Influences of microwave pre-drying and explosion puffing drying induced cell wall polysaccharide modification on physicochemical properties, texture, microstructure and rehydration of pitaya fruit chips

Accepted Manuscript Influences of microwave pre-drying and explosion puffing drying induced cell wall polysaccharide modification on physicochemical properties, texture, microstructure and rehydration of pitaya fruit chips Jianyong Yi, Linyan Zhou, Jinfeng Bi, Xuan Liu, Chen Qinqin, Xinye Wu PII:

S0023-6438(16)30138-4

DOI:

10.1016/j.lwt.2016.03.001

Reference:

YFSTL 5341

To appear in:

LWT - Food Science and Technology

Received Date: 3 October 2015 Revised Date:

28 February 2016

Accepted Date: 1 March 2016

Please cite this article as: Yi, J., Zhou, L., Bi, J., Liu, X., Qinqin, C., Wu, X., Influences of microwave predrying and explosion puffing drying induced cell wall polysaccharide modification on physicochemical properties, texture, microstructure and rehydration of pitaya fruit chips, LWT - Food Science and Technology (2016), doi: 10.1016/j.lwt.2016.03.001. 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.

ACCEPTED MANUSCRIPT 1

Influences of microwave pre-drying and explosion puffing drying induced cell wall

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polysaccharide modification on physicochemical properties, texture, microstructure

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and rehydration of pitaya fruit chips

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Jianyong Yi, Linyan Zhou, Jinfeng Bi∗, Xuan Liu, Chen Qinqin, Xinye Wu

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Institute of Food Science and Technology, Chinese Academy of Agricultural Science (CAAS); Key

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Laboratory of Agro-Products Processing, Ministry of Agriculture, Beijing 100193, China

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E-mail: [email protected] (J. F. Bi)

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Corresponding author. Tel./Fax: +86 010 62812584

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ACCEPTED MANUSCRIPT ABSTRACT

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The effects of microwave (1.0, 2.0, and 4.0 W/g) and explosion puffing combination

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drying (MDx-EPD) on cell wall polysaccharides modification was investigated, and

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the influences of these modification on the physical and physicochemical properties

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of pitaya fruit chips were analyzed. Compared with conventional hot air-explosion

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puffing drying, MDx-EPD significantly increased volume expansion, and yielded

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products with superior porous microstructure and crispier texture. The MDx-EPD

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dried chips showed faster rehydration rates, as well as decreasing Tg (14.01–15.33 °C).

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The anhydrouronic acid contents of the water extractable polysaccharide fractions of

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the MDx-EPD dried chips were increased by 8–16%, and this, together with the

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increase in the amount of pectic neutral sugars for the same fractions, were indicative

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of cell wall polysaccharides solubilization, which could contribute to the

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improvement of rehydration properties. Data from molar mass distribution suggested

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the occurrence of cell wall polysaccharide degradation, which could be partially

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responsible for the decreasing Tg. In conclusion, the relationship between the

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modification in composition, structure and extractability of cell wall polysaccharides

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and the alteration in volume expansion, microstructure, Tg and rehydration behaviors

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confirmed that cell wall polysaccharide played a significant role in the

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physicochemical and physical properties of pitaya fruit chips.

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Keywords: Pitaya fruits; Volume expansion; Glass transition temperature; Cell wall

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polysaccharides; Extractability

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1. Introduction Among many fruit chip production technologies, explosion puffing drying (EPD) is

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an efficient non-fried drying technology with unique advantages. It contributes to a

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typical porous structure and a pleasant crispy taste, which are both important features

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for fruit chips (Huang & Zhang, 2012). Other favorable characteristics of explosion

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puffing are found in color, rehydration, flavor, and production/storage costs (Du, Gao,

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Ji, Ma, Xu & Wang, 2013). Explosion puffing drying has been applied on many fruits,

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for example, apple (Yi, Zhou, Bi, Wang, Liu & Wu, 2015), mango (Zou, Teng, Huang,

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Dai and Wei, 2013), jujube (Du, Gao, Ji, Ma, Xu & Wang, 2013), peach (Lyu, Zhou,

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Bi, Liu & Wu, 2015) and banana (Setyopratomo, Fatmawati, Sutrisna, Savitri & Allaf,

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2012), etc.

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During explosion puffing drying, the amount of steam that generated in the puffing

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phase is largely determined by the initial moisture content of a material, which is in

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most cases around 300 g/kg (Louka & Allaf, 2002). A large amount of generated

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steam completely disintegrates the porous structure, whereas in the opposite case the

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material is not well expanded. Generally, prior to explosion puffing drying, hot air

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drying (AD) is used as a pre-treatment for reducing the moisture content (Louka &

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Allaf, 2002; Lyu et al., 2015). However, severe shrinkage often occurs during AD

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pre-drying stage, leading to adverse effects on final qualities, e.g., limited volume

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expansion. Microwave heating takes place in dielectric materials such as fruit tissues

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due to the polarization effect of electromagnetic radiation. Besides its high efficiency

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on dehydration, it was reported that microwave drying (MD) showed an excellent

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ACCEPTED MANUSCRIPT puffing effect on some agro-products (Lee, Lim, Lim & Lim, 2000; Rakesh & Datta,

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2011). In this sense, microwave could be used as an alternative pre-drying method for

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explosion puffing drying, and this might bring some positive effects on the volume

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expansion and texture of fruit chips.

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Plant cell wall, which plays a important role in the physical properties of many fruit

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and vegetable products, is made up of complex polysaccharides, phenolic compounds

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and proteins stabilized by covalent and non-covalent (e.g. ionic) linkages (Caffall &

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Mohnen, 2009). It is well known that lignin, cellulose and hemicellulose are quite

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stable during common thermal processing (<100 ºC); therefore, a great importance is

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given to pectin, due to the fact that degradation and structural modification could

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occur during thermal treatment (Sila, Smout, Elliot, Loey & Hendrickx, 2006). Pectin

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is a complex cell wall polysaccharide which generally consists of three domains, i.e.

