T2 from 0.05
487
to 1 ms, corresponding to tightly bound water; (ii) T2 > 1 ms, corresponding to weakly
488
bound water. This observation was consistent with our previous work (Chen et al.,
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ACCEPTED MANUSCRIPT 2017a). There was no significant difference (p > 0.05) in the water content of samples
490
fried at initial moisture contents of 20%, 40%, and 60% (Table 2), but a significant
491
increase in both the amount (from 0.0102 to 0.4683 g/g defatted dry solids) and
492
mobility (from 3 to 7 ms) of weakly bound water appeared in the sample fried at 80%
493
moisture content (Table 2). This result suggested that a fraction of the water was not
494
removed from the starchy samples with the highest initial moisture content under the
495
frying conditions used in this study. The initial moisture content had little influence
496
on the mobility of the oil protons, but it did have a major impact on the intensity of
497
the oil peak (Fig. 5A). This result suggested that the initial moisture content did not
498
interfere with the molecular transport of the oil molecules after frying (presumably
499
because the oil and water were in different domains), but it did alter the level of oil
500
absorbed. Quantificationally, the oil content significantly increased from 0.2396 to
501
0.6602 g/g defatted dry solids when the initial moisture content was increased from
502
20% to 40%, but then it decreased to 0.3614 and 0.2531 g/g defatted dry solids when
503
the initial moisture content was further increased to 60 and 80% (Table 2).
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Generally, the absorption of oil by foods during and after frying is controlled by
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505
three major mechanisms: (i) solidification mechanism (ii) condensation mechanism;
506
and (iii) capillary mechanism (Mellema, 2003). First, we consider the solidification
507
mechanism. During frying, the temperature is very high (usually higher than 180℃)
508
and the viscosity of oil is relatively low. However, when the fried samples are
509
removed from the hot oil, the temperature drops causing the oil viscosity to rapidly 27
ACCEPTED MANUSCRIPT increase, thereby leading to high surface adhesion of oil. Obviously, solidification and
511
adhesion of oil on samples happen as soon as samples are removed from the hot oil.
512
Second, we consider the condensation mechanism. When the fried samples are
513
separated from the hot oil, condensation of the water vapor in the pores of the starch
514
sample occurs, thereby generating sub-atmospheric pressure within the pores. As a
515
result, large amounts of oil are sucked into the pores. Third, we consider the capillary
516
mechanism. The porous and rough nature of the starch granule surfaces generates
517
capillary forces that are strongly enough to pull oil into them.
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RI PT
510
In the present work, the initial moisture content influenced the swelling of the starch granules and the destruction of crystalline structures during frying, thereby
520
producing starchy materials with different structural features. These structural
521
changes altered the capillary forces and the contact area of starch for oil, as well as
522
the magnitude of oil solidification and adhesion to the surfaces of the starch samples,
523
thereby affecting oil absorption by starch during frying.
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The initial increase in oil absorption was considered to be a result of the expansion of starch granules (Fig. 1), the destruction of the crystalline structures (Fig.
526
2), the disassembly of double helices (Fig. 3), and the degradation of starch molecules
527
(Fig. 4), which facilitated the ability of the starch granules to absorb oil because of
528
their open granular structures, high capillary forces, and large specific surface.
529
However, the decrease in oil content at higher moisture contents (60% and 80%) was
AC C
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28
ACCEPTED MANUSCRIPT not consistent with the trend reported previously (Chen et al., 2018b). This difference
531
may have originated from the fact that starches with different amylose contents were
532
used in the two studies. Indeed, amylose has previously been reported to affect oil
533
absorption by interfering with the structural evolution of starch or by directly
534
interacting with lipids during frying (Chen et al., 2019b).
535
RI PT
530
In the present work, the decrease in oil content observed at high initial moisture contents may be due to a number of phenomena. First, if all the moisture was not
537
removed from the starch granules during frying there might be less room for the oil to
538
absorb. Considering the high moisture content left in the NMS-80% (Table 2), the
539
low oil content absorbed by starch is expected. Second, the degree of starch granule
540
gelatinization during frying will increased with the initial moisture content. As a
541
result, the starch granules became disrupted and merged together (Fig. 1C), thereby
542
reducing the porosity of starch surface and the specific surface area of starch granules,
543
which decreased the capillary forces and contact area available for absorbing oil.
544
Third, the formation of amylose-lipids complexes at high moisture contents (Fig. 2A)
545
may also have contributed to the reduction in oil absorption during frying. At high
546
moisture contents, the hydrolysis of triglycerides occurred to a greater extent during
547
frying producing more free fatty acids (Dominik, Joanna, & Bartosz, 2018).
548
Moreover, the swollen starch granules might have leached a large amount of amylose
549
(Chen et al., 2018a). As a result of these two effects, the number of interactions
550
between amylose and free fatty acids increased in starchy samples containing high
AC C
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536
29
ACCEPTED MANUSCRIPT initial moisture contents. Unlike the starch gelatinization in water phase, the leached
552
amylose during frying cannot disperse in the oil phase and can only be deposited on
553
the surface of fried starchy samples. Thus, the resulting amylose-lipid complexes
554
around the surface of the fried samples may have worked as a physical layer
555
inhibiting oil absorption.
556
RI PT
551
The T2 relaxation time spectra of samples fried at different temperatures are
shown in Fig. 5B, and the results of the impact of frying temperature on water loss
558
and oil uptake are summarized in Table 2. There was no apparent change in the
559
mobility of protons in both bound water and oil (Fig. 5B), but the final water content
560
of the samples dramatically decreased from 0.1916 to 0.0027 g/g defatted dry solids
561
as the temperature increased from 120 to 210℃ (Table 2). This decrease in water
562
content was attributed to the high heat transfer rates at higher frying temperature,
563
which promoted the removal of water at a faster rate (Su, Zhang, Fang, & Zhang,
564
2017). As for the oil content, the oil content decreased from 0.7591 to 0.4873 g/g
565
defatted dry solids when the temperature increased from 120 to 210℃, however, no
566
significant difference (p > 0.05) in samples fried at 150 and 180℃ was found (Table
567
2), which was in agreement with previous studies (Kita, Lisińska, & Gołubowska,
568
2007). The significant reduction of oil content in the samples fried at 210℃ might be
569
ascribed to the melting and disintegration of the starch granules induced by excessive
570
heating (Fig. 1F). During frying at a limited moisture content, such as the 40% used
571
in this work, several transformations of starch structure occurred at the granular,
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30
ACCEPTED MANUSCRIPT crystal, and molecular levels, including the deformation of starch granules (Fig. 1),
573
the destruction of crystalline structures (Fig. 2), the unfolding of double helices (Fig.
574
3), the degradation of macromolecules (Fig. 4), and the expulsion of amylose (Chen et
575
al., 2018a). Thus, when the starch samples were fried at 210℃ or higher, the excessive
576
heat led to disintegration of the swollen starch granules (Fig. 1F). As a result, they
577
merged together, lost their porous surface, and reduced the effective contact area with
578
oil (Fig. 1F), thereby decreasing the magnitude of the capillary forces and the
579
adsorption capacity that normally hold the oil. Furthermore, the hydrolysis of
580
triglycerides in oil occurred more rapidly at higher temperatures, producing more free
581
fatty acids available for the V-type complexes formation between amylose and fatty
582
acids. Previous reports showed that amylose-fatty acid complexes mainly arranged in
583
the form of faceted crystalline structures or spherocrystalline particles (Zabar,
584
Lesmes, Katz, Shimoni, & Bianco-Peled, 2010). As a result, the V-type microcrystals
585
formed in fried samples might inhibit the oil absorption by starch by increasing the
586
compactness of the fried samples. These results showed that the level of oil absorption
587
can be reduced by increasing the frying temperature. However, this strategy is not
588
commercially feasible because of undesirable chemical reactions that occur at
589
elevated temperatures. For instance, heating above 180℃ has been reported to
590
generate polar materials (Li, Li, Wang, Cao, & Liu, 2017), form acrylamides
591
(Al-Asmar, Naviglio, Giosafatto, & Mariniello, 2018; Grob et al., 2003) and
592
hydrolyze and oxidize oils (Ben Hammouda, Triki, Matthäus, & Bouaziz, 2018; Cui,
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572
31
ACCEPTED MANUSCRIPT 593
Hao, Liu, & Meng, 2017; Mehta & Swinburn, 2001), which all have an adverse effect
594
on food quality, human health, and nutrition.
595
An increase in frying time reduced the final moisture content and oil content of the fried starchy samples (Fig. 5C). Specifically, the final moisture content decreased
597
from 0.1757 to 0.0144 g/g defatted dry solids and the final oil content decreased from
598
1.0755 to 0.5849 g/g defatted dry solids as the frying time increased from 5 to 30 min
599
(Table 2). The decrease in water content with increasing frying time is easy to
600
understand, since there should be a greater amount of water evaporation during the
601
prolonged frying. Previous studies showed that the total oil content of fried foods
602
increased rapidly during the first minute of frying but then remained fairly constant
603
(Durán, Pedreschi, Moyano, & Troncoso, 2007; Pedreschi, Cocio, Moyano, &
604
Troncoso, 2008). This did not appear to be the case for the relatively long frying time
605
(≥ 5 min) used in the present work. With extension of frying time, the water in the
606
samples rapidly evaporated, leaving the dehydrated starch to be heated at high
607
temperatures (180℃). As frying time prolonged, the surface temperature got higher
608
and higher, leading to the fragmentation of swollen starch granules (Fig. 1I), the
609
collapse and stripping of the structure near the surface of the starch granules (Fig. 1I)
610
as well as the degradation of starch molecules (Fig. 4F). These structural changes led
611
to the loss of the porous surface, the reduction of the effective contact area with oil,
612
and the decrease of the magnitude of the capillary forces, which would account for the
613
observed decrease in oil content with increased frying time (Table 2).
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32
ACCEPTED MANUSCRIPT 614
3.6. External oil contents of fried starchy samples The external oil content of the fried starchy samples was determined using
616
ATR-FTIR (Chen et al., 2018b). The ATR-FTIR spectra of fried samples are shown
617
in Fig. 6. The oil content located on the surface layer of the starch granules was
618
calculated by integrating the peak from 1693 to 1793 cm-1, which corresponded to the
619
ester group in the oil, and then substituting the value of the integrated area into the
620
linear regression equation for soybean oil (Chen et al., 2018b). The results obtained
621
from this analysis are summarized in Table 2.
