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Effect of plasma-activated water on the structure and in vitro digestibility of waxy and normal maize starches during heat-moisture treatment
Journal Pre-proofs Effect of plasma-activated water on the structure and in vitro digestibility of waxy and normal maize starches during heat-moisture treatment Yizhe Yan, Linlin Feng, Miaomiao Shi, Chang Cui, Yanqi Liu PII: DOI: Reference:
S0308-8146(19)31713-3 https://doi.org/10.1016/j.foodchem.2019.125589 FOCH 125589
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Food Chemistry
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11 July 2019 18 September 2019 23 September 2019
Please cite this article as: Yan, Y., Feng, L., Shi, M., Cui, C., Liu, Y., Effect of plasma-activated water on the structure and in vitro digestibility of waxy and normal maize starches during heat-moisture treatment, Food Chemistry (2019), doi: https://doi.org/10.1016/j.foodchem.2019.125589
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Effect of plasma-activated water on the structure and in vitro digestibility of waxy and normal maize starches during heat-moisture treatment Yizhe Yana, b, c#, Linlin Fenga#, Miaomiao Shi a*, Chang Cuia, Yanqi Liua*
a School
of Food and Biological Engineering, Zhengzhou University of Light Industry,
Zhengzhou, 450002, PR China b Henan
Key Laboratory of Cold Chain Food Quality and Safety Control, Zhengzhou,
450002, PR China c Collaborative
Innovation Center of Food Production and Safety, Henan Province, PR
China
*Corresponding
author: Tel: +86-13526667259
E-mail: [email protected] (Miaomiao Shi) Postal address: School of Food and Biological Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, PR China **Corresponding
author. Tel: +86-13938228293
E-mail: [email protected] (Yanqi Liu) Postal address: School of Food and Biological Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, PR China
#
The authors contributed equally to this work.
1
Abstract The combined effects of plasma-activated water (PAW) and heat-moisture treatments (HMT) on the structure, physicochemical properties and in vitro digestibility of waxy (WMS) and normal maize starches (NMS) were investigated. X-ray diffraction results revealed that the relative crystallinity of starches treated with PAW-HMT increased without crystalline type transition compared to DW-HMT. Through the Fourier transform infrared spectroscopy and Raman spectroscopy, the short-range order of starches treated with PAW-HMT was improved. Differential scanning calorimetry analysis shown that PAW-HMT increased gelatinization temperatures for NMS while decreasing gelatinization temperatures for WMS. The solubility of starches treated with PAW-HMT was higher than that of DW-HMT while the swelling power decreased. Importantly, the resistant starch (RS) content of starches treated by PAW-HMT increased compared to the starches treated by DW-HMT or native starch. This study provides a novel green method to modify the structure, lower starch digestibility and improve the RS content of starch.
Keywords: plasma-activated water; heat-moisture treatment; structure; digestibility; waxy maize starch; normal maize starch
2
1. Introduction Starch is the main carbohydrate material in human diets with a wide range of industrial applications. It is composed of two basic types of macromolecules: amylose, which is linked linearly by α-1, 4-glycoside bonds, and amylopectin, which is mainly linked by α-1, 4-glycoside bonds but α-1, 6-glycoside bonds at branching sites (Fujita, 2017). According to the X-ray diffraction patterns, the crystalline type of natural starch is generally divided into three types: A, B, and C-type (Buléon, Colonna, Planchot, & Ball, 1998). Cereal starch mainly contains A-type crystal, while B-type crystal mostly exists in root and tuber starch. C-type crystallization is an intermediate crystallization type, between A and B-type crystals. Because natural starch has many disadvantages, the rapid and safe physical modification has attracted attention, such as heat-moisture treatment, annealing, ultra-high pressure, and cold plasma (Ashogbon & Akintayo, 2014). As a kind of green modification technology, heat-moisture treatment (HMT) can not only keep the structure of starch granules intact but also change the physicochemical properties of starch (Zavareze & Dias, 2011). Recently, HMT has been used in combination with other modification technologies, such as acid hydrolysis (Hung, Vien, & Phi, 2016), enzyme hydrolysis (Xie, Li, Chen, & Zhang, 2019), and esterification (Zhang, Li, Xie, & Chen, 2019) to improve the effect of modification. Plasma-activated water (PAW), which is generated from distilled water (DW) by plasma treatment, has the same fluidity and uniformity as DW. PAW contains various reactive species (such as H2O2, O3, OH, NO2−, and NO3−), high oxidation redox potential and low pH value (Liao et al., 2018). Recently, PAW has been widely used for seedling growth (Zhang et al., 2017), eliminating microorganisms (Naumova, 3
Maksimov, & Khlyustova, 2011), and protecting color in meat products (Jung, Lee, Lim, Choe, Yong, & Jo, 2017). As far as we know, PAW has not been employed yet in the starch modification. In this paper, PAW was first combined with HMT to modify waxy maize starch (WMS) and normal maize starch (NMS). The changes in structure, physicochemical properties and in vitro digestibility of WMS and NMS were investigated after PAW-HMT. Notably, this novel modification technology avoided the use of acids or enzymes in HMT combined modification. This study will provide a new strategy for starch modification using HMT combined modification and have the potential application in starch industry. 2. Materials and Methods 2.1. Materials WMS was purchased from Hengrui Starch Import & Export Co. Ltd. (Luohe, China) with 12.1% moisture and 4.5% amylose. NMS was purchased from Xueliu Starch Import & Export Co. Ltd. (Gansu, China) with 11.8% moisture and 23.2% amylose. Absolute ethanol was obtained from Fuyu Chemical Co. Ltd. (Tianjin, China). Pancreatin (8×USP, P7545) and amyloglucosidase from Aspergillus niger (300 U/mL, A7095)
were
provided
by
Sigma
Corporation
of
America.