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homogalacturonan (HG) (smooth region), rhamnogalacturonan I (RG-I) and

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rhamnogalacturonan II (RG-II) (hairy regions) (Mohnen, 2008). Some literature have

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documented that during drying processing, which in most cases is a thermal treatment,

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the modification of pectin structure is correlated to the physicochemical and physical

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properties (e.g. texture and rehydration) of a final product. Femenia, Bestard, Sanjuan,

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Rossello & Mulet (2000) found that the rehydration property of air-dried broccoli was

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substantially affected by the amount and structure of cell wall pectic polysaccharides.

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Latorre, de Escalada, Rojas & Gerschenson (2013) reported that microwave treatment

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modified the structure of cell wall polysaccharides in such a manner that produced an

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increase in their hydrophilicity.

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ACCEPTED MANUSCRIPT The object of this work was to study the influences of cell wall polysaccharides

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modifications induced by microwave-explosion puffing drying (MD-EPD) on the

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physicochemical and physical properties of pitaya fruit chips.

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2. Material and Methods

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2.1. Chemicals

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Chemicals used in the experiment were all analytical grade. In general, chemicals

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were provided by Beijing Beihua Chemicals Co., Ltd (Beijing, China). Commercial

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neutral sugar standards (fucose, rhamnose, arabinose, galactose, glucose, xylose and

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mannose) were provided by Sinopharm Chemical Reagent Beijing Co., Ltd (Beijing,

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China).

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2.2. Sample preparation

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Fully ripened white pitaya fruits (Hylocereus undatus Britt & Rose) were purchased

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from Xinfadi agro-product market in Beijing, China. The fruits were stored at 4 °C

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before used. Prior to drying, fresh pitaya fruits were taken out of storage, peeled and

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cut into slices with average thickness of 7.0 ± 1.0 mm.

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2.3. Drying process

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Hot air drying (AD) was conducted in a convective dryer (DHG-9123A, Jinghong

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Co., Ltd., Shanghai, China). The drying temperature was 65 °C and the air velocity

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was 2.05 m/s. Hot air-explosion puffing drying (AD-EPD) consisted of three steps.

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Firstly, pitaya fruit slices were dried by AD to an intermediate moisture content of 300

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g/kg. Then, the semi-dried samples were transferred to an experimental explosion

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puffing dryer (Qin-de Co. Ltd., Tianjin, China), which was depicted in a previous 5

ACCEPTED MANUSCRIPT study (Bi et al., 2015). Prior to explosion puffing, the samples were equilibrated at

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90 °C for 5 min. Meanwhile, the vacuum chamber was evacuated by a vacuum pump.

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The snuffle valves were opened to obtain a rapid pressure drop (< 0.2 s) to

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approximately 40 Pa (absolute pressure) in the puffing chamber, during which the

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samples were puffed. After puffing, the samples were dried under a continuous

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vacuum at 65 °C.

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Microwave drying (MD) was performed in a laboratory microwave dryer (Sanle

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Co., Nanjing, China) at a power intensity of 2.0 W/g fresh weight (fw). For MD-EPD

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treatment, firstly, pitaya fruit slices were dried by MD to a moisture content of 300

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g/kg. Three microwave intensities (1.0, 2.0, and 4.0 W/g fw) were used for the MD

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pre-drying, namely MD1-, MD2-, and MD4-EPD (MDx-EPD), respectively. Then, the

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semi-dried samples were removed to the puffing chamber for explosion puffing.

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Explosion puffing process was performed at the same condition as the AD-EPD. For

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each drying process, 400 g pitaya fruit slices were used, and the treatments were

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conducted in triplicates. The drying curves are shown in supplemental material (Fig.

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S1).

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2.4. Physicochemical characteristics

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Moisture content was determined by drying a sample to constant weight at 105 °C.

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Water activity (Aw) was measured by using an Aw meter (AquaLab 3, Decagon

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Devices, Inc., Washington, USA) (Giraldo-Zuniga, Arévalo-Pinedo, Rodrigues, Lima

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& Feitosa, 2006). Glass transition temperature (Tg) was determined using a

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differential scanning calorimetry (DSC200PC, Netzsch, Bavaria, Germany) (Zou et 6

ACCEPTED MANUSCRIPT al., 2013). Total soluble solid (TSS) content was measured by using a refractometer

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(MASTER-α, Atago Co. Ltd., Tokyo, Japan). Titratable acidity (TA) was determined

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by titrating a sample with 0.05 mol/L NaOH, and the result was calculated as citric

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acid equivalent (Chien, Sheu & Lin, 2007). All the physicochemical characteristic

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analysis was conducted in triplicates.

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2.5. Volume ratio (VR) and bulk density

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Volume expansion ratio was measured using a Volscan Profiler (VSP 600, Stable

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Micro System Ltd., Godalming, UK). VR was calculated using the equation (Bi et al.,

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2015):

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VR =(Va/Vb)×100%

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(1)

where Va and Vb refer to the volume (mL) of a sample after and before drying,

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respectively. Bulk density was calculated by dividing the mass of a chip to its

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corresponding volume. Volume ratio and bulk density determination were performed

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in triplicates.

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2.6. Texture

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Texture was measured using a TA.XT2i/50 Texture Analyzer (Stable Micro System

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Ltd., Godalming, UK) fitted with a ball probe (P/0.25) according to the method

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described by Lyu et al. (2015). Data were analyzed by the software of Texture

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Exponent 32 (Stable Micro System Ltd., Godalming, UK). Twelve measurements

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were performed for each treatment.