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622
RI PT
615
Similar absorption peaks in the ATR-FTIR spectra of samples were observed regardless of the frying conditions. The molecular information corresponding to
624
functional groups in starch could be clearly seen, including the methyl group (-CH3)
625
in the range of 2995-2886 cm-1, methylene group (-CH2) in the range of 2886-2783
626
cm-1, and the fingerprint region of starch (C-O, C-C and C-O-H stretching and C-O-H
627
bending) in the range of 1200–900 cm-1. However, these absorption peaks might also
628
have originated from the oil. Therefore, the ATR-FTIR spectra of fried starchy
629
samples were a combination of spectrum of starch and oil (Fig. S1). As for the
630
fingerprint of oil, a well resolved peak was distinguishable in the range of 1693-1793
631
cm-1 with no superimposition and interference from starch, which was therefore used
632
as the basis for determining the surface oil using ATR-FTIR (Chen et al., 2018b).
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ACCEPTED MANUSCRIPT 633
As shown in Fig. 6, the peak intensity for ester group of the oil (1693-1793 cm-1) was sensitive to frying conditions, suggesting that the external oil content was also
635
affected by frying parameters. The external oil content of fried starchy samples
636
rapidly increased from 0.1438 to 0.4266 g/g defatted dry solids as the moisture
637
content increased from 20% to 40%, and then decreased with the further increasing of
638
moisture content (Table 2). The peak intensity of the ester group also changed with
639
frying temperature. It increased when the frying temperature was raised from 120 to
640
180℃, but then decreased sharply when it was raised further to 210℃ (Fig. 6B). As
641
discussed above, the gradual gelatinization of starch (Fig. 1), the diminishment of
642
crystalline structures (Fig. 2), the unfoldment of double helices (Fig. 3), and the
643
degradation of starch molecules (Fig. 4) during frying promoted the oil absorption by
644
starchy samples. On the other hand, the fragmentation and aggregation of starch (Fig.
645
1) contributed to the reduced oil content absorbed by starch fried at intensified frying
646
conditions. As frying time prolonged, the peak intensity increased (Fig. 6C),
647
corresponding to the gradual increase of external oil from 0.3570 to 0.5101 g/g
648
defatted dry solids (Table 2). The continuous increase of external oil content with the
649
prolongation of treatment time might be attributed to gradual transform of starch from
650
the compact semi-crystalline structure to the loose un-crystalline structure. At the
651
same time, the preservation of granular morphology (Fig. 1G-I) could further favor
652
the absorption of oil because of the higher specific surface of these granules than the
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34
ACCEPTED MANUSCRIPT 653
aggregate of broken granules as seen in fried samples treated at high moisture content
654
(Fig. 1C) or high temperature (Fig. 1F).
Interestingly, the proportion of external oil increased with increasing initial
656
moisture content, frying temperature, and frying time, with the exception of the
657
samples fried at 210℃. This was attributed to the fact that the heat and mass transfer
658
mainly occurred at the surface of the starch granules, and the extent of these changes
659
enhanced as these frying parameters intensified. This trend was in general agreement
660
with a previous study that showed oil uptake was mainly a surface phenomenon
661
(Bouchon, Hollins, Pearson, Pyle, & Tobin, 2001) and that most of the oil was
662
absorbed and located near the surface of the fried samples (Aguilera & Gloria, 1997;
663
Mellema, 2003).
M AN U
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655
Although the structural changes of samples treated using intensified frying
665
conditions, including the swelling and disintegration of starch granules, the leaching
666
of soluble substance, the collapse of the long-range crystalline structure, the
667
destruction of the short-range double helices, and the degradation of starch
668
macromolecule, were expected to facilitate the absorption of oil both in the external
669
and the internal fractions of starch. However, the expected increase in the proportion
670
of internal oil was not observed experimentally. In fact, after comparing the total oil
671
contents and surface oil contents of samples (Table 2), the proportion of the internal
672
oil in the samples decreased with intensified frying conditions. The unique
AC C
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35
ACCEPTED MANUSCRIPT gelatinization process and the distribution of amylose during frying might be
674
responsible for this effect. The gelatinization of starch in an oil phase is different from
675
that observed in a water phase. During frying, amylose molecules leached from the
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interior region of the granules to the surfaces, but could not move away from the
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surfaces into the surrounding oil phase. In other words, these leached amylose
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molecules could only be deposited on the surface where they could interact with free
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fatty acids, forming a physical layer that prevented oil from penetrating inside the
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samples. As a result, the proportion of internal oil decreased.
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Fried starchy samples treated under different conditions were defatted to further
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assess the affinity of oil with starch within the outer layer. The ATR-FTIR spectra of
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defatted samples are shown in Fig. 7 and the external oil content after defatting are
684
given in Table 2. The oil peak intensity and external oil content dramatically
685
decreased in all samples, and even disappeared completely in some samples (Fig. 7
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and Table 2). The huge reduction (around 68-100%) of external oil content after
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defatting indicated that most of the oil absorbed at the surface was only weakly bound
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with the starch granules through physical adsorption and thus was easily removed by
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organic solvent extraction, which agreed with our previous work (Chen et al., 2018b).
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4. Conclusions
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In the present work, the structural changes and oil absorption of normal maize starch (NMS) as affected by frying conditions were systematically investigated. 36
ACCEPTED MANUSCRIPT During frying, the granular morphology gradually disappeared, the crystalline
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structures destroyed, the double helices disrupted, and the starch molecules especially
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amylopectin degraded, all of which tended to happed at a high extent when frying
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conditions were intensified. Frying also induced a significant variation in the total and
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external oil contents absorbed in the fried starchy samples. The initial moisture
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content in the samples had the most pronounced influence on the amount of oil
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absorbed during frying. The level of oil absorbed first increased but then decreased as
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the initial moisture content increased. The proportion of external oil increased with
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increasing initial moisture content, frying temperature, and frying time. These
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changes in the hierarchical structures of starch contributed to the variation of oil
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absorption in NMS during frying. The improved understanding of the relationship
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between frying conditions, starch structural changes, and oil absorption will assist
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food processors in developing healthier reduced-fat fried foods.
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Conflict of interest
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No conflicts of interest are declared for any of the authors.
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Acknowledgements This study was financially supported by the National Natural Science Foundation of Jiangsu Province - China (No. BK20160052) and the Taishan Industry Leader
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Talent Project - China. The first author Long Chen also greatly appreciates the
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financial support from the Postgraduate Research & Practice Innovation Program of
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Jiangsu Province - China (No. KYLX16_0819), the Outstanding Doctoral Cultivation
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Fund in Jiangnan University - China (No. 1025210172160160) and Postgraduate
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Overseas Research Fund of Jiangnan University - China (No.
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3562050205183520/001).
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Fig. 1 SEM images of NMS fried at different conditions. (A-C) Fried samples treated
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at different moisture contents; (D-F) Fried samples treated at different temperatures;
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(G-I) Fried samples treated for different time.
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Fig. 2 The X-ray diffraction patterns of NMS fried at different conditions. (A) Fried
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Fig. 3 The original (A-C) and deconvoluted (D-F) FTIR spectra (in the range of
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1200-800 cm-1) of NMS fried at different conditions. (A)(D) Fried samples treated at
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different moisture contents; (B)(E) Fried samples treated at different temperatures;
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(C)(F) Fried samples treated for different time.
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Fig. 4 HPSEC-MALLS-RI chromatograms of NMS fried at different conditions. (A),
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(C), and (E) Light scattering (LS) signals and the molecular weight distribution of
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Fig. 5 The CPMG proton distribution of NMS fried at different conditions. (A) Fried
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samples treated at different moisture contents; (B) Fried samples treated at different
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Fig. 6 The ATR-FTIR spectra of NMS fried at different conditions (before defatting).
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(A) Fried samples treated at different moisture contents; (B) Fried samples treated at
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different temperatures; (C) Fried samples treated for different time.
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Fig. 7 The ATR-FTIR spectra of NMS fried (after defatting) at different conditions.
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(A) Fried samples treated at different moisture contents; (B) Fried samples treated at
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different temperatures; (C) Fried samples treated for different time.
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A
Molecular structure R2 (1015/995)
Mw (× 106 g/mol)
Ra (nm)
PDI
0.620 ± 0.010a
C
0.680 ± 0.002e
31.58 ± 1.06a
117.2 ± 3.1a
1.121 ± 0.08e
25.26 ± 0.82d 20.53 ± 1.03e 15.32 ± 1.34g 12.08 ± 1.33h
104.5 ± 1.8d 94.9 ± 2.6e 84.7 ± 2.8g 79.2 ± 1.4h
1.235 ± 0.05cde 1.336 ± 0.08bcd 1.468 ± 0.06ab 1.516 ± 0.17a
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R1 (1045/1015)
B
0.716 ± 0.007d 0.759 ± 0.004ab 0.773 ± 0.005a 0.705 ± 0.009d
0.571 ± 0.010b 0.546 ± 0.007c 0.494 ± 0.008de 0.485 ± 0.005e
0.739 ± 0.006c 0.748 ± 0.010bc 0.740 ± 0.006c 0.749 ± 0.009bc
30.87 ± 1.15abc 28.95 ± 1.32c 19.97 ± 1.03ef 15.33 ± 0.83g
112.3 ± 3.3b 106.7 ± 2.4cd 95.1 ± 2.5e 88.74 ± 1.3f
1.139 ± 0.05e 1.184 ± 0.11de 1.325 ± 0.08bcd 1.462 ± 0.06ab
0.512 ± 0.009d 0.512 ± 0.004d 0.493 ± 0.006de 0.480 ± 0.005e
0.747 ± 0.006bc 0.758 ± 0.007ab 0.760 ± 0.003ab 0.739 ± 0.004c
31.08 ± 0.93ab 29.38 ± 1.06bc 20.08 ± 1.03ef 18.13 ± 1.53f
116.7 ± 2.6a 110.3 ± 2.2bc 95.4 ± 2.0e 91.4 ± 1.7ef
1.121 ± 0.13e 1.101 ± 0.04e 1.329 ± 0.08bcd 1.358 ± 0.11abc
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0.543 ± 0.008c 0.492 ± 0.009de 0.460 ± 0.006f 0.436 ± 0.008g
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Native NMS Different moisture NMS-20% NMS-40% NMS-60% NMS-80% Different temperature NMS-120oC NMS-150oC NMS-180oC NMS-210oC Different time NMS-5 min NMS-10 min NMS-20 min NMS-30 min
Short range order
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Table 1 Short range ordered and molecular structures of NMS fried at different conditions (Moisture; Temperature; Time).