Glucose
oxidase-peroxidase (GOPOD) assay kit was purchased from Megazyme International Ireland Ltd. (Wicklow, Ireland). Chemical reagents utilized in this study were of analytical grade. 2.2. Preparation of PAW In this study, the atmospheric pressure plasma jet (APPJ) device (Easton Geake Automation Equipment Co., Ltd., Shenzhen, China) was used to generate the PAW (Fig. S1 in supplementary material). The high frequency of power supply was about 4
25 kHz and the input power was about 750 W. Distilled water (DW) (100 mL) was placed in a cylindrical tube (High: 12 cm, Diameter: 2.7 cm) and activated by APPJ for 2 min with a gas distance of 25 mm between the plasma jet probe and the water surface to obtain PAW. The pH and oxidation reduction potential (ORP) values were determined by a pH/ORP meter (Yidian Scientific Instrument Co., Ltd., Shanghai, China). The electrical conductivity was determined by an electric conductivity meter (Yidian Scientific Instrument Co. Ltd., Shanghai, China). PAW needs to be used as soon as possible within 12 hours 2.3. HMT of WMS and NMS with PAW or DW WMS and NMS (25.0 g, dry basis) were dried to a moisture content of about 5% in an air oven at 50 °C. The starch moisture was adjusted to 20% with PAW or DW, respectively. The starch samples were placed into hydrothermal reactors (Yikai instrument equipment Co., Ltd., Shanghai, China) with mixing thoroughly. Then the hydrothermal reactors were heated at 120 °C for 12 h in an air oven after equilibrating at room temperature for 12 h. After being cooled to room temperature, the modified starches were washed with distilled water three times and then washed with absolute ethanol once. The washed starch samples were dried at room temperature and finally ground to pass through 100-mesh sieve. Notably, these prepared samples were denoted DW-WMS and DW-NMS, PAW-WMS and PAW-NMS, respectively. All samples were performed in triplicate. 2.4. Scanning electron microscopy The samples with conductive double-sided adhesive were fixed on the loading table and coated with gold (120 s) with a sputter coater (Polaron Sputter Coat System, Model 5001, England) and then observed by a scanning electron microscope (JSM-6490LV, PhilipsXL-3, Rili Co. Ltd., Japan). The morphologies of starch 5
samples were imaged with an accelerating voltage of 20 kV and were taken with magnification of 1500. 2.5. X-ray diffraction All starch samples were equilibrated with saturated NaCl solution at room temperature for one week before testing (Wang, Wang, Wang, & Wang, 2017). The samples (0.5 g) were flattened with a smooth glass sheet and placed at the circular thread of the mold. XRD analysis of starch samples was performed using an X-ray diffractometer (Bruker D8 Advance, Karlsruhe, Germany) under the condition of the tube pressure 40 kV and flow 30 mA. Diffraction patterns were recorded in the range of 5°~35° (2θ), the scanning speed is 2°/ min, scanning step is 0.02°and repetition number is 1. The software Jade 6.0 (Version 6.0, Jade Software Corporation Ltd. Inc., Christchurch, New Zealand) was utilized to calculate the degree of relative crystallinity (RC) of the sample (Nara & Komiya, 1983). 2.6. Fourier transform infrared spectroscopy FTIR spectra of starch samples were recorded using a FTIR spectrometer (Vertex 70, Bruker, Karlsruhe, Germany). KBr and samples (about 100 :1) were dried at 45 °C for 5 h. After stirring evenly, a small amount was taken to make a sheet in the pressing tank. The infrared spectrum was obtained at a scanning wave number ranged from 4000 to 400 cm-1, with a resolution of 4 cm-1 and a scanning time of 64 s. All FTIR measurements were analyzed using OMNIC 8.2 software (Version 8.2, Thermo Nicolet Inc., USA). The spectra were deconvoluted and normalized in the range of 800 to 1200 cm-1 with the peak width was 38 cm-1, and the enhancement factor was 1.9. The ratio of absorbance at 1047/1022 cm-1 was the index of the short-range ordered structure of double helices (Wang, Wang, Zhang, Li, Yu, & Wang, 2015). 2.7. Raman spectroscopy 6
Raman spectra of starch samples were obtained using a manual portable Raman spectrometer (BWS465-785S, B&W, America). The starch samples were placed in the centrifugal tube cover and flattened to form a smooth surface for measurement. The spectra were obtained when the integration time was 10000 ms, and the laser power was 100. The full width at half height (FWHH) of 480 cm-1 characteristic peak was calculated by using BWIQ software (BWTEK Inc., USA) to determine the short-range ordered structure of starch (Mutungi, Passauer, Onyango, Jaros, & Rohm, 2012; Wang, et al., 2015). 2.8. Differential scanning calorimetry The DSC curves of starch samples were obtained by a DSC model thermal analysis. An empty pan was used as the reference. Then the aluminum pan was hermetically sealed and equilibrated at room temperature for 24 h. The analysis conditions were as follows: the scanning from 30 °C up to 120 °C at the rate of 10 °C/min. The data of starches was determined by TA 2000 analysis software (Q20, TA Instruments, USA). 2.9. Swelling power (SP) and solubility (S) Swelling power (g/g) and solubility (%) were determined by the method suggested by Martins, Gutkoski, and Martins (2018) with a slight modification. The starch sample (0.5 g, dry basis) was weighed into centrifuge tube, added with 25 mL of distilled water and homogenized manually. Afterwards, the tubes were placed in water bath at 90 °C for 30 min. Then the samples were cooled and centrifuged at 3000 rpm for 20 min. The supernatant was dried at 105 °C until constant weight, while the gel formed at the bottom of the tube was weighed. The solubility was calculated by dividing the mass of dried supernatant by the mass of starch. The swelling power was calculated as described by Martins et al. (2018). 2.10. In vitro digestibility 7
In vitro digestibility of the samples was analyzed according to the method of Englyst, Kingman, and Cummings (1992) with minor modifications. The mixed enzyme solution should be formed by mixing 0.788 mL of amyloglucosidase, 1.75 mL distilled water and the supernatant prepared by 3 g pancreatin was dispersed in distilled water (20 mL) in a centrifuge tube. The starch sample (200 mg, dry basis), 0.1 mol/L sodium acetate buffer (4 mL, pH = 5.2) and mixed enzyme solution (1 mL) were added into a centrifuge tube (50 mL) at 37 °C for different times at 190 r/min for hydrolysis. The hydrolyzed solution (0.1 mL) was gathered and inactivated with ethanol (4 mL, 70%). 0.1 mL aliquots of the supernatant were mixed with 3 mL of GOPOD at 45 °C for 20 min in water bath. Another 0.1 mL standard glucose solution was treated as the standard, and 0.1 mL distilled water was treated as the blank. Values for the digested starch fractions are expressed as milligrams of glucose multiplying 0.9. Values for rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) were calculated from the G20 and G120 values, using the follow equations: RDS (%) = (G20 – FG) ×0.9 SDS (%) = (G120 – G20) ×0.9 RS (%) = 1 – (RDS + SDS) Where, G20 and G120 were the percentage of glucose in the 20 min and 120 min of the enzymatic hydrolysis, respectively; FG was the amount of glucose in the sample before the enzymatic hydrolysis. 2.11. Statistical analysis The datas were recorded as the mean values ± standard deviations. Analysis of variance (ANOVA) by Duncan’s test (p<0.05) were conducted using the IBM SPSS Statistics 22.0 Software Program (SPSS Inc. Chicago, IL, USA). 8