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2.7. Rehydration

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Rehydration properties were analyzed according to the procedure described by 7

ACCEPTED MANUSCRIPT Markowski & Zielinska (2013). Briefly, each chip was weighed, placed in a tea

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drainer, and then immersed in a water bath at 25 °C for various times. At different

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time intervals, the samples were removed from water. Excess water from the surface

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was gently wiped off using tissues, and the sample was weighed. The rehydration

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ratio (RR) was calculated by the following equation:

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RR = Mr/M0

(2)

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where M0 and Mr are the mass of a sample before and after rehydration (g),

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respectively. Rehydration analysis was performed in quadruplicates.

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2.8. Scanning electron microscopy

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Microstructure characterization were performed using a scanning electron

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microscope (SEM S-570, Hitachi Ltd., Tokyo, Japan) at 150 kV accelerated voltage

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and 10-15 mm working distance.

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2.9. Cell wall polysaccharide extraction and fractionation

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Cell wall polysaccharide was extracted according to the method described by

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Ramírez-Truque, Esquivel & Carle (2011). Pitaya chips (10 g) were homogenized in

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100 mL aqueous ethanol (volume ratio of ethanol/water was 80/20), followed by

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extracting with 100 mL of the abovementioned aqueous ethanol for 2 times and 100

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mL pure acetone for 1 time, respectively. To obtain alcohol insoluble residue (AIR),

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the suspension was filtered and dried at 40 °C for 36 h. AIR fractionation was

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performed according the procedure of Christiaens, Buggenhout, Houben, Chaula, Van

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Loey & Hendrickx (2012). The AIR (0.5 g) was sequentially extracted with 100 mL

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boiling water (pH 6.8) for 5 min, 0.05 mol/L CDTA (cyclohexane-trans-1,2-diamine

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ACCEPTED MANUSCRIPT tetra-acetic acid) in 0.1 mol/L potassium acetate (pH 6.5) at 28 °C for 6 h, 0.05

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mol/L Na2CO3 containing 0.02 mol/L NaBH4 at 4 °C for 16 h, and 4 mol/L KOH

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containing 0.02 mol/L NaBH4 and 35 g/L boric acid at 25 °C for 22 h. The

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respective fractions were designated as water extractable polysaccharides (WEP),

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chelator extractable polysaccharides (CEP), Na2CO3 extractable polysaccharides

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(NEP), and hemicellulose fraction (HC). A portion of each fraction was lyophilized

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and stored in a desiccator over P2O5.

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2.10. Anhydrouronic acid (AUA) content

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The AIR and the fractions obtained thereof (WEP, CEP, NEP, and HC) were first

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hydrolyzed with concentrated sulfuric acid according to the procedure described by

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Ahmed & Labavitch (1978), and Njoroge et al. (2014). AUA content of the

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hydrolysate was determined using a spectrophotometric method (Blumenkrantz &

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Asboe-Hansen, 1973). AUA content measurement was conducted in triplicates.

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2.11. Extent of β-elimination

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Pitaya chips (10 g) were ground and mixed with 20 mL distilled water, then

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centrifuged at 9,000 g for 10 min. The appearance of unsaturated galacturonic acid

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residues was determined by measuring the absorbance at 235 nm. The extent of

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β-elimination (%) was calculated as described by Kravtchenko, Arnould, Voragen

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& Pilnik (1992). Measurement of β-elimination was conducted in triplicates.

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2.12. Neutral sugar compositions

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Neutral sugar analysis was performed according the method described by Njoroge

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et al. (2014). Lyophilized cell wall polysaccharide fractions were hydrolyzed with 4 9

ACCEPTED MANUSCRIPT mol/L trifluoroacetic acid at 110 °C for 1.5 h. Quantification of neutral sugars was

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performed via high performance anion exchange chromatography (Dionex Bio-LC

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system, Dionex Co., Sunnyvale, CA, USA) coupled with pulsed amperometric

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detection (HPAEC-PAD). After equilibrating the system for 5 min with 100 mmol/L

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NaOH and 5 min with 4 mmol/L NaOH, 10 µL of diluted hydrolysate was eluted at

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30 °C on a CarboPac PA20 column (Dionex) with 4 mmol/L NaOH at a flow rate of

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0.5 mL/min. Neutral sugar standards were used for identification and quantification.

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Recovery values for the correction of monosaccharide degradation during acid

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hydrolysis were added (Arnous & Meyer, 2008). Measurement of neutral sugar

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compositions was conducted in duplicates.

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2.13. Molar mass distribution

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Molar mass analysis was performed according the method described by Yang, Gou,

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Chen, An, Chen & Hu (2013). Dialyzed and lyophilized fraction (3.0 mg) was

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dissolved in 1 mL of 0.1 mol/L 4-morpholineethanesulfonic acid monohydrate buffer

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solution (MES), pH 6.5, containing 0.1 mol/L NaCl. Molar mass distribution was

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determined using a high performance size exclusion chromatography (HPSEC)

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coupled with multi-angle laser light scattering (Malls, Dawn-EOS, Wyatt Tech. Co.,

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Santa Barbara, USA) and refractive index (RI) detector (OptiLab-DSP, Wyatt Tech.

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Co., Santa Barbara, USA). The abovementioned solution sample (100 µL) was

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injected and separated by a TSK-Gel G3000SWXL column (7.8 × 300 mm) (Tosoh Co.,

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Tokyo, Japan), eluting with 0.1 mol/L MES buffer (pH 6.5) containing 0.1 mol/L

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NaCl at flow rate of 0.45 mL/min at 35 °C. RI intensity and light scattering intensity

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molar mass distribution, respectively (Vriesmann, Teófilo & de Oliveira Petkowicz,

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2011).

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3. Results

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3.1. Physicochemical characteristics

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The physiochemical characteristics of pitaya fruit chips are presented in Table 1.