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NMS-20% moisture, 40% moisture, 60% moisture, and 80% moisture corresponded to normal maize starch fried at 20%, 40%, 60%, and 80% initial moisture content for samples, respectively. NMS-120oC, 150oC, 180oC, and 210oC corresponded to normal maize starch fried at 120oC, 150oC, 180oC, and 210oC, respectively. NMS-5 min, 10 min, 20 min, and 30 min corresponded to normal maize starch fried at 5 min, 10 min, 20 min, and 30 min, respectively. B R1 and R2 corresponded to the ratios of IR absorbances at 1045 to 1015 cm-1 and 1015 to 995 cm-1. C Data were means ± standard deviations (n = 3). Values in the same column with different lowercase letters were significantly different (p < 0.05) by Tukey’s test.
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Table 2
A
0.0182 ± 0.0070e B 0.0122 ± 0.0022e 0.0102 ± 0.0034e 0.4683 ± 0.0592a
0.2396 ± 0.0155g 0.6602 ± 0.0216c 0.3614 ± 0.0075f 0.2531 ± 0.0128g
0.1916 ± 0.0238b 0.1194 ± 0.0164c 0.0025 ± 0.0005e 0.0027 ± 0.0017e 0.1757 ± 0.0113b 0.0819 ± 0.0122d 0.0161 ± 0.0024e 0.0144 ± 0.0016e
Surface oil contents D (g/g defatted dry solids) Before defatting
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Total oil contents (g/g defatted dry solids)
After defatting N.D. C 0.0171 ± 0.0019c 0.0590 ± 0.0091a 0.0679 ± 0.0171a
0.7591 ± 0.0481b 0.6645 ± 0.0361c 0.6671 ± 0.0187c 0.4873 ± 0.0114e
0.2701 ± 0.0154e 0.2984 ± 0.0152d 0.3852 ± 0.0096c 0.1943 ± 0.0081g
N.D. N.D. 0.0458 ± 0.0077b 0.0610 ± 0.0058a
1.0755 ± 0.0262a 0.6649 ± 0.0290c 0.6733 ± 0.0187c 0.5849 ± 0.0148d
0.3570 ± 0.0221b 0.3749 ± 0.0225c 0.3898 ± 0.0104c 0.5101 ± 0.0139a
N.D. N.D. 0.0372 ± 0.0033b 0.0373 ± 0.0034b
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0.1438 ± 0.0075h 0.4266 ± 0.0141b 0.2918± 0.0055de 0.2329 ± 0.0144f
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Different moisture NMS-20% NMS-40% NMS-60% NMS-80% Different temperature NMS-120oC NMS-150oC NMS-180oC NMS-210oC Different time NMS-5 min NMS-10 min NMS-20 min NMS-30 min
Total water contents (g/g defatted dry solids)
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The total water contents, total oil contents and surface oil contents of NMS fried at different conditions (Moisture; Temperature; Time).
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NMS-20% moisture, 40% moisture, 60% moisture, and 80% moisture corresponded to normal maize starch fried at 20%, 40%, 60%, and 80% initial moisture content for samples, respectively. NMS-120oC, 150oC, 180oC, and 210oC corresponded to normal maize starch fried at 120oC, 150oC, 180oC, and 210oC, respectively. NMS-5 min, 10 min, 20 min, and 30 min corresponded to normal maize starch fried at 5 min, 10 min, 20 min, and 30 min, respectively. B Data were means ± standard deviations (n = 3). Values in the same column with different lowercase letters were significantly different (p < 0.05) by Tukey’s test. C Not detectable. D The internal oil content could be determined by subtracting the external oil content from the total oil content.
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Oil absorption of normal maize starch (NMS) was affected by frying conditions.
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Hierarchical structures of NMS changed a lot during frying. Moisture content had the most pronounced influence on oil absorption.
The proportion of external oil increased as frying treatment intensified.
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Changes in hierarchical structures affected oil absorption by starch during frying.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0268-005X(18)32022-8
DOI:
https://doi.org/10.1016/j.foodhyd.2019.105231
Article Number: 105231 Reference:
FOOHYD 105231
To appear in:
Food Hydrocolloids
Received Date: 12 October 2018 Revised Date:
15 July 2019
Accepted Date: 17 July 2019
Please cite this article as: Chen, L., Ma, R., Zhang, Z., McClements, D.J., Qiu, L., Jin, Z., Tian, Y., Impact of frying conditions on hierarchical structures and oil absorption of normal maize starch, Food Hydrocolloids (2019), doi: https://doi.org/10.1016/j.foodhyd.2019.105231. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Impact of frying conditions on hierarchical structures and oil absorption of normal maize starch Long Chena,b,c, Rongrong Maa, Zipei Zhangc, David Julian McClementsc, Lizhong
a
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Qiud, Zhengyu Jina,b,d, Yaoqi Tiana,b,d* State Key Laboratory of Food Science and Technology, Jiangnan University, 1800
Lihu Road, Wuxi 214122, China
School of Food Science and Technology, Jiangnan University, 1800 Lihu Road,
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Wuxi 214122, China
Department of Food Science, University of Massachusetts, Amherst, MA 01003,
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Zhucheng Xingmao Corn Developing Co., Ltd, Weifang 262200, China
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* Corresponding author
Yaoqi Tian, Professor in Food Science
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Email: [email protected] (Y. Tian); [email protected] (L. Chen)
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Graphical abstract:
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Impact of frying conditions on hierarchical structures and oil
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absorption of normal maize starch
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Long Chena,b,c, Rongrong Maa, Zipei Zhangc, David Julian McClementsc, Lizhong
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Qiud, Zhengyu Jina,b,d, Yaoqi Tiana,b,d*
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a
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Lihu Road, Wuxi 214122, China
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b
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Wuxi 214122, China
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c
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State Key Laboratory of Food Science and Technology, Jiangnan University, 1800
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School of Food Science and Technology, Jiangnan University, 1800 Lihu Road,
Department of Food Science, University of Massachusetts, Amherst, MA 01003,
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USA
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Zhucheng Xingmao Corn Developing Co., Ltd, Weifang 262200, China
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* Corresponding author
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Yaoqi Tian
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Professor in Food Science
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Email: [email protected] (Y. Tian); [email protected] (L. Chen)
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Abstract Knowledge about oil absorption during frying is crucial for the design and production of healthier reduced-fat food products. For this reason, we systematically
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investigated the structural changes and oil absorption of normal maize starch (NMS)
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during simulated frying conditions. In particular, the impact of initial moisture
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content, frying temperature, and frying time were examined. The hierarchical
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structures of the fried samples were firstly characterized using scanning electron
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microscope (SEM), X-ray diffraction (XRD), infrared spectroscopy (FTIR), and size
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exclusion chromatography. Furthermore, the impact of frying conditions on oil
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absorption by NMS was investigated using LF-NMR and ATR-FTIR based methods.
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During frying, the granular morphology of the starch granules was lost, their internal
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crystalline structures were disrupted, their double helices were broken down, and
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starch molecules were degraded. These changes were related to the absorption of oil
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by the starch granules during frying. The initial moisture content of the starch samples
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had the most pronounced influence on the amount of oil absorbed during frying. The
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oil content first increased and then decreased with increasing moisture content, being
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0.2396, 0.6602, 0.3614, and 0.2531 g/g starch for 20, 40, 60 and 80% moisture
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content, respectively. The fraction of oil present at the exterior of the samples after
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frying increased with increasing moisture content, frying temperature, and frying
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time. The variation in oil absorption were attributed to changes in the hierarchical
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structures of the fried samples, including granule morphology, crystalline property,
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double helices, and molecular features.
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Keywords: Fried starchy samples; Frying conditions; Hierarchical structures; Oil
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contents; LF-NMR; ATR-FTIR
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Chemical compounds studied in this article
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Starch (PubChem CID: 24836924); Water (PubChem CID: 962); Amylose (PubChem
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CID: 53477771); Amylopectin (PubChem CID: 439207); MnCl2.4H2O (PubChem
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CID: 643989); n-Hexane (PubChem CID: 8058).
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Please note: Edible oil is not a specific chemical, it is the mixture of triglyceride and
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fatty acid. Thus, soybean oil will not have a specific record in PubChem.
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1. Introduction Frying is a common food preparation method used in both industrial food production and household cooking (Contardo, Parada, Leiva, & Bouchon, 2016;
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Mehta & Swinburn, 2001). In addition to cooking, frying also plays an important role
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in the dehydration and sterilization of foods (Durán, Pedreschi, Moyano, & Troncoso,
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2007). The boiling and evaporation of water during frying mainly occur at the
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surfaces of foods, which makes room for hot oil to be absorbed (Dana & Saguy,
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2006). The amount of oil taken up by foods depends on the level of dehydration
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during frying, and may fluctuate between about 8 to 40% by total weight (Mehta &
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Swinburn, 2001; Mellema, 2003). The overconsumption of highly palatable fried
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foods is of concern to many consumers and medical professionals because of their
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potential to promote obesity and related diseases (Guo, Ye, Bellissimo, Singh, &
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Rousseau, 2017; Kurek, Ščetar, & Galić, 2017). Despite this, fried foods are still
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popular in both developed and developing countries. Consequently, it is important for
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food scientists to understand the factors that impact the fat content of fried foods so
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that high quality versions with better health profiles can be developed.
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Frying involves a combination of heat and mass transfer processes that are
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impacted by frying conditions and food properties (Mehta & Swinburn, 2001;
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Naghavi, Dehghannya, & Ghanbarzadeh, 2018). Numerous studies have been carried
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out to establish the effects of these parameters on oil absorption and food quality so as
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role in oil uptake, which has mainly been attributed to a combination of condensation
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and capillary mechanisms (Mellema, 2003). Typically, a higher initial moisture
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content leads to a higher oil uptake (Chen et al., 2018b; Mehta & Swinburn, 2001).
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The frying temperature is another important factor affecting oil uptake with higher
78
temperatures typically leading to a lower oil content (Abd Rahman, Abdul Razak,
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Lokmanalhakim, Taip, & Mustapa Kamal, 2017; Moyano & Pedreschi, 2006).