3. Results and discussion 3.1. Physicochemical properties of PAW According to Table 1, the pH value of DW decreased rapidly from 6.46 to 2.62 after plasma treatment for two minutes, which obviously showed that plasma discharge could lead to water acidification. During the plasma experiment, the dissolution of nitrogen oxides generated from O2 and N2 would not only come into being NO3− and NO2−, but also produce H+ that decreases the pH of PAW (Zhou et al., 2018). This might be the cause of water acidification. Conductivity was also measured to detect the presence of active ions in water. After plasma activation, the value of conductivity increased from 2.58 to 895.33 μS/cm. This was because plasma activation could generate the soluble active groups and ions (ROS or RNS) in water, which improved the conductivity (Xu, Tian, Ma, Liu, & Zhang, 2016). The ORP is determined to be an important factor affecting microbial inactivation (Ma, Wang, Tian, Wang, Zhang, & Fang, 2015). The ORP of PAW increased linearly with the prolongation of plasma activation time. And the value of ORP increased from 322.33 to 593.33 mV after plasma activation for 2 min. That might be due to the generation of ROS, such as H2O2 or O3 by plasma treatment (Wu et al., 2017). 3.2. Granular morphology The SEM micrographs of treated and untreated starch samples were shown in Fig. 1. The granular size of NMS and WMS was uniform, mostly spherical and polyhedral. The surface was smooth, full and glossy. For NMS treated with DW and PAW, the starch granules treated were not broken. However, the surface of DW-WMS and PAW-WMS was cracked with some pits. Notably, there was no obvious difference between the granular morphology of starches modified with DW and PAW. 9
3.3. Crystalline properties XRD patterns and relative crystallinity of starch samples were shown in Fig. 2. The characteristic diffraction peaks of all the starch primarily displayed at 2θ = 15.2°, 17.2°, 18.0°, and 23.1°, which belonged to the A-type starch structure (Shi, Gao, & Liu, 2018). The crystallinity of HMT-modified starches (DW-WMS and DW-NMS) were lower than that of native starch. This change might be because this treatment can promote the rupture of amylopectin, weakening its double helices and causing a transition from crystalline to amorphous (Oliveira, Bet, Bisinella, Waiga, Colman, & Schnitzler, 2018). Chen et al. (2017) also found that HMT caused starch degradation with degraded areas represented by shallow indentations along the growth ring and longitudinal grooves originating from the hilum. Moreover, the RC of maize starch by PAW-HMT (PAW-WMS and PAW-NWS) was higher than that by DW-HMT (DW-WMS and DW-NWS), indicating that PAW could increase the RC of maize starch. This could be attributed to the preferential hydrolysis of the amorphous regions by the acidic ingredient of PAW. In addition, this behavior might be ascribed to better orientation of the crystallites for the hydrolysis starch (Rafiq, Singh, & Saxena, 2016; Zhang, Hou, Liu, Wang, & Dong, 2019). 3.4. Fourier transform infrared spectroscopy analysis The infrared spectra of native and modified starches were presented in Fig. S2 (supplementary material). It can be seen from Fig. S2a that the spectra shown similar trend for all the starches, indicating that there was no obvious formation of new functional groups. The fundamental region of starch corresponds to the stretching vibration of broad band at 3400 cm-1 confirmed by the presence of O-H vibrations (Rafiq et al., 2016). The characteristic peaks at 2934 cm-1 were attributed to C-H deformation vibration of glucose element (Flores-morales, Jiménez-estrada & 10
Mora-escobedo, 2012). The peak at 1649 cm-1 was related to the bending vibration of O-H in water and associated to the amorphous region of starch (Wang et al., 2017). The peaks at 1156, 1080 and 1019 cm-1 corresponded to the asymmetric C-O-C, C-O and C-C skeleton stretching vibrations, respectively (Kizil, Irudayaraj, & Seetharaman, 2002). The IR spectra of starch was also sensitive to the short-range ordered structure of double helices and independent of the composition of double helices related to long-range structure (Soest, Tournois, Wit, & Vliegenthart, 1995). The IR bands at 1047 and 1022 cm-1 in Fig. S2b were related to the ordered and amorphous structure of starch, respectively. The absorbance ratio of the two could be used as the parameter of the short-range order of double helices (Shingel, et al., 2002). From Table 2, the absorbance ratio of DW-WMS decreased by 0.036. This might be double helices were destroyed and transformed into amorphous components (Chen et al., 2017). However, the absorbance ratio of PAW-WMS increased by 0.026 (Table 2), which was basically consistent with XRD result. For NMS, the parameter of the short-range order performed a similar trend as WMS. This was because PAW could produce high amount of short starch chains, which could increase the mobility of starch chains and allow more efficient rearrangement during HMT (Zhang, Li, Xie, & Chen, 2019). 3.5. Raman spectroscopy analysis Raman spectra of starch samples were shown in Fig. S3 (supplementary material). It was obvious that the peak strength of the modified starch became weaker. The peak at 2913 cm-1 was attributed to C-H stretching vibration, the band at 1343 cm-1 was dominated by C-OH modes, and the spectral bands at 1260 cm-1 were correlated with the C-H bending vibration of CH2. The band at 1123 cm-1 was associated to the C-O stretching and C-O-H deformation. The band at 943 cm-1 was assigned to the 11
symmetric stretching vibration of the α-1,4-glycosidic linkage. The characteristic peak of 865 cm-1 was associated to the C-O-C stretching vibration. Moreover, the bands in the 479 cm-1 were the skeletal-mode vibration region, which was usually used to characterize the molecular order of starch (Mutungi, et al., 2012; Flores-Morales et al., 2012; Liu, Xu, Yan, Hu, Yang, & Shen, 2015). FWHH is widely used to characterize the structural changes of starch samples, and band narrowing is the index of narrower bond energy distribution in the more ordered samples (Wang, et al., 2015). The FWHH of the band at 480 cm-1 was utilized to characterize the molecular order of starch. In generally, with the increase of short-range order of starch, the FWHH of starch decreased at 480 cm-1 (Fechner, Wartewig, Kleinebudde, & Neubert, 2005). From Table 2, the FWHH of the starch treated with DW-HMT was obviously higher than that of the native starch or the starch treated with PAW-HMT. These observations were in agreement with the results of XRD and FTIR. 3.6. Thermal properties The DSC curves of waxy and normal maize starches were presented in Fig. S4 (supplementary material). The datas obtained for transition temperatures including onset temperature (To), peak temperature (Tp), conclusion temperature (Tc) and gelatinization enthalpy (∆H) were summarized in Table 2. HMT has a significant effect on the thermal properties of starch, and an increase of gelatinization temperature (To, Tp and Tc) was observed for DW-WMS and DW-NMS (Table 2). The reason for the increase in gelatinization temperature was the enhanced interactions between amylose and amylopectin (Oliveira et al., 2018). On the other hand, the decrease of gelatinization enthalpy (∆H) could be attributed to the double
12