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The moisture contents of the final products were between 5.7 and 6.3 g/100 g. The

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TSS contents of the MDx-EPD dried samples were slightly higher than that of the

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AD-EPD dried samples. No significant differences were found in titratable acidity

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among different samples, as well as in the Aw among the four puffed samples. Tg is

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closely related to moisture content, Aw and chemical constitution of a material. Water

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can decrease Tg as an effective plasticizing agent, while matrix with low molar mass

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exhibit low Tg due to its high molecular mobility (Roos, 2010). The Tg of the

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MDx-EPD samples were lower than that of the AD and MD dried samples, and the

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lowest Tg (14.01 °C) was observed in the MD4-EPD dried samples. Generally, a high

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moisture content and Aw are correlated to a low Tg. However, the results of moisture

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content and Aw were not fully agreed with the alteration in Tg. Therefore, it is

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postulated that the modification of chemical constitution might also played a role in

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the changing Tg.

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3.2. Volume ratio, bulk density and texture

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The maximum volume ratio and the minimum bulk density were observed in the

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MD2-EPD and MD4-EPD dried samples, followed by the MD1-EPD and AD-EPD 11

ACCEPTED MANUSCRIPT dried samples (Table 2). During the decompression phase of explosion puffing,

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internal water vaporized and exploded rapidly to the vacuum environment (Mounir,

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Allaf, Mujumdar & Allaf, 2012), corresponding to the sharp drop in the AD-EPD

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drying curve (Fig. S1); meanwhile, the volume of pitaya fruit slices was expanded. It

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was reported that microwave can be used to expand or puff barley (Altan, 2014); the

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puffing effect of microwave might render a better microstructure for the pre-dried

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pitaya fruit slices as well, which might be a partial explanation for the fact that the

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volume ratio of the MDx-EPD treatment was higher than that of the AD-EPD

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treatment. Apparent shrinkages (79–83%) were observed after the AD and MD

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pre-drying, but the volumes of these semi-dried samples were recovered to some

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extent during explosion puffing.

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Texture is one of the most importance physical features for fruit chips, and it is

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closely related with porosity. Table 2 also shows the texture properties of pitaya fruit

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chips. The maximum compression force represents hardness, while the number of

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compression peaks is associated with crispness (Tabtiang, Prachayawarakon &

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Soponronnarit, 2012). For the AD-EPD and MDx-EPD dried chips, the maximum

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forces reached 44 and 31–36 N, respectively, and showed dozens of small peaks until

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reaching the zero point. Such textural characteristics revealed that all the puffed

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products obtained typical crispy mouth feel (Chen & Opara, 2013). In addition, more

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compression peaks were observed in the MDx-EPD dried chips than in the AD-EPD

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dried chips, implying that MDx-EPD yielded products with crispier texture. It was

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observed that the outer layers of pitaya fruit slices became rigid to some extent after

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ACCEPTED MANUSCRIPT AD pre-drying, and this outer layer might acquire considerable strength and could be

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a hindrance for volume expansion, resulting in relatively high hardness and low

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crispness for the AD-EPD dried chips.

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3.3. Microstructure

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Four representative microstructure images of pitaya fruit chips are shown in Fig. 1.

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Typical porous microstructures were both observed in the AD-EPD and MD2-EPD

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dried samples (Fig. 1c and 1d). The microstructures of the MD1-EPD and MD4-EPD

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dried chips were similar to that of the MD2-EPD dried product (photo not shown). A

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substantial increase in cell size was found in the AD-EPD and MD2-EPD dried

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samples. The expansion was difficult to quantify due to the large differences in pore

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size, but explosion puffing formed cavities which diameters reached approximate 150

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µm at the maximum. The microstructure of the MD2-EPD dried sample was more

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expanded and porous than that of the AD-EPD dried samples. In addition, un-puffed

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areas with dense tissue material (marked with arrows) were clearly visible in Fig. 1c,

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revealing that some parts of the material was not puffed. The un-puffed areas in the

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AD-EPD dried samples, together with their relatively small porous size, were

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consistent with the results of volume ratio (Table 2). Furthermore, these un-puffed

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dense areas were expected to fortify the structural strength of AD-EPD dried chips,

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contributing to their relatively high hardness and low crispness.

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3.4. Rehydration

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The rehydration rates of pitaya fruit chips deceased in the sequence of MD4-, MD2-, 13

ACCEPTED MANUSCRIPT MD1-, and AD-EPD treatments, followed by MD and AD treatments (Fig. 2). The

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rehydration kinetics of a dried fruit is strongly related to porosity, and filling of the

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capillaries and cavities near the surface yields quick saturation (Marabi & Saguy 2004;

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Rahman, 2001). The AD-EPD dried chips were not able to absorb water as quick as

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the MDx-EPD dried chips, which could be partially due to the number and size of

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available capillary paths or pores of AD-EPD finished samples were relatively smaller

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(Fig. 1c and 1d), thus not facilitating a fast capillary suction. Rehydration capacity is

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mainly dependent on the microstructures and osmolality of a material (Van der Sman,

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2014). No significant differences were found in rehydration capacities among the

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three MDx-EPD dried samples, but they were higher than that of the AD-EPD dried

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product. Compared with the partly un-puffed microstructure in the AD-EPD dried

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chips, the fully puffed microstructures in the MDx-EPD dried chips could be a partial

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explanation for their higher rehydration capacities.