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However, some studies have reported an increase in oil absorption with increasing
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frying temperature (Gamble, Rice, & Selman, 1987; Garayo & Moreira, 2002). The
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level and distribution of oil absorbed by a food product are also affected by the
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duration of frying. Increasing the frying time has been reported to increase the oil
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content of potato chips (Cruz et al., 2018). This may partly be because the progressive
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evaporation of water during frying leads to an increase in the calculated oil content
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when reported on a wet-weight basis. Conversely, frying time and temperature have
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been reported to have little impact on the oil content of corn chips (Yuksel et al.,
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2017). One of the main reasons for the conflicting findings reported in different
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studies is that real foods are often used as test materials, which have different
90
compositions and structures. Another in-negligible reason for these contrary
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conclusions between studies is that the results are often reported using different
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concentration units, i.e., wet- or dry-weight basis. Typically, the oil content is higher
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when the results are reported on a dry-weight basis rather than a wet-weight basis. It
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This is because the total weight of samples (oil+solids+water) will vary with the
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temperature and treatment duration (caused by the evaporation of water during frying)
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when calculated on a wet basis, confusing the comparison and analysis of related data
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in different studies.
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Many different analytical techniques have been used to determine the level, migration, and distribution of oil in foods, including the Soxhlet extraction,
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differential scanning calorimetry (Aguilera & Gloria, 1997), confocal microscopy
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(Zhu, Zou, Shi, Zhao, & Huang, 2017), Raman spectroscopy (Dong, Wu, Chen, &
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Liu, 2017), and infrared microscopy (Bouchon, Hollins, Pearson, Pyle, & Tobin,
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2001). These techniques provide valuable information about the properties of oil
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during frying but they are time-consuming, required sophisticated sample processing,
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and/or have high susceptibility to interference from other food components.
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Conversely, low field nuclear magnetic resonance (LF-NMR) is a rapid, non-invasive,
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and low-cost technology that can be used to detect the distribution and mobility of the
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protons of certain small molecules in complex food systems (Li et al., 2015).
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LF-NMR has been successfully used for the characterization of water in starchy gel
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(Chen, Tian, Tong, Zhang, & Jin, 2017b), the measurement of oil in botanical seed
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(Niu, Li, Chen, & Xu, 2014), and the real-time monitoring of the dynamic change in
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the lipid concentration in the cells during microalgal fermentation processes (Wang et
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al., 2016). In our previous work, a LF-NMR based method has been established for
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(Chen et al., 2017a). However, LF-NMR could only analyze the overall oil content of
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the samples, rather than the distribution of oil between the internal and external
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portions. Therefore, an ATR-FTIR based method has been used to determine the level
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of oil on the external surfaces of fried starch granules, which is based on the limited
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penetration depth of the infrared beam from the ATR accessory (Chen et al., 2018b).
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Starch is the main component in many fried foods, including potato chips, wheat dough, corn paste, instant noodles, and coating flour. Little information is, however,
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currently available about the impact of frying conditions on the structural changes and
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oil absorption of starch. This lack of knowledge is hindering the effective control of
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oil absorption by starch-based foods during frying. For this reason, a starch-oil-water
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model system with a well-defined composition and structure has been developed to
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facilitate the performance and interpretation of frying studies (Chen et al., 2018a;
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Chen et al., 2018b).
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In the present work, the hierarchical structures of the fried starch samples,
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including granule morphology, crystallinity, double helix formation, and molecular
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properties, were characterized using scanning electron microscope (SEM), X-ray
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diffraction (XRD), infrared spectroscopy (FTIR), and size exclusion chromatography,
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respectively. We postulated that integration of the information obtained from these
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different analytical methods would lead to a more comprehensive understanding of
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mobility and total oil content of the fried starch samples were determined using a
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LF-NMR method, whereas the external oil content was determined using an
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ATR-FTIR method. The information obtained from these methods is useful for
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understanding the influence of frying conditions on oil absorption.
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Overall, this study should therefore provide valuable insights into the major
factors impacting oil uptake during the frying of starchy foods, which may be useful
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for developing healthier fried foods with lower fat contents.
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2. Materials and methods
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2.1. Materials
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Co., Ltd. (Suzhou, China). It contained 27.3% amylose, 13.1% moisture content,
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0.95% free lipids, and 0.1% proteins. Soybean oil produced by Yi Hai Kerry Co., Ltd.
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(Shanghai, China) was purchased from the local supermarket. MnCl2.4H2O and
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n-hexane of chromatographical purity were supplied by Sinopharm Chemical Reagent
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Co., Ltd. (Shanghai, China). Petroleum ether with a boiling range of 30 to 60°C was
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purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other
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chemicals and reagents were of analytical grade unless otherwise stated.
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2.2. Preparation of fried starchy samples under different conditions The heat-moisture treatments of starch in oil phase under different conditions
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were performed as described in our previous studies (Chen et al., 2019a, b, c; Chen et
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al., 2018a) with some modifications. Initially, model samples with different moisture
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contents were prepared as follows. The NMS was first hydrated to water contents of
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20%, 40%, 60%, and 80%, in sealed plastic bags for 10 h to achieve moisture
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equilibrium. Then, the hydrated NMS (5 g starch, dry basis) was directly dispersed
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into 100 mL of soybean oil at 20°C using a magnetic stirrer (C-MAG HS 7, IKA Co.,
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Germany). Subsequently, the starch-oil-water mixtures were treated at 180°C in an oil
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bath for 20 min to mimic frying. Afterward, the hot samples were rapidly removed
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from the hot oil by vacuum filtration. The total and external oil contents of the fresh
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fried samples were rapidly analyzed using LF-NMR and ATR-FTIR methods,
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respectively. Then, the fried samples were defatted by Soxhlet extraction using
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petroleum ether as an organic solvent, dried in a vacuum drying oven at 40°C for 12
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h, milled to pass through a 100-mesh sieve, and preserved in vacuum bags prior to
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further structural analysis.
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Samples fried at different temperatures were prepared as follows. The NMS was
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first hydrated to 40% moisture content in a sealed plastic bag for 10 h, then dispersed
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into 100 mL of soybean oil, and then heated at 120, 150, 180, and 210°C for 20 min
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using an oil bath. Subsequently, the fresh fried samples were promptly isolated from
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the hot oil using vacuum filtration. The post-treatment process of fried samples was
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the same as above.
Samples fried for different durations were prepared as follows. The NMS was
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firstly hydrated to 40% moisture content in a sealed plastic bag for 10 h, dispersed
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into 100 mL of soybean oil, and then heated at 180°C in an oil bath for 5, 10, 20, and
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30 min, respectively. Subsequently, the fresh fried samples were promptly isolated
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from the hot oil using vacuum filtration. The post-treatment process of the fried
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samples was then the same as described earlier.
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2.3. Scanning electron microscope (SEM)
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The morphology of the starch granules after frying was monitored using SEM (Quanta 200, FEI Inc., Hillsboro, OR, United States). The fried starchy samples were
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defatted by Soxhlet extraction, dehydrated by vacuum drying, spread on a specimen
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platform, and then coated with gold palladium. Finally, their microstructures were
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observed at 1200 × resolution at a low voltage of 5.0 kV to avoid the sample damage
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induced by the electron beam.
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2.4. X-ray diffraction analysis (XRD)
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Before measurement, the fresh fried samples were first defatted using Soxhlet
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extraction. The long-range ordered structural properties of defatted starchy samples
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were then determined using a Bruker X-ray diffractometer (D2 PHASER, Bruker
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CuKα radiation, with λ= 1.5406 angstrom (monochromatic). Diffractograms were
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recorded from 4 to 40° (2θ) with a step size of 0.005° to improve the resolution of
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diffraction peaks. It is worth noting that both elastic and inelastic scattering coexist in
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starchy samples because of the fact that both of them are diffraction phenomena. It is
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not, however, possible to separate the signals of elastic and inelastic scattering in the
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X-ray diffraction pattern. Therefore, a calculation of the percent crystalline material
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present is not accurate (Rodriguez-Garcia, Londoño-Restrepo, Ramirez-Gutierrez, &
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Millan-Malo, 2018).
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2.5. Attenuated total reflection - Fourier transform infrared spectroscopy
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(ATR-FTIR)
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The short-range ordered structures of fried starchy samples were studied using a FTIR spectrometer (IS10, ThermoNicolet Inc., Waltham, MA, United States).
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Generally, the structural changes of starch during frying started from the surface,
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therefore, in the present work ATR-FTIR was used to detect the possible short-range
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structural changes on the external region of the starch. The IR spectra of an empty cell
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(air as the control) and defatted sample were collected at a resolution of 4 cm-1 for 32
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scans. Spectra acquired in the range of 1200-800 cm-1 are sensitive to the degree of
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molecular order of starch and so they were used to calculate the short-range ordered
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structure parameters, i.e. the ratios of IR absorbances at 1047 to 1015 cm-1 (R1) and
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spectrum was first smoothed and then deconvoluted at a half-width of 19 cm-1 and a
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resolution enhancement factor of 1.9. Subsequently, the R1 and R2 values of the
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samples were calculated.
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2.6. Size Exclusion Chromatography
Size exclusion chromatography was used to evaluate the molecular structures of
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the fried samples as affected by frying conditions according to our previous work with
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some slight modifications (Sun, Tian, Chen, & Jin, 2017). Briefly, samples (5 mg, dry
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basis) were dissolved in 5 mL 90% DMSO by heating in a boiling water bath with
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continuous stirring for 12 h. Then, the starch molecules were collected by alcohol
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precipitation using an 8-fold volume of ethanol. The precipitate was collected and the
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extra ethanol was removed using the vortex nitrogen sweeping method. The starch
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was then dispersed in the mobile phase (0.1 M acetate buffer containing 0.02% (w/v)
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NaN3 and 50 mM NaNO3), and injected into a high-performance liquid
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chromatography (HPLC) system equipped with a multi-angle laser light-scattering
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(MALLS) detector (Wyatt Technologies, Santa Barbara, CA, USA), a
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refractive-index (RI) detector (Wyatt Technologies, Santa Barbara, CA, USA), and
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three Phenogel columns (guard column, Shodex OHpak SB-806 HQ, Shodex OHpak
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SB-804 HQ) (Showa Denko K.K., Kawasaki, Japan). The flow rate of the mobile
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phase was set at 0.6 mL/min and the temperature in the column was maintained at
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instrument software (Astra version 5.3.4, Wyatt Technologies, Santa Barbara, CA,
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USA).
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2.7. Determination of total water and oil contents of fried samples using
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LF-NMR
Low-field nuclear magnetic resonance (LF-NMR) measurements were conducted
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using a 23 MHz NMR analyzer (NMI20-015V-I, Niumag Co., Ltd., Suzhou, China)
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according to our previous work (Chen et al., 2017a; Chen, Tian, Tong, Zhang, & Jin,
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2017b). Samples (2-3 g) were accurately weighed into 10 mL glass sample bottles that
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were then sealed with three layers of Teflon tape. The sample bottle was then
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carefully transferred into an NMR test tube of 25 mm diameter. Spin-spin relaxation
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time (T2) signals of the samples were then acquired at 32°C using the
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Carr-Purcell-Meiboom-Gill (CPMG) sequence.