helices dissociation of the crystalline and amorphous regions in the particles (Lv et al., 2018). For NMS, the gelatinization temperature of PAW-NMS further increased because of the reorientation of starch granules and crystalline recombination. The increase of conclusion temperature might be attributed to formation of more stable crystalline during HMT (Chen et al., 2017). The value of gelatinization enthalpy increased from 10.65 (DW-NMS) to 12.69 (PAW-NMS), which might be associated with the greater extent of degradation by acid that resulted in more heterogeneous crystallites (Alimi & Workneh, 2018). However, compared with DW-WMS, a decrease of gelatinization temperature (To, Tp and Tc) was observed for PAW-WMS. This might be due to the heterogeneity of the crystallites formed from different entities, such as crystalline amylopectin side chains and retrograded amylose, after acid hydrolysis (Wang & Copeland, 2015). The values of gelatinization enthalpy increased from 11.06 (DW-WMS) to 15.17 (PAW-WMS), which was attributed to higher the crystalline structure, the crystallinity and the double helices structure (Zhang, Li, Liu, Xie, & Chen, 2013). 3.7. Swelling power (SP) and solubility (S) For all samples, the swelling power of the starch by DW-HMT decreased compared to that of the native starch (Table 2), which might be related to the decrease of water absorption and retention ability of amylopectin. The increase of starch solubility after DW-HMT was due to the destruction of the double helices structure of amylopectin and the stronger leaching ability of amylopectin from thermal damaged particles (Rocha-Villarreal, Hoffmann, Vanier, Serna-Saldivar, & García-Lara, 2018). The swelling power of modified starch with PAW-HMT was lower than that with DW-HMT, which might be due to the acid hydrolysis of amorphous region reduced 13
the water binding ability of starch molecules (Hung, My, & Phi, 2014). In terms of solubility (Table 2), the solubility of starch treated with PAW-HMT was higher than that with DW-HMT. The increase of starch solubility was due to the high content of amylose produced by hydrolysis of acidic substances in water activated by plasma, which was easy to be separated and diffused from particles in the process of expansion (Yan, Wu, Li, Yin, Ren, & Tao, 2019). 3.8. In vitro digestibility The digestion properties of native and modified starches were presented in Fig. 3. The contents of RDS, SDS, and RS of the native and treated starches were present in Table 2. The results revealed that HMT had an obvious influence on starch digestibility. Compared to native WMS, the RS content of DW-WMS increased to 24.58%. Similar trend was also found in native NMS and DW-NMS. The increase of thermal stable RS might be explained that some interactions formed during DW-HMT might survive after gelatinization, thus partially limiting the availability of starch chain to hydrolase (Chung, Liu, & Hoover, 2009). Moreover, PAW-HMT significantly resulted in the formation of higher amount of RS than DW-HMT for WMS and NMS. The reason might be that acidic component in PAW could produce lower-molecular-weight hydrolysates (both branched and linear structures of amylose and amylopectin) and these starch components are resistant to the hydrolysis of the enzyme by forming double helices and compartmentalizing amylose-amylose, amylopectin-amylopectin and amylose-amylopectin chains during HMT (Hung, et al., 2016). It was noteworthy that RS content of starch modified by PAW-HMT was higher than that of both native starch and starches by DW-HMT. 4. Conclusions In summary, the modification of PAW combined with HMT had a significant 14
effect on structure and properties of WMS and NMS. Compared to DW-HMT, the long-range and short-range order structure of starches treated with PAW-HMT increased while the granule structure did not change. The physicochemical properties of WMS and NMS were also obviously changed after PAW-HMT. Furthermore, PAW-HMT resulted in the decrease of in vitro digestibility and the increase of RS content in comparison to DW-HMT. It is concluded that the acidic component in PAW could play an important role during HMT. In a word, PAW-HMT will be expected to be a promising technology to modify the muti-scale structure and improve resistant starch content in starch industry. Acknowledgments This work was supported by the National Natural Science Foundation of China (21502177), the Program for Science and Technology Innovation Talents in Universities of Henan Province (20HASTIT037), the Science and Technology Basic Research
Program
of
Henan
Province
(182102110248,
182102310903,
192102110104, 192102110213), the Basic Research Plan of Higher Education School Key Scientific Research Project of Henan Province (19zx012), and the Higher Education School Young Backbone Teacher Training Program of Henan Province (2018GGJS093). Declarations of interest The authors declare no conflict of interest.
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Figure Captions
Fig. 1. SEM images of native and modified starches. DW-distilled water; PAW-plasma-activated water; WMS-waxy maize starch; NMS-normal maize starch. Fig. 2. XRD patterns of native and modified starches. DW-distilled water; PAW-plasma-activated water; WMS-waxy maize starch; NMS-normal maize starch. Fig. 3. In vitro digestibility of native and modified starches. a: in vitro digestibility of WMS, DW-WMS and PAW-WMS; b: in vitro digestibility of NMS, DW-NMS and PAW-NMS; DW-distilled water; PAW-plasma-activated water; WMS-waxy maize starch; NMS-normal maize starch.
22
Table 1 pH, electric conductivity, and ORP of PAW Water
pH
Conductivity (μS/cm)
ORP (mV)
DW
6.46±0.02a
2.58±0.06b
322.33±0.58b
PAW
2.62±0.01b
895.33±6.02a
593.33±0.58a
DW-distilled water; PAW-plasma-activated water; ORP-oxidation reduction potential. Values are the mean of triplicate measurements ± standard deviation (SD); values with different lowercase letters in the same column indicated a significant difference at p<0.05.
23
Table 2 Short-range order structure, physical properties, thermal properties, RDS, SDS, and RS of native and modified starches Samples
WMS
DW-WMS
PAW-WMS
NMS
DW-NMS
PAW-NMS
1047/1022
0.991±0.003a
0.955±0.004b
0.981±0.006a
0.945±0.002a
0.914±0.005b
0.938±0.004a
FWHH (cm-1)
17.08±0.05b
17.53 ±0.23a
16.39±0.02a
17.51±0.30b
19.09±0.54a
18.49±0.28a
S (%)
2.46±0.25c
32.39 ±0.13b
37.18±0.26a
3.09±0.37c
7.21±0.90b
10.11±0.52a
SP (g/g)
25.94±0.28a
3.90 ±0.39b
3.83±0.10b
13.14±0.87a
8.19±0.19b
6.37±0.24c
To (℃)
62.76±0.37c
75.90±0.15a
70.69±0.15b
64.31±0.10c
72.87±0.21b
73.88±0.11a
Tp (℃)
69.86±0.24c
81.68±0.01a
76.38±0.14b
69.53±0.01c
78.05±0.51b
79.78±0.15a
Tc (℃)
79.02±1.84b
92.16±0.81a
80.57±2.72b
75.14±0.54a
82.74±2.93a
87.54±0.45a
∆H (J/g)
16.61±0.31a
11.06±1.36b
15.17±0.40a
12.49±0.19a
10.65±0.19b
12.69±0.23a
RDS (%)
39.71±0.33a
37.73±2.80ab
33.92±0.82c
32.33±0.99b
39.98±1.48a
37.73±0.49a
SDS (%)
39.63±0.22a
37.69±1.98a
36.87±0.93a
40.49±1.10a
28.60±1.76b
26.89±0.99b
RS (%)
20.66±0.55c
24.58±0.82b
29.20±0.11a
27.18±0.11c
31.42±0.28b
35.38±0.50a
DW-distilled water; PAW-plasma-activated water; WMS-waxy maize starch; NMS-normal maize starch. FWHH-the full width at half height; SP-swelling power; S-solubility; To-onset temperature, Tp-peak temperature, Tc-conclusion temperature, ∆H-enthalpy;
RDS-rapidly
digestible
starch;
SDS-slowly
digestible
starch;
RS-resistant starch. Values are the mean of triplicate measurements ± standard deviation (SD); values with different lowercase letters in the same line for the same starch indicated a significant difference at p<0.05. 24
WMS
NMS
DW-WMS
DW-NMS
PAW-WMS
PAW-NMS Fig. 1
25
17.2 18.0
Relative intensity
15.2
23.1 WMS, RC=37.5% DW-WMS, RC=34.9% PAW-WMS, RC=37.1% NMS, RC=33.6% DW-NMS, RC=30.6% PAW-NMS, RC=33.3%
5
10
15
20
25
Diffraction angle ( 2θ ) Fig. 2
26
30
35
a
Hydrolysis percentage (%)
100
80
60
40
20
WMS DW-WMS PAW-WMS
0 60
0
120
180
240
300
360
Time (min)
b
Hydrolysis percentage (%)
100
80
60
40
20
NMS DW-NMS PAW-NMS
0 0
60
120
180
Time (min) Fig. 3
27
240
300
360
Highlights
Combined effect of plasma-activated water with heat-moisture treatment was studied.