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3.5. AUA contents and neutral sugar compositions

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Analysis of the composing sugars of different factions allowed us to make an

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overall fingerprint of cell wall polysaccharides. The AUA contents and neutral sugar

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composition of pitaya fruit chips are shown in Table 3. High amounts of AUA, Rha,

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Ara and Gal were observed in the WEP, CEP and NEP fractions of all the samples,

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indicating the predominate presence of pectic polysaccharides in these fractions,

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possibly HG and/or RG backbones branched with arabinogalactan, arabinan and

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galactan. Similar sugar composition was reported by Ramírez-Truque et al. (2011),

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who found that pitaya fruit was dominated by Ara, Glc and Gal, while Fuc and Xyl

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were minor. Small amount of AUA and pectic sugars were detected in HC fractions,

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implying that the HC contained, besides hemicellulose polymers, residual pectin

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that were strongly bound to cellulose or hemicellulose. Alteration in AUA and neutral sugar composition in different fractions reflects

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the structural modification of cell wall polysaccharides (Christiaens et al., 2012).

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WEP is generally made up of highly esterified pectic polysaccharides that are

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loosely bound to cell wall through non-covalent and non-ionic bonds, while CEP

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contains mainly low-esterified pectin that are held in cell wall by calcium bridges,

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and NEP is predominantly linked to cell wall through covalent ester bonds. In

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general, the sugar contents for the AIR of MD-EDP treatments were higher than

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that of the AD and MD treatment, which might be related to the improvement of

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sample porosity, thus increasing the extraction of cell wall polysaccharides.

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Remarkable differences in AUA contents were noticed among the cell wall

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polysaccharide fractions of pitaya chip dried by different methods. Compared with

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the AD treated samples, the AUA contents of the WEP fractions of the AD-, MD1-,

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MD2-, and MD4-EPD dried pitaya chips were increased by 19, 24, 32 and 36%,

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respectively. Together with the relatively low AUA contents in CEP and NEP

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fractions for the same samples, it was suggested that certain amount of pectic ionic

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and/or ester linkages in cell wall polysaccharides were disconnected during

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AD-EPD and MDx-EPD treatment. This disconnection could lead to the weakening

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of pectin related intermolecular interactions, consequently, increasing pectin

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releasing during hot water extraction. Similar increases were found in the contents

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ACCEPTED MANUSCRIPT of Rha, Ara, Gal and Xyl, proving that the major portions of these neutral sugars were

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co-extracted with pectic polysaccharides. This result indicates these neutral sugars

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could belong to highly branched pectic structure and/or non-pectic polysaccharides

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that are tightly bound to pectin. The contents of Man and Glc, which in most cases

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derive from hemicellulose and cellulose, were relatively stable in different factions,

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corroborating that their extractability was less susceptible to drying process.

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3.6. Extractability of pectic polysaccharides

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The relative amounts of the pectic polysaccharides in WEP, CEP, NEP, and HC

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fractions are expressed in extraction yields (percentage of AIR) and presented in Table

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3. The amounts of the WEP fractions in the AD- and MDx-EPD dried products were

328

significantly higher than that in the AD and MD dried products; however, the amounts

329

of CEP and NEP fractions were slightly lower in AD- and MDx-EPD expanded

330

products. The alterations in the amounts of different fractions suggested that the four

331

puffing treatments substantially increased the water extractability of cell wall

332

polysaccharides. In addition, the amounts of the WEP fraction of the MDx-EPD dried

333

samples were generally higher than that of the AD-EPD dried samples, indicating that

334

microwave significantly increased the water extractability of cell wall polysaccharides.

335

Yeoh, Shi & Langrish (2008) also found that microwave treatment give

336

approximately double the amount of citrus peel pectin extracted by water, possibly by

337

breaking down cell/middle lamella structure (Latorre et al., 2013) and improving

338

capillary-porous structure. These structure modifications might facilitate water

339

penetration during extraction, thus improving the water extractability of cell wall

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16

ACCEPTED MANUSCRIPT polysaccharide. Furthermore, the amounts of the WEP fractions of the MD1-, MD2-,

341

and MD4-EPD dried chips were increased by 8, 11 and 16%, respectively,

342

indicative of the positive effect of microwave intensity on WEP extraction. Wang,

343

Chen, Wu, Wang, Liao & Hu (2007) found that microwave power displayed a

344

significantly quadratic effect on the yield of apple pectin. Besides, a positive

345

correlation (R2 0.851) was found between the amount of WEP fractions and the

346

rehydration rate of pitaya fruit chips, indicating that the water extractability of cell

347

wall polysaccharide played a significant role in rehydration behavior.

348

3.7. Molar mass distribution

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340

Fig. 3 illustrates the molar mass distribution and concentration profiles of cell

350

wall polysaccharides in the WEP and NEP fractions of pitaya fruit chips. The molar

351

mass distributions of cell wall polysaccharides extracted from different samples

352

were similar, for WEP and NEP fractions, respectively. The MALLS and RI signals

353

of CEP fractions were very low (data not shown). The elution times of the WEP

354

fractions of the AD and MD dried chips (around 37 min) were generally earlier than

355

that of the four puffed samples (39–42 min), suggesting that the molar mass of

356

WEP fraction from AD and MD sample were higher than that from the AD-EPD

357

and MDx-EPD samples. The explosion puffing process enhanced the extent of

358

polysaccharide degradation. In addition, peak shifting to higher elution times in the

359

WEP fractions were observed with increasing microwave intensities, implying the

360

extent of cell wall polysaccharide degradation was raised correspondingly. The

361

NEP fractions for different samples were all eluted at 37 min. However, the peak

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17

ACCEPTED MANUSCRIPT concentrations of the MDx-EPD dried samples were significantly lower than that of

363

the AD-EPD dried samples. This, together with the apparent increasing concentration

364

at elution time after 43 min, was an indicative of decreasing molar mass, which

365

further confirmed the occurrence of cell wall polysaccharide degradation in the

366

MDx-EPD dried samples. The depolymerization of pectin might be attributed to

367

β-elimination (Table 1).