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system, 3% MnCl2·4H2O aqueous solution and soybean oil were used as the standard
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materials in the preparation of calibration curves for water and oil, respectively.
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Briefly, the T2 signals from reference substances of known mass were collected and
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inverted to T2 distribution curves. For LF-NMR analysis, the areas under the curves
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corresponding to specific T2 ranges are proportional to the number of protons in
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different environments. This approach was used to determine the amount of water and
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(Chen et al, 2017a). All samples were measured under the same operating conditions
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and all data were normalized by sample weight to ensure the signals were comparable
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in terms of intensities. The fried samples were defatted and then dried to a constant
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weight. To ensure the accuracy of the analysis, the results in present work were
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expressed on a dried-weight basis, i.e. (grams of oil or water) / (grams of defatted dry
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solids). Furthermore, for the purpose of comparison, Soxhlet extraction (AOAC,
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1990) was also used to measure the total oil contents of the fried samples.
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2.8. Determination of external oil contents of fried samples using ATR-FTIR
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Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) analysis was used to determine the level of surface oil on the fried samples due to the short
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penetration depth of infrared light (Chen et al., 2018b). A commercial FTIR
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spectrometer (IS10, ThermoNicolet Inc., America) combined with a 50 µL ATR
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accessory was used to acquire the spectra. Soybean oil was dispersed into n-hexane to
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prepare oil reference substances with varying oil concentrations. Then, the soybean
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oil dispersions or the fried starchy samples were rapidly daubed onto the surface of
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the ATR crystal, and the FTIR spectra were measured from 600-4000 cm-1 at a
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resolution of 4 cm-1. After baseline subtraction and smoothing, the peak area for 1743
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cm-1 was calculated by integrating from 1693 to 1793 cm-1, and the calibration curves
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for the quantitative analysis of surface oil were obtained by linear regression between
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starchy samples were calculated by substituting the peak area values for each fried
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sample in the linear regression equation. Both the fried starchy samples and their
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defatted counterparts were analyzed to distinguish the oil fractions that associated
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strongly or weakly with the starch molecules close to the surface. The fried samples
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were defatted and then dried to a constant weight, and the results were expressed as
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(grams of oil) / (grams of defatted dry solids).
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2.9. Statistical analysis
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Means ± standard deviations of triplicate determinations were used unless otherwise stated. One-way ANOVA (Tukey’s test) was applied to assess the statistical
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significance of data using SPSS 20.0 (SPSS Inc., Chicago, USA). The value of p <
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0.05 was considered to be statistically significant.
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3. Results and discussion
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3.1. Morphological changes of fried starchy samples
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shown in Fig. 1. Compared to native NMS, no obvious changes could be observed in
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the samples initially containing 20% moisture content after frying (Fig. 1A), while
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considerable distortion and deformation of the starch granules could be observed for
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the sample initially containing 40% moisture content (Fig. 1B). The starch granules in
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granules with pinholes and flaws on their surface (Fig. 1A), which was consistent
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with previous studies (Chen et al., 2018a; Dhital, Shelat, Shrestha, & Gidley, 2013).
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The expansion of the starch granules was limited due to the inhibition of swelling and
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gelatinization caused by frying (Chen et al., 2018a). When the sample was treated at
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40% moisture content, the pores and flaws on the granule surface disappeared and
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membranous or fragmented structures were observed (Fig. 1B). It is worth noting that
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the gelatinization of starch in oil is different from the classical gelatinization of starch
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in water. The hydrophilic amylose molecules that leach from the starch granules
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during frying cannot simply diffuse into the oil phase. Instead, they deposit onto the
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surfaces of the starch granules, which would account for the observed disappearance
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of the porous structure on the starch granule surfaces. Additionally,
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erythrocyte-shaped granules with concave cores and thick edges were observed after
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frying (Fig. 1B), supporting the hypothesis that the core of the starch granules was
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less densely packed and less thermally stable than the outer layer of granules
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(Copeland, Blazek, Salman, & Tang, 2009). When the moisture content increased to
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60%, the majority of the starch granules dissociated into small fragments and then
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merged into large clumps (Fig. 1C).
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As the frying temperature was increased, the fraction of granules that had lost
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their integrity increased. When fried at 150℃, most of the starch granules were still
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visible, but pores and flaws on their surfaces disappeared (Fig. 1D). At 180℃, many 17
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some membranous or fragmented structures on their surfaces (Fig. 1E). The
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morphology of the fried starchy samples then only changed a little as the temperature
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was further increased from 180 to 210℃ (Fig. 1F).
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Intact starch granules with membrane-like materials on their surfaces were also
observed in samples fried at different time. However, the treatment time did not have
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a major impact on morphology of the granules (Fig. 1G-I). Even so, there did appear
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to be slightly more disruption and merging of the starch granules for the longest
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treatment (Fig. 1I).
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The crystalline regions in native starch granules are composed of oriented double-helices of amylopectin side chains, while the amorphous regions are made up
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of amylose and the branching points of amylopectin (Tester, Karkalas, & Qi, 2004).
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The internal structure of the starch granules is believed to be largely held together by
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hydrogen bonds between the starch molecules. According to previous studies, the
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gelatinization of starch in a low moisture content environment (such as frying) could
327
more reasonably be defined as the “melting” (Liu, Xie, Yu, Chen, & Li, 2009).
328
During the initial stage of frying, the presence of water molecules in samples
329
promotes the swelling of the amorphous regions, which leads to the disruption of the
330
hydrogen bonds holding the starch molecules together. In addition, the swelling of the
331
amorphous regions around the crystalline regions could pull apart the crystalline
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18
ACCEPTED MANUSCRIPT structures by stripping of starch chains from the crystallites (Donovan, 1979). When
333
the frying conditions are intensified, the amount of water present diminishes and the
334
internal temperature increases. Under these conditions, starch molecules become more
335
flexible, the hydrogen bonds between the starch molecules are progressively ruptured,
336
the crystalline regions melt, and eventually the granules become deformed, fractured,
337
and aggregated (Fig. 1).
338
3.2. Long-range crystalline structures of fried starchy samples
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XRD was applied to further investigate the impact of frying parameters on the
340
long-range crystalline structure of the fried samples. The XRD patterns of the defatted
341
samples are presented in Fig. 2. The X-ray diffraction pattern of starch clearly
342
depended on the nature of the frying conditions used, suggesting that there were
343
pronounced changes in the long-range molecular organization of the starch molecules.
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The initial moisture content had an important impact on the crystalline characteristics of the fried starch (Fig. 2A). The native NMS exhibited broad
346
diffraction peaks (Fig. 2), indicating the existence of nanocrystals and lamellar
347
structures in the starch granules (Londoño-Restrepo, Rincón-Londoño,
348
Contreras-Padilla, Millan-Malo, & Rodriguez-Garcia, 2018). The native NMS and
349
sample fried at 20% moisture exhibited features of a A-type pattern, with distinct
350
peaks at 2θ values of 15, 17, 19 and 23° (Popov et al., 2009; Londoño-Restrepo,
351
Rincón-Londoño, Contreras-Padilla, Millan-Malo, & Rodriguez-Garcia, 2018). In
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ACCEPTED MANUSCRIPT addition, the small peaks at 2θ values of 6.5 and 21.5° (Fig. 2) indicated the
353
occurrence of oleic or linoleic acids in the samples, which were the main components
354
of soybean oil. These results suggested that the oil absorbed by starch during frying
355
cannot be completely removed during the defatting process. The diffraction peaks
356
corresponding to the crystalline structures in the native NMS gradually decreased as
357
the initial moisture content of the samples increased. This result showed that the
358
crystalline structure was destroyed during frying. The higher the initial moisture
359
content, the more the crystal structures were destroyed. It is well known that water is
360
a good plasticizer for starch (Paris, Bizot, Emery, Buzaré, & Buléon, 1999), therefore,
361
more water molecules will promote changes in the structure of starch granules during
362
frying, including the expansion of the amorphous area, the disintegration of
363
crystalline zone, and the exudation of starch molecules (especially amylose).
364
The diffraction patterns of starch also indicated that the semi-crystalline
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structures in the samples were gradually destroyed when the frying temperature was
366
increased (Fig. 2B). The diffraction intensity for the peaks progressively weakened
367
when the frying temperature was increased from 120℃ to 210℃. Obviously, the
368
higher the temperature, the more heat was transferred into the starch granules. For this
369
reason, more hydrogen bonds are broken at higher temperatures, leading to the
370
destruction and meltdown of the crystalline regions in the starch.
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ACCEPTED MANUSCRIPT The intensity of the X-ray diffraction peaks gradually decreased with increasing
372
heating time (Fig. 2C), which was indicative of greater destruction and meltdown of
373
the crystalline structures within the starch granules. Obviously, the longer the time,
374
the more heat was transferred into the starch granules, promoting the destruction and
375
meltdown of crystalline structures in starch.
376
3.3. Short-range structures of fried NMS-PUL mixtures
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377
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The short-range order of the external region of the starch samples was evaluated using ATR-FTIR. The original and deconvoluted FTIR spectra in the range of
379
1200-800 cm-1 are shown in Fig. 3 and the ratios of absorbances at 1047/1015 cm-1
380
(R1) and 1015/995 cm-1 (R2) are summarized in Table 1.
The FTIR spectrum of starch in the range of 1200 to 800 cm-1 is known to be
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sensitive to changes in short-range order. Specifically, the absorbances at 1047 cm-1
383
and 1015 cm-1 are indicative of the amount of the crystalline and amorphous
384
structures, respectively (Miao, Zhang, Mu, & Jiang, 2010). For this reason,
385
measurement of the R1 value is widely used to quantify the degree of ordered
386
structure in starch.
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As shown in Fig. 3A-C, the bands at 1047, 1015, and 995 cm-1 in the original
388
spectra overlapped with each other, making it difficult to accurately calculate the R1
389
and R2. After deconvolution, the bands that overlapped with each other were
21
ACCEPTED MANUSCRIPT 390
successfully uncoupled (Fig. 3D-F). The bands at 1047, 1015, and 995 cm-1 could be
391
clearly distinguished.