Structure and properties of waxy and normal maize starches were obviously modified.
Resistant starch content of starches by combined modification was increased.
Acidic species in plasma-activated water is an important factor for modification.
28
S0308-8146(19)31713-3 https://doi.org/10.1016/j.foodchem.2019.125589 FOCH 125589
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
11 July 2019 18 September 2019 23 September 2019
Please cite this article as: Yan, Y., Feng, L., Shi, M., Cui, C., Liu, Y., Effect of plasma-activated water on the structure and in vitro digestibility of waxy and normal maize starches during heat-moisture treatment, Food Chemistry (2019), doi: https://doi.org/10.1016/j.foodchem.2019.125589
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© 2019 Published by Elsevier Ltd.
Effect of plasma-activated water on the structure and in vitro digestibility of waxy and normal maize starches during heat-moisture treatment Yizhe Yana, b, c#, Linlin Fenga#, Miaomiao Shi a*, Chang Cuia, Yanqi Liua*
a School
of Food and Biological Engineering, Zhengzhou University of Light Industry,
Zhengzhou, 450002, PR China b Henan
Key Laboratory of Cold Chain Food Quality and Safety Control, Zhengzhou,
450002, PR China c Collaborative
Innovation Center of Food Production and Safety, Henan Province, PR
China
*Corresponding
author: Tel: +86-13526667259
E-mail: [email protected] (Miaomiao Shi) Postal address: School of Food and Biological Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, PR China **Corresponding
author. Tel: +86-13938228293
E-mail: [email protected] (Yanqi Liu) Postal address: School of Food and Biological Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, PR China
#
The authors contributed equally to this work.
1
Abstract The combined effects of plasma-activated water (PAW) and heat-moisture treatments (HMT) on the structure, physicochemical properties and in vitro digestibility of waxy (WMS) and normal maize starches (NMS) were investigated. X-ray diffraction results revealed that the relative crystallinity of starches treated with PAW-HMT increased without crystalline type transition compared to DW-HMT. Through the Fourier transform infrared spectroscopy and Raman spectroscopy, the short-range order of starches treated with PAW-HMT was improved. Differential scanning calorimetry analysis shown that PAW-HMT increased gelatinization temperatures for NMS while decreasing gelatinization temperatures for WMS. The solubility of starches treated with PAW-HMT was higher than that of DW-HMT while the swelling power decreased. Importantly, the resistant starch (RS) content of starches treated by PAW-HMT increased compared to the starches treated by DW-HMT or native starch. This study provides a novel green method to modify the structure, lower starch digestibility and improve the RS content of starch.
Keywords: plasma-activated water; heat-moisture treatment; structure; digestibility; waxy maize starch; normal maize starch
2
1. Introduction Starch is the main carbohydrate material in human diets with a wide range of industrial applications. It is composed of two basic types of macromolecules: amylose, which is linked linearly by α-1, 4-glycoside bonds, and amylopectin, which is mainly linked by α-1, 4-glycoside bonds but α-1, 6-glycoside bonds at branching sites (Fujita, 2017). According to the X-ray diffraction patterns, the crystalline type of natural starch is generally divided into three types: A, B, and C-type (Buléon, Colonna, Planchot, & Ball, 1998). Cereal starch mainly contains A-type crystal, while B-type crystal mostly exists in root and tuber starch. C-type crystallization is an intermediate crystallization type, between A and B-type crystals. Because natural starch has many disadvantages, the rapid and safe physical modification has attracted attention, such as heat-moisture treatment, annealing, ultra-high pressure, and cold plasma (Ashogbon & Akintayo, 2014). As a kind of green modification technology, heat-moisture treatment (HMT) can not only keep the structure of starch granules intact but also change the physicochemical properties of starch (Zavareze & Dias, 2011). Recently, HMT has been used in combination with other modification technologies, such as acid hydrolysis (Hung, Vien, & Phi, 2016), enzyme hydrolysis (Xie, Li, Chen, & Zhang, 2019), and esterification (Zhang, Li, Xie, & Chen, 2019) to improve the effect of modification. Plasma-activated water (PAW), which is generated from distilled water (DW) by plasma treatment, has the same fluidity and uniformity as DW. PAW contains various reactive species (such as H2O2, O3, OH, NO2−, and NO3−), high oxidation redox potential and low pH value (Liao et al., 2018). Recently, PAW has been widely used for seedling growth (Zhang et al., 2017), eliminating microorganisms (Naumova, 3
Maksimov, & Khlyustova, 2011), and protecting color in meat products (Jung, Lee, Lim, Choe, Yong, & Jo, 2017). As far as we know, PAW has not been employed yet in the starch modification. In this paper, PAW was first combined with HMT to modify waxy maize starch (WMS) and normal maize starch (NMS). The changes in structure, physicochemical properties and in vitro digestibility of WMS and NMS were investigated after PAW-HMT. Notably, this novel modification technology avoided the use of acids or enzymes in HMT combined modification. This study will provide a new strategy for starch modification using HMT combined modification and have the potential application in starch industry. 2. Materials and Methods 2.1. Materials WMS was purchased from Hengrui Starch Import & Export Co. Ltd. (Luohe, China) with 12.1% moisture and 4.5% amylose. NMS was purchased from Xueliu Starch Import & Export Co. Ltd. (Gansu, China) with 11.8% moisture and 23.2% amylose. Absolute ethanol was obtained from Fuyu Chemical Co. Ltd. (Tianjin, China). Pancreatin (8×USP, P7545) and amyloglucosidase from Aspergillus niger (300 U/mL, A7095)
were
provided
by
Sigma
Corporation
of
America.