368

4. Discussion

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A schematic presentation of the influences of cell wall polysaccharides

370

modification induced by microwave-explosion puffing drying on the physicochemical

371

and physical properties of pitaya fruit chips are shown in Fig. 4. Compared with

372

AD-EPD treatment, MDx-EPD treatments significantly affected the qualities of pitaya

373

fruit chips. The increases in the volume expansion of the MDx-EPD dried chips were

374

partially attributed to the puffing effect of microwave pre-drying (Altan, 2014;

375

Rakesh & Datta, 2011). It was postulated that microwave treatment rendered a faster

376

internal water transfer toward outside during MD pre-drying, due to the heat that

377

generated through the penetration of electromagnetic wave. This might be in favor of

378

presenting more pores or capillary paths in semi-dried pitaya fruit, and consequently

379

could bring positive effects on volume expansion during following explosion puffing

380

process. Besides, these capillary paths could also enhance water infiltration during

381

immersing, contributing to a faster rehydration rate for the MDx-EPD dried product.

382

The extent of puffing sets up the porosity of a fruit chip (Lewicki, 1998), in other

383

words, the volume expansion during decompression phase extensively determines the

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18

ACCEPTED MANUSCRIPT number of pores and pore size, i.e. microstructure. Thereafter, maintenance of the

385

microstructure at the expanded state is dependent on further dehydration and

386

hardening (Louka & Allaf, 2002), as well as some physicochemical properties, such

387

as moisture content, Aw and Tg. The MDx-EPD dried chips showed superior porous

388

microstructure, and this was in line with their relatively low hardness and high

389

crispness. Generally, at a temperature below Tg, food components such as water,

390

enzymes, small molecular chemicals and biopolymers in a matrix exhibit minimal

391

mobility; therefore, organoleptic qualities are quite stable at this state (Roos, 2010).

392

The MDx-EPD dried pitaya chips showed a relatively low Tg, indicating the system

393

require a lower temperature (14.01-15.33 °C) to reach glass state.

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MDx-EPD treatments substantially influenced the composition, structure and

395

extractability of cell wall polysaccharides. Data from AUA contents and neutral

396

sugar compositions confirmed that the cell wall polysaccharide of pitaya fruit

397

contained large amounts of pectic polysaccharides, which structure is quite

398

susceptible to thermal processing. A sequential extraction enables analysis of

399

extractability, cross-linking and/or the interaction of pectic polymers involved in

400

cell wall network. In the case of MDx-EPD dried pitaya chips, the increases in the

401

amounts of AUA and some of the neutral sugars of WEP fractions revealed the

402

improvement of the water extractability of cell wall polysaccharide. This is

403

believed to be caused by the disconnection of some ionic and/or ester bond related

404

to cell wall pectic polysaccharide cross-link, which consequently, could enhance

405

cell wall polysaccharides liberation and releasing during water extraction. Besides,

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19

ACCEPTED MANUSCRIPT the degradation of cell wall polysaccharides produced certain amount of small

407

molecules, which could render an increasing osmolality for the material. Therefore,

408

on one hand, it is speculated that the hydrophilicity of cell wall polysaccharides was

409

increased after MDx-EPD treatment. On the other hand, the occurrence of

410

polysaccharide depolymerization induced by pectin β-elimination and the

411

modification of cell wall polysaccharide intermolecular interactions suggested that

412

cell wall polysaccharide network might be disorganized and misaligned to certain

413

extent due to the drying process. This could damage the integrity of the primary cell

414

wall and/or middle lamella of pitaya fruit slices, leading to a decrease in the strength

415

of intercellular adhesion.

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406

The abovementioned modifications in cell wall polysaccharides, consequently,

417

could affect the physicochemical and physical properties of pitaya fruit chips. Firstly,

418

reduction in intercellular adhesion strength might be in favor of tissue/cell separation

419

and reduce the internal structural resistance for volume expansion during puffing

420

phase. This, consequently, could contribute to a more porous microstructure as well as

421

a crispier texture for the MDx-EPD dried chips. The effects of microwave treatment

422

on reducing cell wall network integrity and enhancing cell separation were also

423

reported by Prothon, Ahrné, Funebo, Kidman, Langton & Sjöholm (2001). Secondly,

424

polysaccharide degradation and solubilization could lead to a better hydrophilicity for

425

cell wall polysaccharides, which was a partial explanation for the improvement of

426

rehydration rate and capacity. This are in good agreement with the report of Latorre et

427

al. (2013), who found that the modification of cell wall polysaccharide structure

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20

ACCEPTED MANUSCRIPT induced by microwave drying resulted in an significant increase in cell wall

429

polysaccharide hydrophilicity. In addition, the more porous microstructure in the

430

MDx-EPD samples could facilitate a faster capillary suction during immersing,

431

which was another reason for the superior rehydration properties. Thirdly, cell wall

432

polysaccharide deploymerization produced reaction products with smaller

433

molecular mass, which could increase the molecular mobility of a system, thus

434

decreasing Tg. Moreover, the damage and misalignment of cell wall polysaccharide

435

network might liberate some structural polysaccharides from cell wall, which could

436

also increase molecular mobility and contribute to the decreasing Tg. Besides, the

437

increase in TSS might be associated with cell wall polysaccharide degradation and

438

solubilization as well, possibly by increasing the amount of water soluble

439

components.