392
As shown in Fig. 3D-F, the intensity of the band at 1047 cm-1 gradually decreased, while the intensity of the band at 1015 cm-1 increased with the intensifying
394
of frying conditions. Fried samples had lower R1 values than that of their native
395
counterpart (Table 1). For instance, the R1 significantly decreased from 0.620 in
396
NMS to 0.543 in NMS-20%, 0.492 in NMS-40%, 0.460 in NMS-60%, and 0.436 in
397
NMS-80%. R1 also significantly decreased with the increasing frying temperature
398
(Table 1), but it did not change appreciably with frying time (Table 1). The observed
399
changes in band intensity and R1 values suggested that the short-range order of the
400
starch molecules at the exterior of the granules gradually became disordered during
401
frying. The short-range order detected by FTIR is mainly related to the level of double
402
helices present, while the long-range order of starch detected by XRD is mainly
403
related to the packing of the double helices. Therefore, the decreased R1 observed in
404
the fried samples indicated that the double helices in the external region of the starch
405
granules gradually uncoiled during frying, especially at higher moisture contents and
406
higher temperature. As discussed earlier, more water was available at higher initial
407
moisture contents to plasticize the starch molecules. Moreover, at higher frying
408
temperatures the starch molecules are more likely to become disordered due to
409
configurational entropy effects. Thus, the hydrogen bonds between the starch
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ACCEPTED MANUSCRIPT 410
molecules that normally hold the double helices together are gradually destroyed,
411
thereby accounting for the observed decreased R1 in Table 1.
412
The absorbance peak at 995 cm-1 is sensitive to water and so the R2 value can be used to evaluate the interaction between starch and water (Van Soest, Tournois, De
414
Wit, & Vliegenthart, 1995). In the present work, the R2 values of fried samples were
415
significantly higher than that of their native counterpart (Table 1). For instance, the
416
R2 values of NMS-120℃, NMS-150℃, NMS-180℃, and NMS-210℃ were 0.739,
417
0.748, 0.740, and 0.749, respectively, which were all higher than the R2 value of NMS
418
(0.680). This result was due to the partial gelatinization of starch during frying, which
419
increased the interaction between starch molecules and water.
420
3.4. Molecular weight and size distribution
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The HPSEC-MALLS-RI chromatograms of native and fried starchy samples
422
treated at different conditions are illustrated in Fig. 4. The molecular weight and size
423
parameters including Mw, Ra, and PDI are also summarized in Table 1. In the refractive index (RI) chromatograms (Fig. 4B), native NMS showed a
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425
typical bimodal molecular weight distribution, with the left peak corresponding to
426
amylopectin and the right shoulder peak corresponding to amylose. The Mw, Ra, and
427
PDI values of NMS were 31.58×106 g/mol, 117.2 nm, and 1.121, respectively (Table
428
1).
23
ACCEPTED MANUSCRIPT 429
As shown in Fig. 4A, C, and E, the peak of fried starchy samples shifted to the right and the signal intensified at high elution volume, suggesting that degradation of
431
starch molecules had occurred during frying. Furthermore, in Fig. 4B, D, and F, the
432
right shoulder peak became more and more pronounced, indicating the formation of a
433
higher fraction of small molecules during frying. The ratio of the left peak to right
434
peak also decreased after frying, implicating that some amylopectin molecules were
435
degraded during frying, which was consistent with our previous work (Chen et al.,
436
2018).
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437
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With the intensifying of frying conditions, starch molecules degraded to a higher extent. For instance, the Mw decreased from 31.58×106 g/mol in NMS to 25.26×106
439
g/mol in NMS-20%, further decreased to 20.53×106 g/mol, 15.32×106 g/mol, and
440
12.08×106 g/mol when the initial moisture contents increased to 40%, 60%, and 80%,
441
respectively (Table 1). The Mw also significantly decreased with increasing frying
442
temperature and duration (Table 1). In addition, the apparent decrease in Ra and the
443
increase in the PDI (Table 1) proved the apparent degradation of starch molecules
444
and the enlarged structural heterogeneity of starch mass.
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Previous studies have shown that thermal treatment, with or without shear, can
446
induce the breakdown of starch molecules, especially for amylopectin (Liu, Halley, &
447
Gilbert, 2010; Van den Einde, Akkermans, Van der Goot, & Boom, 2004; Van Den
448
Einde, Van Der Goot, & Boom, 2003). The degradation of starch molecules during
24
ACCEPTED MANUSCRIPT frying results from two types of reaction, thermal hydrolysis (which occurs during
450
heating of moistened starch) and dry thermal depolymerization (which occurs during
451
heating of dry starch) (Van Den Einde, Van Der Goot, & Boom, 2003). It is
452
reasonable to speculate that the thermal hydrolysis reaction dominated during the
453
initial stage of frying, while the dry thermal depolymerization reaction dominated at
454
the later stage because most of the water disappeared. Regardless of the degradation
455
mode, both degradation processes of starch molecules enhanced when the frying
456
conditions were intensified, inducing a reduction in Mw and Ra, and an increase in
457
PDI (Table 1).
458
3.5. Total water and oil contents of fried starchy samples
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LF-NMR was applied to evaluate the effects of frying parameters on oil absorption in fried starchy samples in the present work. The spin-spin relaxation time
461
(T2) distinguishes proton signals from water and oil and can therefore be used to
462
simultaneously determine these two components in fried starchy systems (Chen et al.,
463
2017a). The T2 relaxation time spectra of fried starchy samples treated using different
464
frying conditions are shown in Fig. 5. The quantitative analysis of the oil and water
465
content calculated from the calculation curves using a method described earlier (Chen
466
et al., 2017a) are shown in Table 2. The oil contents obtained by LF-NMR analysis
467
were slightly higher than that given by Soxhlet extraction (Table S1), but this
468
difference was not significant in most samples except for the samples treated at 80%
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ACCEPTED MANUSCRIPT moisture or 210℃ (Table S1). This difference might result from the shortcoming of
470
Soxhlet extraction as discussed previously (Chen et al., 2017a). In samples treated at
471
intensified frying conditions, a small fraction of fatty acids was tightly combined with
472
starch molecules (Table 2, Fig. 2), and was not easily removed by organic solvent
473
extraction, thus leading to an underestimate of the oil contents in fried samples by
474
Soxhlet extraction.
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The peaks in the T2 relaxation time spectra of fried starchy samples could be roughly divided into two groups with T2 values < 10 ms and T2 values > 20 ms,
477
respectively (Fig. 5). According to our previous work, peaks with T2 values < 10 ms
478
corresponded to bound water, whereas those with T2 values > 20 ms corresponded to
479
oil (Chen et al., 2017a). Thus, the variation in the molecular mobility and distribution
480
of water and oil absorbed in the fried starch samples could easily be investigated. The
481
T2 relaxation time spectra of fried starchy samples changed appreciably with frying
482
parameters (Fig. 5).
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Compared to frying temperature and heating time, the initial moisture content had the most pronounced influence on the T2 distribution of both the bound water and oil
485
in the fried starchy samples (Fig. 5A). Two fractions of bound water could be
486
discerned in the fried samples with different initial moisture contents: (i) T2 from 0.05
487
to 1 ms, corresponding to tightly bound water; (ii) T2 > 1 ms, corresponding to weakly
488
bound water. This observation was consistent with our previous work (Chen et al.,
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ACCEPTED MANUSCRIPT 2017a). There was no significant difference (p > 0.05) in the water content of samples
490
fried at initial moisture contents of 20%, 40%, and 60% (Table 2), but a significant
491
increase in both the amount (from 0.0102 to 0.4683 g/g defatted dry solids) and
492
mobility (from 3 to 7 ms) of weakly bound water appeared in the sample fried at 80%
493
moisture content (Table 2). This result suggested that a fraction of the water was not
494
removed from the starchy samples with the highest initial moisture content under the
495
frying conditions used in this study. The initial moisture content had little influence
496
on the mobility of the oil protons, but it did have a major impact on the intensity of
497
the oil peak (Fig. 5A). This result suggested that the initial moisture content did not
498
interfere with the molecular transport of the oil molecules after frying (presumably
499
because the oil and water were in different domains), but it did alter the level of oil
500
absorbed. Quantificationally, the oil content significantly increased from 0.2396 to
501
0.6602 g/g defatted dry solids when the initial moisture content was increased from
502
20% to 40%, but then it decreased to 0.3614 and 0.2531 g/g defatted dry solids when
503
the initial moisture content was further increased to 60 and 80% (Table 2).
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Generally, the absorption of oil by foods during and after frying is controlled by
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505
three major mechanisms: (i) solidification mechanism (ii) condensation mechanism;
506
and (iii) capillary mechanism (Mellema, 2003). First, we consider the solidification
507
mechanism. During frying, the temperature is very high (usually higher than 180℃)
508
and the viscosity of oil is relatively low. However, when the fried samples are
509
removed from the hot oil, the temperature drops causing the oil viscosity to rapidly 27
ACCEPTED MANUSCRIPT increase, thereby leading to high surface adhesion of oil. Obviously, solidification and
511
adhesion of oil on samples happen as soon as samples are removed from the hot oil.
512
Second, we consider the condensation mechanism. When the fried samples are
513
separated from the hot oil, condensation of the water vapor in the pores of the starch
514
sample occurs, thereby generating sub-atmospheric pressure within the pores. As a
515
result, large amounts of oil are sucked into the pores. Third, we consider the capillary
516
mechanism. The porous and rough nature of the starch granule surfaces generates
517
capillary forces that are strongly enough to pull oil into them.
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510
In the present work, the initial moisture content influenced the swelling of the starch granules and the destruction of crystalline structures during frying, thereby
520
producing starchy materials with different structural features. These structural
521
changes altered the capillary forces and the contact area of starch for oil, as well as
522
the magnitude of oil solidification and adhesion to the surfaces of the starch samples,
523
thereby affecting oil absorption by starch during frying.
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The initial increase in oil absorption was considered to be a result of the expansion of starch granules (Fig. 1), the destruction of the crystalline structures (Fig.
526
2), the disassembly of double helices (Fig. 3), and the degradation of starch molecules
527
(Fig. 4), which facilitated the ability of the starch granules to absorb oil because of
528
their open granular structures, high capillary forces, and large specific surface.
529
However, the decrease in oil content at higher moisture contents (60% and 80%) was
AC C
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28
ACCEPTED MANUSCRIPT not consistent with the trend reported previously (Chen et al., 2018b). This difference
531
may have originated from the fact that starches with different amylose contents were
532
used in the two studies. Indeed, amylose has previously been reported to affect oil
533
absorption by interfering with the structural evolution of starch or by directly
534
interacting with lipids during frying (Chen et al., 2019b).