Glucose
oxidase-peroxidase (GOPOD) assay kit was purchased from Megazyme International Ireland Ltd. (Wicklow, Ireland). Chemical reagents utilized in this study were of analytical grade. 2.2. Preparation of PAW In this study, the atmospheric pressure plasma jet (APPJ) device (Easton Geake Automation Equipment Co., Ltd., Shenzhen, China) was used to generate the PAW (Fig. S1 in supplementary material). The high frequency of power supply was about 4
25 kHz and the input power was about 750 W. Distilled water (DW) (100 mL) was placed in a cylindrical tube (High: 12 cm, Diameter: 2.7 cm) and activated by APPJ for 2 min with a gas distance of 25 mm between the plasma jet probe and the water surface to obtain PAW. The pH and oxidation reduction potential (ORP) values were determined by a pH/ORP meter (Yidian Scientific Instrument Co., Ltd., Shanghai, China). The electrical conductivity was determined by an electric conductivity meter (Yidian Scientific Instrument Co. Ltd., Shanghai, China). PAW needs to be used as soon as possible within 12 hours 2.3. HMT of WMS and NMS with PAW or DW WMS and NMS (25.0 g, dry basis) were dried to a moisture content of about 5% in an air oven at 50 °C. The starch moisture was adjusted to 20% with PAW or DW, respectively. The starch samples were placed into hydrothermal reactors (Yikai instrument equipment Co., Ltd., Shanghai, China) with mixing thoroughly. Then the hydrothermal reactors were heated at 120 °C for 12 h in an air oven after equilibrating at room temperature for 12 h. After being cooled to room temperature, the modified starches were washed with distilled water three times and then washed with absolute ethanol once. The washed starch samples were dried at room temperature and finally ground to pass through 100-mesh sieve. Notably, these prepared samples were denoted DW-WMS and DW-NMS, PAW-WMS and PAW-NMS, respectively. All samples were performed in triplicate. 2.4. Scanning electron microscopy The samples with conductive double-sided adhesive were fixed on the loading table and coated with gold (120 s) with a sputter coater (Polaron Sputter Coat System, Model 5001, England) and then observed by a scanning electron microscope (JSM-6490LV, PhilipsXL-3, Rili Co. Ltd., Japan). The morphologies of starch 5
samples were imaged with an accelerating voltage of 20 kV and were taken with magnification of 1500. 2.5. X-ray diffraction All starch samples were equilibrated with saturated NaCl solution at room temperature for one week before testing (Wang, Wang, Wang, & Wang, 2017). The samples (0.5 g) were flattened with a smooth glass sheet and placed at the circular thread of the mold. XRD analysis of starch samples was performed using an X-ray diffractometer (Bruker D8 Advance, Karlsruhe, Germany) under the condition of the tube pressure 40 kV and flow 30 mA. Diffraction patterns were recorded in the range of 5°~35° (2θ), the scanning speed is 2°/ min, scanning step is 0.02°and repetition number is 1. The software Jade 6.0 (Version 6.0, Jade Software Corporation Ltd. Inc., Christchurch, New Zealand) was utilized to calculate the degree of relative crystallinity (RC) of the sample (Nara & Komiya, 1983). 2.6. Fourier transform infrared spectroscopy FTIR spectra of starch samples were recorded using a FTIR spectrometer (Vertex 70, Bruker, Karlsruhe, Germany). KBr and samples (about 100 :1) were dried at 45 °C for 5 h. After stirring evenly, a small amount was taken to make a sheet in the pressing tank. The infrared spectrum was obtained at a scanning wave number ranged from 4000 to 400 cm-1, with a resolution of 4 cm-1 and a scanning time of 64 s. All FTIR measurements were analyzed using OMNIC 8.2 software (Version 8.2, Thermo Nicolet Inc., USA). The spectra were deconvoluted and normalized in the range of 800 to 1200 cm-1 with the peak width was 38 cm-1, and the enhancement factor was 1.9. The ratio of absorbance at 1047/1022 cm-1 was the index of the short-range ordered structure of double helices (Wang, Wang, Zhang, Li, Yu, & Wang, 2015). 2.7. Raman spectroscopy 6
Raman spectra of starch samples were obtained using a manual portable Raman spectrometer (BWS465-785S, B&W, America). The starch samples were placed in the centrifugal tube cover and flattened to form a smooth surface for measurement. The spectra were obtained when the integration time was 10000 ms, and the laser power was 100. The full width at half height (FWHH) of 480 cm-1 characteristic peak was calculated by using BWIQ software (BWTEK Inc., USA) to determine the short-range ordered structure of starch (Mutungi, Passauer, Onyango, Jaros, & Rohm, 2012; Wang, et al., 2015). 2.8. Differential scanning calorimetry The DSC curves of starch samples were obtained by a DSC model thermal analysis. An empty pan was used as the reference. Then the aluminum pan was hermetically sealed and equilibrated at room temperature for 24 h. The analysis conditions were as follows: the scanning from 30 °C up to 120 °C at the rate of 10 °C/min. The data of starches was determined by TA 2000 analysis software (Q20, TA Instruments, USA). 2.9. Swelling power (SP) and solubility (S) Swelling power (g/g) and solubility (%) were determined by the method suggested by Martins, Gutkoski, and Martins (2018) with a slight modification. The starch sample (0.5 g, dry basis) was weighed into centrifuge tube, added with 25 mL of distilled water and homogenized manually. Afterwards, the tubes were placed in water bath at 90 °C for 30 min. Then the samples were cooled and centrifuged at 3000 rpm for 20 min. The supernatant was dried at 105 °C until constant weight, while the gel formed at the bottom of the tube was weighed. The solubility was calculated by dividing the mass of dried supernatant by the mass of starch. The swelling power was calculated as described by Martins et al. (2018). 2.10. In vitro digestibility 7
In vitro digestibility of the samples was analyzed according to the method of Englyst, Kingman, and Cummings (1992) with minor modifications. The mixed enzyme solution should be formed by mixing 0.788 mL of amyloglucosidase, 1.75 mL distilled water and the supernatant prepared by 3 g pancreatin was dispersed in distilled water (20 mL) in a centrifuge tube. The starch sample (200 mg, dry basis), 0.1 mol/L sodium acetate buffer (4 mL, pH = 5.2) and mixed enzyme solution (1 mL) were added into a centrifuge tube (50 mL) at 37 °C for different times at 190 r/min for hydrolysis. The hydrolyzed solution (0.1 mL) was gathered and inactivated with ethanol (4 mL, 70%). 0.1 mL aliquots of the supernatant were mixed with 3 mL of GOPOD at 45 °C for 20 min in water bath. Another 0.1 mL standard glucose solution was treated as the standard, and 0.1 mL distilled water was treated as the blank. Values for the digested starch fractions are expressed as milligrams of glucose multiplying 0.9. Values for rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) were calculated from the G20 and G120 values, using the follow equations: RDS (%) = (G20 – FG) ×0.9 SDS (%) = (G120 – G20) ×0.9 RS (%) = 1 – (RDS + SDS) Where, G20 and G120 were the percentage of glucose in the 20 min and 120 min of the enzymatic hydrolysis, respectively; FG was the amount of glucose in the sample before the enzymatic hydrolysis. 2.11. Statistical analysis The datas were recorded as the mean values ± standard deviations. Analysis of variance (ANOVA) by Duncan’s test (p<0.05) were conducted using the IBM SPSS Statistics 22.0 Software Program (SPSS Inc. Chicago, IL, USA). 8