440

5. Conclusions

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428

Pitaya fruit chips were produced by AD, MD, AD-EPD, and MDx-EPD,

442

respectively. The influences of the modification in cell wall polysaccharide

443

composition, structure and extractability induced by MDx-EPD on the volume

444

expansion, microstructure, Tg and rehydration behavior, etc. confirmed that cell

445

wall polysaccharides modification played a significant role in the physicochemical

446

and physical properties of pitaya fruit chips. Cell wall polysaccharide degradation

447

was in favor of tissue/cell separation and volume expansion during explosion

448

puffing phase, consequently, contributing to a superior porous structure and crispier

449

texture. Cell wall polysaccharide deploymerization, as well as the damage and

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21

ACCEPTED MANUSCRIPT misalignment of cell wall polysaccharide network, could be responsible for the

451

decreasing Tg. Since Tg is a critical property of fruit chips which might determine the

452

hardening of porous structure after puffing, the influences of cell wall polysaccharide

453

modification during different stages of processing (e.g. pre-treatment, pre-drying and

454

explosion puffing) on Tg can be better understood by further study using model

455

system.

456

Acknowledgment

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450

The authors acknowledge the Ministry of Agriculture of P.R.C. for financial support

458

from the project No. 201303077. J. Y. Yi is a Postdoctoral Researcher funded by

459

China Scholarship Council (No. 201403250025).

460

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Femenia, A., Bestard, M. J., Sanjuan, N., Rossello, C., & Mulet, A. (2000). Effect of

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Markowski, M., & Zielinska, M. (2013). Influence of drying temperature and rehydration on

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Effects of combined osmotic and microwave dehydration of apple on texture,

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dehydration kinetics, physical properties and nutritional content of banana textured by

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of banana slices. Drying Technology, 30, 20-28.

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Van der Sman, R. G. M., Vergeldt, F. J., Van As, H., Van Dalen, G., Voda, A., & Van

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nitric acid-mediated extraction of pectin from cacao pod husks (Theobroma cacao L.)

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using response surface methodology. Carbohydrate Polymers, 84, 1230-1236.

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Wang, S., Chen, F., Wu, J., Wang, Z., Liao, X., & Hu, X. (2007). Optimization of pectin

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extraction assisted by microwave from apple pomace using response surface methodology.

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Journal of Food Engineering, 78, 693-700.

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Yang, C., Gou, Y., Chen, J., An, J., Chen, W., & Hu, F. (2013). Structural characterization

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and antitumor activity of a pectic polysaccharide from Codonopsis pilosula. Carbohydrate

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Polymers, 98, 886-895. 26

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Yeoh, S., Shi, J., & Langrish, T. (2008). Comparisons between different techniques for water-based extraction of pectin from orange peels. Desalination, 218, 229-237. Yi, J., Zhou, L., Bi, J., Wang, P., Liu, X., & Wu, X. (2015). Influence of number of puffing

563

times on physicochemical, color, texture, and microstructure of explosion puffing dried

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apple chips. Drying Technology, DOI: 10.1080/07373937.2015.1076838.

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Zou, K., Teng, J., Huang, L., Dai, X., & Wei, B. (2013). Effect of osmotic pretreatment on

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quality of mango chips by explosion puffing drying. LWT-Food Science and Technology,

567

51, 253-259.

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Figure captions

569

Fig. 1. Scanning electron micrographs of pitaya fruit chips dried by (a) hot air drying,

570

(b)

571

microwave-explosion puffing drying at microwave intensity of 2.0 W/g.

572

Fig. 2. Rehydration properties of pitaya fruit chips dried by different methods. –■–

573

hot air drying; –●– hot air-explosion puffing drying; –▲– microwave drying;

574

microwave-explosion puffing drying at microwave intensities of 1.0 (–▼–), 2.0

575

(–◄–), and 4.0 (–►–) W/g fresh weight, respectively. Bars indicate the standard

576

deviation (n = 4).

577

Fig. 3. Molar mass distribution and concentration of cell wall polysaccharides in (a)

578

water extractable polysaccharides and (b) Na2CO3 extractable polysaccharides

579

fractions of pitaya fruit chips dried by different methods. AD (–■–): hot air drying;

580

AD-EPD (–●–): hot air-explosion puffing drying; MD (–▲–): microwave drying;

581

MD1-EPD (–▼–), MD2-EPD (–◄–), and MD4-EPD (–►–): microwave-explosion

582

puffing drying at microwave intensities of 1.0, 2.0 and 4.0 W/g fresh weight,

583

respectively.

584

Fig. 4. Schematic presentation of the influences of cell wall polysaccharides

585

modification induced by microwave-explosion puffing drying (MD-EPD) on the

586

physicochemical and physical properties of pitaya fruit chips.

hot

air-explosion

puffing

drying,

and

(d)

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(c)

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588

drying,

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587

microwave

589

Supplemental material

590

Fig. S1. Evolution of the moisture content of pitaya fruit slices during drying. AD

591

(–■–): hot air drying; AD-EPD (–●–): hot air-explosion puffing drying; MD (–▲–):

592

microwave drying; MD1-EPD (–▼–), MD2-EPD (–◄–), and MD4-EPD (–►–):

593

microwave-explosion puffing drying at microwave intensities of 1.0, 2.0, and 4.0 W/g

594

fresh weight, respectively; VD: vacuum drying.

28

ACCEPTED MANUSCRIPT Table 1 Physicochemical characteristics of pitaya fruit chips dried by different methods Samples

AD

AD-EPD a

6.3 ± 0.2

5.7 ± 0.1

TSS (g/100 g)

83.0±0.2c

TA (g/100 g)

MD1-EPD a

MD2-EPD b

MD4-EPD ab

5.8 ± 0.2b

6.3 ± 0.2

5.9 ± 0.2

83.7±0.3b

83.4±0.4bc

84.1±0.3ab

84.4±0.3a

84.2±0.2a

1.02±0.05a

1.04±0.02a

1.02±0.06a

1.02±0.07a

1.04±0.06a

1.03±0.04a

Aw

0.53 ± 0.01a

0.50 ± 0.01b

0.54 ± 0.02a

0.52 ± 0.02ab

0.50 ± 0.01b

0.49 ± 0.02b

Tg (°C)

16.78±0.32a

16.02±0.31b

16.65±0.53ab

15.33±0.27c

14.58±0.40d

14.01±0.49d

50 ± 1a

53 ± 1b

50 ± 1a

51 ± 2ab

54 ± 1bc

57 ± 2c

4.1 ± 0.5e

6.0 ± 0.3c

4.7 ± 0.4d

5.9 ± 0.3c

7.7 ± 0.5b

9.4 ± 0.3a

AIR (mg/g db)

β-elimination (%)

6.0 ± 0.3

RI PT

Moisture (g/100 g db)

MD b

AD: hot air drying; AD-EPD: hot air-explosion puffing drying; MD: microwave drying; MD1-EPD,

SC

MD2-EPD, and MD4-EPD: microwave-explosion puffing drying at microwave intensities of 1.0, 2.0, and 4.0 W/g fresh weight, respectively; db: dry basis; TSS: total soluble solid; TA: titratable acidity;

M AN U

AIR: alcohol insoluble residue; AUA: andydrouronic acid. Each value is expressed as mean value ±

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standard deviation (n = 3). Different letters in the same raw indicate significant differences at p < 0.05.

ACCEPTED MANUSCRIPT

Table 2 Physical characteristics of pitaya fruit chips dried by different methods Texture∗∗ Volume ratio (%)∗

Maximum

(g/cm3)∗

Number of

compression force (N) a

a

compression peaks

a

17 ± 2

0.94 ± 0.03

64 ± 4

AD-EPD

54 ± 3c

0.31 ± 0.02c

44 ± 3c

MD

22 ± 2b

0.76 ± 0.04b

56 ± 3b

MD1-EPD

60 ± 2d

0.26 ± 0.02d

31 ± 2e

MD2-EPD

74 ± 4e

0.22 ± 0.02e

34 ± 4de

MD4-EPD

70 ± 4e

0.23 ± 0.04de

36 ± 2d

5 ± 2a

21 ± 3c

10 ± 2b

28 ± 4d 31 ± 3d

30 ± 3d

SC

AD

RI PT

Samples

Bulk density

AD: hot air drying; AD-EPD: hot air-explosion puffing drying; MD: microwave drying; MD1-EPD,

M AN U

MD2-EPD, and MD4-EPD: microwave-explosion puffing drying at microwave intensities of 1.0, 2.0 and 4.0 W/g fresh weight, respectively. ∗ Each value is expressed as mean value ± standard deviation (n = 3).

∗∗

Each value is expressed as mean value ± standard deviation (n = 12). Different letters in the

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same column indicate significant differences at p < 0.05.

ACCEPTED MANUSCRIPT Table 3 AUA contents and neutral sugar compositions of the fractions of the alcohol insoluble residues (AIR) extracted from pitaya fruit chips dried by different methods (mg/g AIR dry weight)

AIR Fractions

Fuc

AIR WEP

75

45

20

5

94

3

85

169

181

22

19

4

141

70

3

61

102

59

7

111

3

93

397

264

27

HC

87

37

2

15

68

80

339

2

36

99

6

4

20

8

157

135

18

81

47

24

7

22

5

6

3

29

21

8

153

113

19

43

WEP

185

109

4

98

177

197

22

CEP

131

60

3

53

83

39

6

NEP

134

100

3

83

336

232

HC

89

35

2

14

63

68

326

3

WEP

168

102

4

CEP

140

59

3

NEP

136

97

3

HC

90

41

1

334

2

23

6

93

36

167

95

186

77

26

21

4

55

91

26

6

7

4

81

361

243

22

21

8

14

53

71

158

120

19

37

103

78

47

26

5

93

182

203

25

23

6

36

85

57

6

6

3

WEP

182

115

2

CEP

132

62

2

NEP

133

95

3

79

335

233

25

21

8

HC

88

32

2

13

43

63

156

114

20

340

2

37

106

80

50

24

7

18.6

122

3

101

198

221

26

24

5

CEP

13.4

59

2

43

73

46

6

6

3

NEP

13.1

88

2

63

313

229

20

21

8

32

2

11

51

53

145

103

20

351

3

39

110

84

51

23

7

AIR WEP

HC AIR WEP

8.5

195

124

2

101

206

221

24

22

4

AC C

MD4-EPD

94

13.8

AIR

Man

34

CEP

AIR

Xyl

3

TE D

MD2-EPD

Glc

NEP

AIR

MD1-EPD

Gal

320

EP

MD

Ara

SC

AD-EPD

163

Rha

M AN U

AD

Neutral sugars AUA

Yield (mg/g)

RI PT

Samples

CEP

131

57

2

33

69

41

5

4

4

NEP

127

85

2

62

296

225

24

20

9

HC

84

33

2

13

56

66

150

111

21

AD: hot air drying; AD-EPD: hot air-explosion puffing drying; MD: microwave drying; MD1-EPD, MD2-EPD, and MD4-EPD: microwave-explosion puffing drying at microwave intensities of 1.0, 2.0, and 4.0 W/g fresh weight, respectively. WEP: water extractable polysaccharides; CEP: chelator extractable polysaccharides; NEP: Na2CO3 extractable polysaccharides; HC: hemicellulose fractions; AUA: andydrouronic acid; Fuc: fucose; Rha: rhamnose; Ara: arabinose; Gal: galactose; Glc: glucose; Xyl: xylose; Man: mannose. Results obtained from duplicates.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

Highlights

Microwave-explosion puffing resulted in superior microstructure and texture.

•

Cell wall polysaccharides played a significant role in texture of pitaya chips.

•

Decreasing intercellular adhesion strength enhanced volume expansion.

•

Increasing cell wall polysaccharides hydrophilicity improved rehydration property.

•

Cell wall polysaccharides depolymerization contributed to a decreasing Tg.

AC C

EP

TE D

M AN U

SC

RI PT

•