535
RI PT
530
In the present work, the decrease in oil content observed at high initial moisture contents may be due to a number of phenomena. First, if all the moisture was not
537
removed from the starch granules during frying there might be less room for the oil to
538
absorb. Considering the high moisture content left in the NMS-80% (Table 2), the
539
low oil content absorbed by starch is expected. Second, the degree of starch granule
540
gelatinization during frying will increased with the initial moisture content. As a
541
result, the starch granules became disrupted and merged together (Fig. 1C), thereby
542
reducing the porosity of starch surface and the specific surface area of starch granules,
543
which decreased the capillary forces and contact area available for absorbing oil.
544
Third, the formation of amylose-lipids complexes at high moisture contents (Fig. 2A)
545
may also have contributed to the reduction in oil absorption during frying. At high
546
moisture contents, the hydrolysis of triglycerides occurred to a greater extent during
547
frying producing more free fatty acids (Dominik, Joanna, & Bartosz, 2018).
548
Moreover, the swollen starch granules might have leached a large amount of amylose
549
(Chen et al., 2018a). As a result of these two effects, the number of interactions
550
between amylose and free fatty acids increased in starchy samples containing high
AC C
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536
29
ACCEPTED MANUSCRIPT initial moisture contents. Unlike the starch gelatinization in water phase, the leached
552
amylose during frying cannot disperse in the oil phase and can only be deposited on
553
the surface of fried starchy samples. Thus, the resulting amylose-lipid complexes
554
around the surface of the fried samples may have worked as a physical layer
555
inhibiting oil absorption.
556
RI PT
551
The T2 relaxation time spectra of samples fried at different temperatures are
shown in Fig. 5B, and the results of the impact of frying temperature on water loss
558
and oil uptake are summarized in Table 2. There was no apparent change in the
559
mobility of protons in both bound water and oil (Fig. 5B), but the final water content
560
of the samples dramatically decreased from 0.1916 to 0.0027 g/g defatted dry solids
561
as the temperature increased from 120 to 210℃ (Table 2). This decrease in water
562
content was attributed to the high heat transfer rates at higher frying temperature,
563
which promoted the removal of water at a faster rate (Su, Zhang, Fang, & Zhang,
564
2017). As for the oil content, the oil content decreased from 0.7591 to 0.4873 g/g
565
defatted dry solids when the temperature increased from 120 to 210℃, however, no
566
significant difference (p > 0.05) in samples fried at 150 and 180℃ was found (Table
567
2), which was in agreement with previous studies (Kita, Lisińska, & Gołubowska,
568
2007). The significant reduction of oil content in the samples fried at 210℃ might be
569
ascribed to the melting and disintegration of the starch granules induced by excessive
570
heating (Fig. 1F). During frying at a limited moisture content, such as the 40% used
571
in this work, several transformations of starch structure occurred at the granular,
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30
ACCEPTED MANUSCRIPT crystal, and molecular levels, including the deformation of starch granules (Fig. 1),
573
the destruction of crystalline structures (Fig. 2), the unfolding of double helices (Fig.
574
3), the degradation of macromolecules (Fig. 4), and the expulsion of amylose (Chen et
575
al., 2018a). Thus, when the starch samples were fried at 210℃ or higher, the excessive
576
heat led to disintegration of the swollen starch granules (Fig. 1F). As a result, they
577
merged together, lost their porous surface, and reduced the effective contact area with
578
oil (Fig. 1F), thereby decreasing the magnitude of the capillary forces and the
579
adsorption capacity that normally hold the oil. Furthermore, the hydrolysis of
580
triglycerides in oil occurred more rapidly at higher temperatures, producing more free
581
fatty acids available for the V-type complexes formation between amylose and fatty
582
acids. Previous reports showed that amylose-fatty acid complexes mainly arranged in
583
the form of faceted crystalline structures or spherocrystalline particles (Zabar,
584
Lesmes, Katz, Shimoni, & Bianco-Peled, 2010). As a result, the V-type microcrystals
585
formed in fried samples might inhibit the oil absorption by starch by increasing the
586
compactness of the fried samples. These results showed that the level of oil absorption
587
can be reduced by increasing the frying temperature. However, this strategy is not
588
commercially feasible because of undesirable chemical reactions that occur at
589
elevated temperatures. For instance, heating above 180℃ has been reported to
590
generate polar materials (Li, Li, Wang, Cao, & Liu, 2017), form acrylamides
591
(Al-Asmar, Naviglio, Giosafatto, & Mariniello, 2018; Grob et al., 2003) and
592
hydrolyze and oxidize oils (Ben Hammouda, Triki, Matthäus, & Bouaziz, 2018; Cui,
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572
31
ACCEPTED MANUSCRIPT 593
Hao, Liu, & Meng, 2017; Mehta & Swinburn, 2001), which all have an adverse effect
594
on food quality, human health, and nutrition.
595
An increase in frying time reduced the final moisture content and oil content of the fried starchy samples (Fig. 5C). Specifically, the final moisture content decreased
597
from 0.1757 to 0.0144 g/g defatted dry solids and the final oil content decreased from
598
1.0755 to 0.5849 g/g defatted dry solids as the frying time increased from 5 to 30 min
599
(Table 2). The decrease in water content with increasing frying time is easy to
600
understand, since there should be a greater amount of water evaporation during the
601
prolonged frying. Previous studies showed that the total oil content of fried foods
602
increased rapidly during the first minute of frying but then remained fairly constant
603
(Durán, Pedreschi, Moyano, & Troncoso, 2007; Pedreschi, Cocio, Moyano, &
604
Troncoso, 2008). This did not appear to be the case for the relatively long frying time
605
(≥ 5 min) used in the present work. With extension of frying time, the water in the
606
samples rapidly evaporated, leaving the dehydrated starch to be heated at high
607
temperatures (180℃). As frying time prolonged, the surface temperature got higher
608
and higher, leading to the fragmentation of swollen starch granules (Fig. 1I), the
609
collapse and stripping of the structure near the surface of the starch granules (Fig. 1I)
610
as well as the degradation of starch molecules (Fig. 4F). These structural changes led
611
to the loss of the porous surface, the reduction of the effective contact area with oil,
612
and the decrease of the magnitude of the capillary forces, which would account for the
613
observed decrease in oil content with increased frying time (Table 2).
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32
ACCEPTED MANUSCRIPT 614
3.6. External oil contents of fried starchy samples The external oil content of the fried starchy samples was determined using
616
ATR-FTIR (Chen et al., 2018b). The ATR-FTIR spectra of fried samples are shown
617
in Fig. 6. The oil content located on the surface layer of the starch granules was
618
calculated by integrating the peak from 1693 to 1793 cm-1, which corresponded to the
619
ester group in the oil, and then substituting the value of the integrated area into the
620
linear regression equation for soybean oil (Chen et al., 2018b). The results obtained
621
from this analysis are summarized in Table 2.
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622
RI PT
615
Similar absorption peaks in the ATR-FTIR spectra of samples were observed regardless of the frying conditions. The molecular information corresponding to
624
functional groups in starch could be clearly seen, including the methyl group (-CH3)
625
in the range of 2995-2886 cm-1, methylene group (-CH2) in the range of 2886-2783
626
cm-1, and the fingerprint region of starch (C-O, C-C and C-O-H stretching and C-O-H
627
bending) in the range of 1200–900 cm-1. However, these absorption peaks might also
628
have originated from the oil. Therefore, the ATR-FTIR spectra of fried starchy
629
samples were a combination of spectrum of starch and oil (Fig. S1). As for the
630
fingerprint of oil, a well resolved peak was distinguishable in the range of 1693-1793
631
cm-1 with no superimposition and interference from starch, which was therefore used
632
as the basis for determining the surface oil using ATR-FTIR (Chen et al., 2018b).
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ACCEPTED MANUSCRIPT 633
As shown in Fig. 6, the peak intensity for ester group of the oil (1693-1793 cm-1) was sensitive to frying conditions, suggesting that the external oil content was also
635
affected by frying parameters. The external oil content of fried starchy samples
636
rapidly increased from 0.1438 to 0.4266 g/g defatted dry solids as the moisture
637
content increased from 20% to 40%, and then decreased with the further increasing of
638
moisture content (Table 2). The peak intensity of the ester group also changed with
639
frying temperature. It increased when the frying temperature was raised from 120 to
640
180℃, but then decreased sharply when it was raised further to 210℃ (Fig. 6B). As
641
discussed above, the gradual gelatinization of starch (Fig. 1), the diminishment of
642
crystalline structures (Fig. 2), the unfoldment of double helices (Fig. 3), and the
643
degradation of starch molecules (Fig. 4) during frying promoted the oil absorption by
644
starchy samples. On the other hand, the fragmentation and aggregation of starch (Fig.
645
1) contributed to the reduced oil content absorbed by starch fried at intensified frying
646
conditions. As frying time prolonged, the peak intensity increased (Fig. 6C),
647
corresponding to the gradual increase of external oil from 0.3570 to 0.5101 g/g
648
defatted dry solids (Table 2). The continuous increase of external oil content with the
649
prolongation of treatment time might be attributed to gradual transform of starch from
650
the compact semi-crystalline structure to the loose un-crystalline structure. At the
651
same time, the preservation of granular morphology (Fig. 1G-I) could further favor
652
the absorption of oil because of the higher specific surface of these granules than the
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34
ACCEPTED MANUSCRIPT 653
aggregate of broken granules as seen in fried samples treated at high moisture content
654
(Fig. 1C) or high temperature (Fig. 1F).
Interestingly, the proportion of external oil increased with increasing initial
656
moisture content, frying temperature, and frying time, with the exception of the
657
samples fried at 210℃. This was attributed to the fact that the heat and mass transfer
658
mainly occurred at the surface of the starch granules, and the extent of these changes
659
enhanced as these frying parameters intensified. This trend was in general agreement
660
with a previous study that showed oil uptake was mainly a surface phenomenon
661
(Bouchon, Hollins, Pearson, Pyle, & Tobin, 2001) and that most of the oil was
662
absorbed and located near the surface of the fried samples (Aguilera & Gloria, 1997;
663
Mellema, 2003).
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655
Although the structural changes of samples treated using intensified frying
665
conditions, including the swelling and disintegration of starch granules, the leaching
666
of soluble substance, the collapse of the long-range crystalline structure, the
667
destruction of the short-range double helices, and the degradation of starch
668
macromolecule, were expected to facilitate the absorption of oil both in the external
669
and the internal fractions of starch. However, the expected increase in the proportion
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of internal oil was not observed experimentally. In fact, after comparing the total oil
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contents and surface oil contents of samples (Table 2), the proportion of the internal
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oil in the samples decreased with intensified frying conditions. The unique
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responsible for this effect. The gelatinization of starch in an oil phase is different from
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that observed in a water phase. During frying, amylose molecules leached from the
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interior region of the granules to the surfaces, but could not move away from the
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surfaces into the surrounding oil phase. In other words, these leached amylose
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molecules could only be deposited on the surface where they could interact with free
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fatty acids, forming a physical layer that prevented oil from penetrating inside the
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samples. As a result, the proportion of internal oil decreased.