3. Results and discussion 3.1. Physicochemical properties of PAW According to Table 1, the pH value of DW decreased rapidly from 6.46 to 2.62 after plasma treatment for two minutes, which obviously showed that plasma discharge could lead to water acidification. During the plasma experiment, the dissolution of nitrogen oxides generated from O2 and N2 would not only come into being NO3− and NO2−, but also produce H+ that decreases the pH of PAW (Zhou et al., 2018). This might be the cause of water acidification. Conductivity was also measured to detect the presence of active ions in water. After plasma activation, the value of conductivity increased from 2.58 to 895.33 μS/cm. This was because plasma activation could generate the soluble active groups and ions (ROS or RNS) in water, which improved the conductivity (Xu, Tian, Ma, Liu, & Zhang, 2016). The ORP is determined to be an important factor affecting microbial inactivation (Ma, Wang, Tian, Wang, Zhang, & Fang, 2015). The ORP of PAW increased linearly with the prolongation of plasma activation time. And the value of ORP increased from 322.33 to 593.33 mV after plasma activation for 2 min. That might be due to the generation of ROS, such as H2O2 or O3 by plasma treatment (Wu et al., 2017). 3.2. Granular morphology The SEM micrographs of treated and untreated starch samples were shown in Fig. 1. The granular size of NMS and WMS was uniform, mostly spherical and polyhedral. The surface was smooth, full and glossy. For NMS treated with DW and PAW, the starch granules treated were not broken. However, the surface of DW-WMS and PAW-WMS was cracked with some pits. Notably, there was no obvious difference between the granular morphology of starches modified with DW and PAW. 9
3.3. Crystalline properties XRD patterns and relative crystallinity of starch samples were shown in Fig. 2. The characteristic diffraction peaks of all the starch primarily displayed at 2θ = 15.2°, 17.2°, 18.0°, and 23.1°, which belonged to the A-type starch structure (Shi, Gao, & Liu, 2018). The crystallinity of HMT-modified starches (DW-WMS and DW-NMS) were lower than that of native starch. This change might be because this treatment can promote the rupture of amylopectin, weakening its double helices and causing a transition from crystalline to amorphous (Oliveira, Bet, Bisinella, Waiga, Colman, & Schnitzler, 2018). Chen et al. (2017) also found that HMT caused starch degradation with degraded areas represented by shallow indentations along the growth ring and longitudinal grooves originating from the hilum. Moreover, the RC of maize starch by PAW-HMT (PAW-WMS and PAW-NWS) was higher than that by DW-HMT (DW-WMS and DW-NWS), indicating that PAW could increase the RC of maize starch. This could be attributed to the preferential hydrolysis of the amorphous regions by the acidic ingredient of PAW. In addition, this behavior might be ascribed to better orientation of the crystallites for the hydrolysis starch (Rafiq, Singh, & Saxena, 2016; Zhang, Hou, Liu, Wang, & Dong, 2019). 3.4. Fourier transform infrared spectroscopy analysis The infrared spectra of native and modified starches were presented in Fig. S2 (supplementary material). It can be seen from Fig. S2a that the spectra shown similar trend for all the starches, indicating that there was no obvious formation of new functional groups. The fundamental region of starch corresponds to the stretching vibration of broad band at 3400 cm-1 confirmed by the presence of O-H vibrations (Rafiq et al., 2016). The characteristic peaks at 2934 cm-1 were attributed to C-H deformation vibration of glucose element (Flores-morales, Jiménez-estrada & 10
Mora-escobedo, 2012). The peak at 1649 cm-1 was related to the bending vibration of O-H in water and associated to the amorphous region of starch (Wang et al., 2017). The peaks at 1156, 1080 and 1019 cm-1 corresponded to the asymmetric C-O-C, C-O and C-C skeleton stretching vibrations, respectively (Kizil, Irudayaraj, & Seetharaman, 2002). The IR spectra of starch was also sensitive to the short-range ordered structure of double helices and independent of the composition of double helices related to long-range structure (Soest, Tournois, Wit, & Vliegenthart, 1995). The IR bands at 1047 and 1022 cm-1 in Fig. S2b were related to the ordered and amorphous structure of starch, respectively. The absorbance ratio of the two could be used as the parameter of the short-range order of double helices (Shingel, et al., 2002). From Table 2, the absorbance ratio of DW-WMS decreased by 0.036. This might be double helices were destroyed and transformed into amorphous components (Chen et al., 2017). However, the absorbance ratio of PAW-WMS increased by 0.026 (Table 2), which was basically consistent with XRD result. For NMS, the parameter of the short-range order performed a similar trend as WMS. This was because PAW could produce high amount of short starch chains, which could increase the mobility of starch chains and allow more efficient rearrangement during HMT (Zhang, Li, Xie, & Chen, 2019). 3.5. Raman spectroscopy analysis Raman spectra of starch samples were shown in Fig. S3 (supplementary material). It was obvious that the peak strength of the modified starch became weaker. The peak at 2913 cm-1 was attributed to C-H stretching vibration, the band at 1343 cm-1 was dominated by C-OH modes, and the spectral bands at 1260 cm-1 were correlated with the C-H bending vibration of CH2. The band at 1123 cm-1 was associated to the C-O stretching and C-O-H deformation. The band at 943 cm-1 was assigned to the 11
symmetric stretching vibration of the α-1,4-glycosidic linkage. The characteristic peak of 865 cm-1 was associated to the C-O-C stretching vibration. Moreover, the bands in the 479 cm-1 were the skeletal-mode vibration region, which was usually used to characterize the molecular order of starch (Mutungi, et al., 2012; Flores-Morales et al., 2012; Liu, Xu, Yan, Hu, Yang, & Shen, 2015). FWHH is widely used to characterize the structural changes of starch samples, and band narrowing is the index of narrower bond energy distribution in the more ordered samples (Wang, et al., 2015). The FWHH of the band at 480 cm-1 was utilized to characterize the molecular order of starch. In generally, with the increase of short-range order of starch, the FWHH of starch decreased at 480 cm-1 (Fechner, Wartewig, Kleinebudde, & Neubert, 2005). From Table 2, the FWHH of the starch treated with DW-HMT was obviously higher than that of the native starch or the starch treated with PAW-HMT. These observations were in agreement with the results of XRD and FTIR. 3.6. Thermal properties The DSC curves of waxy and normal maize starches were presented in Fig. S4 (supplementary material). The datas obtained for transition temperatures including onset temperature (To), peak temperature (Tp), conclusion temperature (Tc) and gelatinization enthalpy (∆H) were summarized in Table 2. HMT has a significant effect on the thermal properties of starch, and an increase of gelatinization temperature (To, Tp and Tc) was observed for DW-WMS and DW-NMS (Table 2). The reason for the increase in gelatinization temperature was the enhanced interactions between amylose and amylopectin (Oliveira et al., 2018). On the other hand, the decrease of gelatinization enthalpy (∆H) could be attributed to the double
12
helices dissociation of the crystalline and amorphous regions in the particles (Lv et al., 2018). For NMS, the gelatinization temperature of PAW-NMS further increased because of the reorientation of starch granules and crystalline recombination. The increase of conclusion temperature might be attributed to formation of more stable crystalline during HMT (Chen et al., 2017). The value of gelatinization enthalpy increased from 10.65 (DW-NMS) to 12.69 (PAW-NMS), which might be associated with the greater extent of degradation by acid that resulted in more heterogeneous crystallites (Alimi & Workneh, 2018). However, compared with DW-WMS, a decrease of gelatinization temperature (To, Tp and Tc) was observed for PAW-WMS. This might be due to the heterogeneity of the crystallites formed from different entities, such as crystalline amylopectin side chains and retrograded amylose, after acid hydrolysis (Wang & Copeland, 2015). The values of gelatinization enthalpy increased from 11.06 (DW-WMS) to 15.17 (PAW-WMS), which was attributed to higher the crystalline structure, the crystallinity and the double helices structure (Zhang, Li, Liu, Xie, & Chen, 2013). 3.7. Swelling power (SP) and solubility (S) For all samples, the swelling power of the starch by DW-HMT decreased compared to that of the native starch (Table 2), which might be related to the decrease of water absorption and retention ability of amylopectin. The increase of starch solubility after DW-HMT was due to the destruction of the double helices structure of amylopectin and the stronger leaching ability of amylopectin from thermal damaged particles (Rocha-Villarreal, Hoffmann, Vanier, Serna-Saldivar, & García-Lara, 2018). The swelling power of modified starch with PAW-HMT was lower than that with DW-HMT, which might be due to the acid hydrolysis of amorphous region reduced 13
the water binding ability of starch molecules (Hung, My, & Phi, 2014). In terms of solubility (Table 2), the solubility of starch treated with PAW-HMT was higher than that with DW-HMT. The increase of starch solubility was due to the high content of amylose produced by hydrolysis of acidic substances in water activated by plasma, which was easy to be separated and diffused from particles in the process of expansion (Yan, Wu, Li, Yin, Ren, & Tao, 2019). 3.8. In vitro digestibility The digestion properties of native and modified starches were presented in Fig. 3. The contents of RDS, SDS, and RS of the native and treated starches were present in Table 2. The results revealed that HMT had an obvious influence on starch digestibility. Compared to native WMS, the RS content of DW-WMS increased to 24.58%. Similar trend was also found in native NMS and DW-NMS. The increase of thermal stable RS might be explained that some interactions formed during DW-HMT might survive after gelatinization, thus partially limiting the availability of starch chain to hydrolase (Chung, Liu, & Hoover, 2009). Moreover, PAW-HMT significantly resulted in the formation of higher amount of RS than DW-HMT for WMS and NMS. The reason might be that acidic component in PAW could produce lower-molecular-weight hydrolysates (both branched and linear structures of amylose and amylopectin) and these starch components are resistant to the hydrolysis of the enzyme by forming double helices and compartmentalizing amylose-amylose, amylopectin-amylopectin and amylose-amylopectin chains during HMT (Hung, et al., 2016). It was noteworthy that RS content of starch modified by PAW-HMT was higher than that of both native starch and starches by DW-HMT. 4. Conclusions In summary, the modification of PAW combined with HMT had a significant 14
effect on structure and properties of WMS and NMS. Compared to DW-HMT, the long-range and short-range order structure of starches treated with PAW-HMT increased while the granule structure did not change. The physicochemical properties of WMS and NMS were also obviously changed after PAW-HMT. Furthermore, PAW-HMT resulted in the decrease of in vitro digestibility and the increase of RS content in comparison to DW-HMT. It is concluded that the acidic component in PAW could play an important role during HMT. In a word, PAW-HMT will be expected to be a promising technology to modify the muti-scale structure and improve resistant starch content in starch industry. Acknowledgments This work was supported by the National Natural Science Foundation of China (21502177), the Program for Science and Technology Innovation Talents in Universities of Henan Province (20HASTIT037), the Science and Technology Basic Research
Program
of
Henan
Province
(182102110248,
182102310903,
192102110104, 192102110213), the Basic Research Plan of Higher Education School Key Scientific Research Project of Henan Province (19zx012), and the Higher Education School Young Backbone Teacher Training Program of Henan Province (2018GGJS093). Declarations of interest The authors declare no conflict of interest.
15
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Figure Captions
Fig. 1. SEM images of native and modified starches. DW-distilled water; PAW-plasma-activated water; WMS-waxy maize starch; NMS-normal maize starch. Fig. 2. XRD patterns of native and modified starches. DW-distilled water; PAW-plasma-activated water; WMS-waxy maize starch; NMS-normal maize starch. Fig. 3. In vitro digestibility of native and modified starches. a: in vitro digestibility of WMS, DW-WMS and PAW-WMS; b: in vitro digestibility of NMS, DW-NMS and PAW-NMS; DW-distilled water; PAW-plasma-activated water; WMS-waxy maize starch; NMS-normal maize starch.
22
Table 1 pH, electric conductivity, and ORP of PAW Water
pH
Conductivity (μS/cm)
ORP (mV)
DW
6.46±0.02a
2.58±0.06b
322.33±0.58b
PAW
2.62±0.01b
895.33±6.02a
593.33±0.58a
DW-distilled water; PAW-plasma-activated water; ORP-oxidation reduction potential. Values are the mean of triplicate measurements ± standard deviation (SD); values with different lowercase letters in the same column indicated a significant difference at p<0.05.
23
Table 2 Short-range order structure, physical properties, thermal properties, RDS, SDS, and RS of native and modified starches Samples
WMS
DW-WMS
PAW-WMS
NMS
DW-NMS
PAW-NMS
1047/1022
0.991±0.003a
0.955±0.004b
0.981±0.006a
0.945±0.002a
0.914±0.005b
0.938±0.004a
FWHH (cm-1)
17.08±0.05b
17.53 ±0.23a
16.39±0.02a
17.51±0.30b
19.09±0.54a
18.49±0.28a
S (%)
2.46±0.25c
32.39 ±0.13b
37.18±0.26a
3.09±0.37c
7.21±0.90b
10.11±0.52a
SP (g/g)
25.94±0.28a
3.90 ±0.39b
3.83±0.10b
13.14±0.87a
8.19±0.19b
6.37±0.24c
To (℃)
62.76±0.37c
75.90±0.15a
70.69±0.15b
64.31±0.10c
72.87±0.21b
73.88±0.11a
Tp (℃)
69.86±0.24c
81.68±0.01a
76.38±0.14b
69.53±0.01c
78.05±0.51b
79.78±0.15a
Tc (℃)
79.02±1.84b
92.16±0.81a
80.57±2.72b
75.14±0.54a
82.74±2.93a
87.54±0.45a
∆H (J/g)
16.61±0.31a
11.06±1.36b
15.17±0.40a
12.49±0.19a
10.65±0.19b
12.69±0.23a
RDS (%)
39.71±0.33a
37.73±2.80ab
33.92±0.82c
32.33±0.99b
39.98±1.48a
37.73±0.49a
SDS (%)
39.63±0.22a
37.69±1.98a
36.87±0.93a
40.49±1.10a
28.60±1.76b
26.89±0.99b
RS (%)
20.66±0.55c
24.58±0.82b
29.20±0.11a
27.18±0.11c
31.42±0.28b
35.38±0.50a
DW-distilled water; PAW-plasma-activated water; WMS-waxy maize starch; NMS-normal maize starch. FWHH-the full width at half height; SP-swelling power; S-solubility; To-onset temperature, Tp-peak temperature, Tc-conclusion temperature, ∆H-enthalpy;
RDS-rapidly
digestible
starch;
SDS-slowly
digestible
starch;
RS-resistant starch. Values are the mean of triplicate measurements ± standard deviation (SD); values with different lowercase letters in the same line for the same starch indicated a significant difference at p<0.05. 24
WMS
NMS
DW-WMS
DW-NMS
PAW-WMS
PAW-NMS Fig. 1
25
17.2 18.0
Relative intensity
15.2
23.1 WMS, RC=37.5% DW-WMS, RC=34.9% PAW-WMS, RC=37.1% NMS, RC=33.6% DW-NMS, RC=30.6% PAW-NMS, RC=33.3%
5
10
15
20
25
Diffraction angle ( 2θ ) Fig. 2
26
30
35
a
Hydrolysis percentage (%)
100
80
60
40
20
WMS DW-WMS PAW-WMS
0 60
0
120
180
240
300
360
Time (min)
b
Hydrolysis percentage (%)
100
80
60
40
20
NMS DW-NMS PAW-NMS
0 0
60
120
180
Time (min) Fig. 3
27
240
300
360
Highlights
Combined effect of plasma-activated water with heat-moisture treatment was studied.
Structure and properties of waxy and normal maize starches were obviously modified.
Resistant starch content of starches by combined modification was increased.
Acidic species in plasma-activated water is an important factor for modification.
28