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Fried starchy samples treated under different conditions were defatted to further
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assess the affinity of oil with starch within the outer layer. The ATR-FTIR spectra of
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defatted samples are shown in Fig. 7 and the external oil content after defatting are
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given in Table 2. The oil peak intensity and external oil content dramatically
685
decreased in all samples, and even disappeared completely in some samples (Fig. 7
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and Table 2). The huge reduction (around 68-100%) of external oil content after
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defatting indicated that most of the oil absorbed at the surface was only weakly bound
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with the starch granules through physical adsorption and thus was easily removed by
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organic solvent extraction, which agreed with our previous work (Chen et al., 2018b).
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4. Conclusions
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In the present work, the structural changes and oil absorption of normal maize starch (NMS) as affected by frying conditions were systematically investigated. 36
ACCEPTED MANUSCRIPT During frying, the granular morphology gradually disappeared, the crystalline
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structures destroyed, the double helices disrupted, and the starch molecules especially
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amylopectin degraded, all of which tended to happed at a high extent when frying
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conditions were intensified. Frying also induced a significant variation in the total and
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external oil contents absorbed in the fried starchy samples. The initial moisture
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content in the samples had the most pronounced influence on the amount of oil
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absorbed during frying. The level of oil absorbed first increased but then decreased as
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the initial moisture content increased. The proportion of external oil increased with
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increasing initial moisture content, frying temperature, and frying time. These
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changes in the hierarchical structures of starch contributed to the variation of oil
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absorption in NMS during frying. The improved understanding of the relationship
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between frying conditions, starch structural changes, and oil absorption will assist
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food processors in developing healthier reduced-fat fried foods.
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Conflict of interest
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No conflicts of interest are declared for any of the authors.
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Acknowledgements This study was financially supported by the National Natural Science Foundation of Jiangsu Province - China (No. BK20160052) and the Taishan Industry Leader
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Talent Project - China. The first author Long Chen also greatly appreciates the
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financial support from the Postgraduate Research & Practice Innovation Program of
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Jiangsu Province - China (No. KYLX16_0819), the Outstanding Doctoral Cultivation
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Fund in Jiangnan University - China (No. 1025210172160160) and Postgraduate
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Overseas Research Fund of Jiangnan University - China (No.
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3562050205183520/001).
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Fig. 1 SEM images of NMS fried at different conditions. (A-C) Fried samples treated
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at different moisture contents; (D-F) Fried samples treated at different temperatures;
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(G-I) Fried samples treated for different time.
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Fig. 2 The X-ray diffraction patterns of NMS fried at different conditions. (A) Fried
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temperatures; (C) Fried samples treated for different time.
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Fig. 3 The original (A-C) and deconvoluted (D-F) FTIR spectra (in the range of
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1200-800 cm-1) of NMS fried at different conditions. (A)(D) Fried samples treated at
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different moisture contents; (B)(E) Fried samples treated at different temperatures;
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(C)(F) Fried samples treated for different time.
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Fig. 4 HPSEC-MALLS-RI chromatograms of NMS fried at different conditions. (A),
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(C), and (E) Light scattering (LS) signals and the molecular weight distribution of
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samples; (B), (D), and (F) The refractive index (RI) chromatograms of samples.
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Fig. 5 The CPMG proton distribution of NMS fried at different conditions. (A) Fried
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samples treated at different moisture contents; (B) Fried samples treated at different
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temperatures; (C) Fried samples treated for different time.
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Fig. 6 The ATR-FTIR spectra of NMS fried at different conditions (before defatting).
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(A) Fried samples treated at different moisture contents; (B) Fried samples treated at
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different temperatures; (C) Fried samples treated for different time.
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Fig. 7 The ATR-FTIR spectra of NMS fried (after defatting) at different conditions.
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(A) Fried samples treated at different moisture contents; (B) Fried samples treated at
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different temperatures; (C) Fried samples treated for different time.
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A
Molecular structure R2 (1015/995)
Mw (× 106 g/mol)
Ra (nm)
PDI
0.620 ± 0.010a
C
0.680 ± 0.002e
31.58 ± 1.06a
117.2 ± 3.1a
1.121 ± 0.08e
25.26 ± 0.82d 20.53 ± 1.03e 15.32 ± 1.34g 12.08 ± 1.33h
104.5 ± 1.8d 94.9 ± 2.6e 84.7 ± 2.8g 79.2 ± 1.4h
1.235 ± 0.05cde 1.336 ± 0.08bcd 1.468 ± 0.06ab 1.516 ± 0.17a
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R1 (1045/1015)
B
0.716 ± 0.007d 0.759 ± 0.004ab 0.773 ± 0.005a 0.705 ± 0.009d
0.571 ± 0.010b 0.546 ± 0.007c 0.494 ± 0.008de 0.485 ± 0.005e
0.739 ± 0.006c 0.748 ± 0.010bc 0.740 ± 0.006c 0.749 ± 0.009bc
30.87 ± 1.15abc 28.95 ± 1.32c 19.97 ± 1.03ef 15.33 ± 0.83g
112.3 ± 3.3b 106.7 ± 2.4cd 95.1 ± 2.5e 88.74 ± 1.3f
1.139 ± 0.05e 1.184 ± 0.11de 1.325 ± 0.08bcd 1.462 ± 0.06ab
0.512 ± 0.009d 0.512 ± 0.004d 0.493 ± 0.006de 0.480 ± 0.005e
0.747 ± 0.006bc 0.758 ± 0.007ab 0.760 ± 0.003ab 0.739 ± 0.004c
31.08 ± 0.93ab 29.38 ± 1.06bc 20.08 ± 1.03ef 18.13 ± 1.53f
116.7 ± 2.6a 110.3 ± 2.2bc 95.4 ± 2.0e 91.4 ± 1.7ef
1.121 ± 0.13e 1.101 ± 0.04e 1.329 ± 0.08bcd 1.358 ± 0.11abc
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0.543 ± 0.008c 0.492 ± 0.009de 0.460 ± 0.006f 0.436 ± 0.008g
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Native NMS Different moisture NMS-20% NMS-40% NMS-60% NMS-80% Different temperature NMS-120oC NMS-150oC NMS-180oC NMS-210oC Different time NMS-5 min NMS-10 min NMS-20 min NMS-30 min
Short range order
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Table 1 Short range ordered and molecular structures of NMS fried at different conditions (Moisture; Temperature; Time).
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NMS-20% moisture, 40% moisture, 60% moisture, and 80% moisture corresponded to normal maize starch fried at 20%, 40%, 60%, and 80% initial moisture content for samples, respectively. NMS-120oC, 150oC, 180oC, and 210oC corresponded to normal maize starch fried at 120oC, 150oC, 180oC, and 210oC, respectively. NMS-5 min, 10 min, 20 min, and 30 min corresponded to normal maize starch fried at 5 min, 10 min, 20 min, and 30 min, respectively. B R1 and R2 corresponded to the ratios of IR absorbances at 1045 to 1015 cm-1 and 1015 to 995 cm-1. C Data were means ± standard deviations (n = 3). Values in the same column with different lowercase letters were significantly different (p < 0.05) by Tukey’s test.
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Table 2
A
0.0182 ± 0.0070e B 0.0122 ± 0.0022e 0.0102 ± 0.0034e 0.4683 ± 0.0592a
0.2396 ± 0.0155g 0.6602 ± 0.0216c 0.3614 ± 0.0075f 0.2531 ± 0.0128g
0.1916 ± 0.0238b 0.1194 ± 0.0164c 0.0025 ± 0.0005e 0.0027 ± 0.0017e 0.1757 ± 0.0113b 0.0819 ± 0.0122d 0.0161 ± 0.0024e 0.0144 ± 0.0016e
Surface oil contents D (g/g defatted dry solids) Before defatting
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Total oil contents (g/g defatted dry solids)
After defatting N.D. C 0.0171 ± 0.0019c 0.0590 ± 0.0091a 0.0679 ± 0.0171a
0.7591 ± 0.0481b 0.6645 ± 0.0361c 0.6671 ± 0.0187c 0.4873 ± 0.0114e
0.2701 ± 0.0154e 0.2984 ± 0.0152d 0.3852 ± 0.0096c 0.1943 ± 0.0081g
N.D. N.D. 0.0458 ± 0.0077b 0.0610 ± 0.0058a
1.0755 ± 0.0262a 0.6649 ± 0.0290c 0.6733 ± 0.0187c 0.5849 ± 0.0148d
0.3570 ± 0.0221b 0.3749 ± 0.0225c 0.3898 ± 0.0104c 0.5101 ± 0.0139a
N.D. N.D. 0.0372 ± 0.0033b 0.0373 ± 0.0034b
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0.1438 ± 0.0075h 0.4266 ± 0.0141b 0.2918± 0.0055de 0.2329 ± 0.0144f
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Different moisture NMS-20% NMS-40% NMS-60% NMS-80% Different temperature NMS-120oC NMS-150oC NMS-180oC NMS-210oC Different time NMS-5 min NMS-10 min NMS-20 min NMS-30 min
Total water contents (g/g defatted dry solids)
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The total water contents, total oil contents and surface oil contents of NMS fried at different conditions (Moisture; Temperature; Time).
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NMS-20% moisture, 40% moisture, 60% moisture, and 80% moisture corresponded to normal maize starch fried at 20%, 40%, 60%, and 80% initial moisture content for samples, respectively. NMS-120oC, 150oC, 180oC, and 210oC corresponded to normal maize starch fried at 120oC, 150oC, 180oC, and 210oC, respectively. NMS-5 min, 10 min, 20 min, and 30 min corresponded to normal maize starch fried at 5 min, 10 min, 20 min, and 30 min, respectively. B Data were means ± standard deviations (n = 3). Values in the same column with different lowercase letters were significantly different (p < 0.05) by Tukey’s test. C Not detectable. D The internal oil content could be determined by subtracting the external oil content from the total oil content.
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Oil absorption of normal maize starch (NMS) was affected by frying conditions.
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Hierarchical structures of NMS changed a lot during frying. Moisture content had the most pronounced influence on oil absorption.
The proportion of external oil increased as frying treatment intensified.
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Changes in hierarchical structures affected oil absorption by starch during frying.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: