Accepted Manuscript

Title: Osmotic-ultrasound dehydration pretreatment improvesmoisture adsorption isotherms water state ofmicrowave-assisted vacuum fried purple-fleshed sweet potatoslices

Authors: Kai Fan, Min Zhang, Bhesh Bhandari

PII: S0960-3085(18)30293-1DOI: https://doi.org/10.1016/j.fbp.2019.03.011Reference: FBP 1058

To appear in: Food and Bioproducts Processing

Received date: 22 May 2018Revised date: 14 December 2018Accepted date: 27 March 2019

Please cite this article as: Fan K, Zhang M, Bhandari B, Osmotic-ultrasound dehydrationpretreatment improves moisture adsorption isotherms water state of microwave-assistedvacuum fried purple-fleshed sweet potato slices, Food and Bioproducts Processing(2019), https://doi.org/10.1016/j.fbp.2019.03.011

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Osmotic-ultrasound dehydration pretreatment improves moisture

adsorption isotherms water state of microwave-assisted vacuum fried

purple-fleshed sweet potato slices

Kai Fana,b, Min Zhanga,c,* min@jiangnan.edu.cn, Bhesh Bhandarid

aState Key Laboratory of Food Science and Technology, Jiangnan University, 214122

Wuxi, Jiangsu, China

bInternational Joint Laboratory on Food Safety, Jiangnan University, China

cJiangsu Province Key Laboratory of Advanced Food Manufacturing Equipment and

Technology, Jiangnan University, China

dSchool of Agriculture and Food Sciences, University of Queensland, Brisbane, QLD,

Australia

*Corresponding author: Professor Min Zhang, School of Food Science and

Technology, Jiangnan University, 214122 Wuxi, Jiangsu Province, China.

Fax: 0086-(0)510-8580797

Highlights

Adsorption isotherms of microwave-assisted vacuum fried purple-fleshed

sweet potato were determined.

GAB model showed a good fit to describe adsorption isotherms of fried

purple-fleshed sweet potato.

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Osmotic-ultrasound dehydration pretreatment reduced equilibrium moisture

content

LF-NMR showed the distribution state of absorbed water during storage.

Abstract

Effect of osmotic-ultrasound dehydration pretreatment on the moisture adsorption

isotherms of microwave-assisted vacuum fried purple-fleshed sweet potato (PSP) was

determined at 30, 45 and 60 ºC and fitted with six models. Absorbed water state of

microwave-assisted vacuum fried purple-fleshed sweet potato was measured by

low-field nuclear magnetic resonance (LF-NMR). Results indicated that the optimized

conditions of osmotic-ultrasound dehydration pretreatment were 11.21 min for

ultrasound time, 56.99% sucrose concentration and 74.84 min for osmotic

dehydration time. Moisture adsorption isotherms showed type II sigmoid shape. GAB

model had the best fit evaluated by the higher values of R2 (> 0.9868) and the lower

values of RMSE (< 0.0074) and χ2 (< 9.4893×10-5) for untreated and pretreated fried

purple-fleshed sweet potato slices. Monolayer moisture content from GAB model

decreased from 0.0598 at 30 ºC to 0.0452 at 60 ºC. The net isosteric heat of

adsorption for untreated and pretreated fried PSP was calculated by the

Clausius-Clapeyron equation and decreased with increasing moisture contents. The

correlation between the bound water population (T21) peak areas of absorbed water

and equilibrium moisture content was respectively high for untreated PSP (R2 >

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0.9595) and pretreated fried PSP (R2 > 0.9845) by using LF-NMR.

Keywords: Osmotic-ultrasound dehydration; Moisture adsorption isotherms; Sorption

models; Net isosteric heat; LF-NMR

1. Introduction

Purple-fleshed sweet potato (PSP) has a large number of anthocyanins as natural

pigment. Anthocyanins from PSP are useful for improving antioxidant capacity, sight

acuteness, and memory (Liu et al., 2013). The global production of sweet potato was

about 120 million tons in 2003 and China accounts for about 90% (108 million tons)

of worldwide sweet potato production (Abegunde et al., 2013). PSP is processed

around the world to provide the market demand (de Aguiar Cipriano et al., 2015). PSP

is transformed into many forms such as baked, juice, dried, dairy, confection, cooked

puree, powdered and fried products (Xu et al., 2015). Fried potato products are widely

consumed in the market. The increasing demand for low fat fried snacks by the

consumers has encouraged a combination of traditional vacuum frying and new

technology (Su et al., 2015). Microwave as heating source assisted vacuum frying

(MVF) can obtain better quality and less oil absorption for fruits and vegetables than

conventional vacuum frying (Su et al., 2016). Some pretreatment methods such as

blanching, osmotic dehydration and pre-drying applied before frying process can

improve the product quality (Fan et al., 2006; Su et al., 2017; Troncoso and Pedreschi,

2009). The osmotic dehydration treatment is a process to remove part of water from

raw material. Ultrasound as a new technology is used extensively and can produce

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“sponge effect” creating internal microscopic channels (Fernandes et al., 2008;

Fonteles et al., 2016). Moreover, this process causes different degrees of damage to

the cells (Nowacka et al., 2017). The use of ultrasound prior to osmotic dehydration

can increase moisture fast removal and improve stability of the products during

storage and marketing (Noshad et al., 2012; Nowacka et al., 2014).

Water activity (aw) is very important to control microbial and physiochemical

stability of food during storage. The relationship between equilibrium moisture

content and water activity of the products at a constant temperature and pressure is

called as moisture sorption isotherm (Polatoğlu et al., 2011). The knowledge of

moisture sorption isotherm is good for the reasonable design of frying equipment,

understanding heat and mass transfer of frying process, selecting the appropriate

mode of packaging and storage, determining the safety moisture content of food

during storage, predicting the kinetic parameters of sorption process and shelf life of

product, and reflecting microstructure of water on the surface of the products

(Al-Mahasneh et al., 2007; Choudhury et al., 2011; Fan et al., 2014; Kumar et al.,

2012; Moreira et al., 2010; Sobukola et al., 2007).

Numerous mathematical models such as GAB, BET, Peleg, Halsey, Caurie, Oswin,

Hendenson, Smith described sorption isotherm of different products (Choudhury et al.,

2011). BET and GAB models are the popular isotherm equations of products. The

BET equation is used to fit experimental data for different products when the water

activity is less than 0.45 (Sawhney et al., 2011). GAB equation is used for predicting

moisture sorption isotherm for different products in comparison to the BET equation

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in the wide water activity range from 0.1 to 0.9 (Moreira et al., 2010). As reported by

Tungsangprateep and Jindal (2004), GAB model had the best fit to experimental data

of fried cassava-shrimp chips in the wide range of water activity. Sobukola et al.

(2007) found that GAB model can also well predict the equilibrium moisture content

of fried yam chips.

Isosteric heat of sorption can be used to determine binding energy of absorbed

water in the solid product. The net isosteric heat of sorption by using the

Clausius-Clapeyron equation can estimate the state of absorbed water of different

products during storage (Fang et al., 2013; Sawhney et al., 2011). In order to further

understand the state of absorbed water, low-field nuclear magnetic resonance

(LF-NMR) has been adopted as an analytical technique to determine the state of

absorbed water within food materials and can offer a distinction between free,

physically bound, and chemically bound water (Cheng et al., 2014; Rodríguez et al.,

2014; Xin et al., 2013).

Some researchers have investigated on the sorption isotherm of vacuum fried

products such as cassava-shrimp chips (Tungsangprateep and Jindal, 2004), carrot

chips (Fan et al., 2005), yam chips (Sobukola et al., 2007). However, there are few

studies on the sorption isotherm of PSP slices after microwave-assisted vacuum frying.

In addition, ultrasound and osmotic dehydration may change microstructural of the

product resulting in the change of sorption property. It was expected that ultrasound

and osmotic dehydration pretreatment can reduce equilibrium moisture content of the

fried product during storage. Thus, the objectives of this study are to investigate the

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effect of osmotic-ultrasound dehydration pretreatment on adsorption isotherms of PSP

after microwave-assisted vacuum frying at different temperatures, to select the most

suitable model describing the isotherms, to calculate the net isosteric heat of sorption

and to determine distribution state of absorbed water using LF-NMR.

2. Materials and methods

2.1. Raw materials

Fresh purple-fleshed sweet potato (PSP; Ipomoea batatas L.) and soybean oil

(Yihai Kerry Company, Shanghai, China) were purchased from a local supermarket in

Wuxi, China. Fresh PSP samples were stored in a dark place at 4 ºC for 24 h until

experiment. Fresh PSP was prepared by peeling and cutting into thicknesses of about

4 ± 0.3 mm manually with a stainless steel knife. Diameter of the sample was 36 ± 1

mm. The initial moisture content of PSP was 65.75 ± 1.63% (wet basis, w.b.), which

was measured by oven method at 105 ºC for 24 h (Rahman et al., 2009).

2.2. Ultrasound treatment

PSP slices samples of 50 g were placed in a 250 mL glass beaker to avoid

interference between the samples and 200 mL of distilled water was added to achieve

small soluble solids content in ultrasound treatment at room temperature (25 ºC)

(Fernandes and Rodrigues, 2007). The glass beaker was then immersed into an

ultrasound bath (SB-5200-DTN, Ningbo Scientz Biotechnology Co., Ltd., Ningbo,

China; internal dimensions: 300 × 240 × 150 mm). The ultrasound frequency was 40

kHz, and the power was 250 W (the power intensity of 14.64 W/L) (Xin et al., 2013).

The sample without treatment of ultrasound was dipped in distilled water as control

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sample. The other samples were processed for 10, 20 and 30 min of ultrasound

treatment time at a constant temperature (25 ºC) using a heat exchanger connected to

a thermostatic bath (Fernandes and Rodrigues, 2007). The increase of temperature

during the experiments was below 2 ºC after 30 min of ultrasound treatment. After

each ultrasound treatment time, the samples were taken out the glass beakers and

blotted with filter paper to remove the excess water from the surface, then weighed

and determined for moisture contents. These samples were subsequently transferred to

osmotic solution.

2.3. Osmotic dehydration

The samples after ultrasound treatment were immersed in the osmotic solution for

60, 90 and 120 min according to Nowacka et al. (2017). The osmotic solution at room

temperature (25 ºC) was prepared by mixing food grade sucrose with distilled water to

give the concentrations of 30, 45 and 60 % (w/w). The ratio of solution to sample

mass was 4:1 (weight basis) to avoid changes of solution concentration (Kek et al.,

2013). Osmotic dehydration was performed in a water bath (HH-4, Jintan Ronghua

Instrument Manufacture Co., Changzhou, China) at 50 ºC according to Noshad et al.

(2011). The samples were removed from solution and flushed with 200 mL distilled

water and blotted with filter paper to remove the excess solution, then weighed and

dried in an oven at 105 ºC to determine moisture and solid contents. The water loss

(WL, g water/g) and solid gain (SG, g solid/g) were calculated by the Eqs. (1) and (2),

respectively (Fernandes and Rodrigues, 2007; Kek et al., 2013).

(1)

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(2)

where wo, wt, Xo, Xt, Xso and Xst are initial mass (g), mass after pretreatment (g),

initial moisture content (g water/g, w.b.), moisture content after pretreatment (g

water/g, w.b.), initial solid content (g solid/g, w.b.) and solid content after

pretreatment (g solid/g, w.b.), respectively. The experimental data are average values

of the triplicate experiments.

2.4. Experimental design and statistical analysis

Response surface methodology (RSM) was used to evaluate the effect of the

process variables on WL and SG of PSP slices. Box-Behnken design (BBD) with

ultrasound time (X1), sucrose concentration (X2) and osmotic time (X3) at three levels

(Table 1) was chosen to optimize parameters for ultrasound and osmotic dehydration

process. The design generated 15 experimental groups (Table 2). The RSM was

applied by using the software Design-Expert 8.0 (Stat-Ease, Inc., Minneapolis, USA).

A second-order polynomial regression model was used to describe relationship

between response (Y) and three variables (X1, X2 and X3) as follows:

(3)

where Y is the response (WL and SG), bi is regression coefficients. The fitting

goodness of the model was evaluated by the coefficient of determination (R2) and the

analysis of variance (ANOVA) and F-test. The significance was identified at p < 0.05.

BBD combined with Derringer’s desirability function was applied to obtain

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optimization condition for multiple responses. Desirability function can transform

each response (di) into the range from 0 to 1. The desirability function (D) was

determined by using the geometric mean (di), which is the maximum value under the

optimized parameter. The di = 0 and di = 1 represent undesirable response and ideal

response, respectively (Amami et al., 2017; Dranca and Oroian, 2016). The samples

treated with optimized condition were used for microwave-assisted vacuum frying as

described in section 2.5 below.

2.5. Microwave-assisted vacuum frying

Microwave-assisted vacuum frying equipment (ORW3S-600U; Nanjing Orient

Microwave Technology Co., Ltd., Nanjing, China) was employed in this experiment

and described by Su et al. (2015). Fig. 1 shows the schematic diagram of this

equipment. Experiments were done at microwave power level of 1000 W, temperature

of 90 ºC, vacuum degree of 0.090 MPa for 15 min according to Su et al. (2018).

Soybean oil (5 L) was firstly heated to reach set temperature. 50 g of fresh PSP and

pretreated PSP with optimized osmotic-ultrasound dehydration condition was fried in

the experiment, respectively. Fried PSP slices were centrifuged at 10 ×g for 5 min to

remove surface excess oil.

2.6. Moisture absorption isotherms measurements

The equilibrium moisture contents for adsorption isotherms of untreated and

pretreated fried PSP slices were determined by using static gravimetric method. As

reported by Greenspan (1977) and Mclaughlin and Magee (1998), saturated salt

solutions can provide different water activities at different temperatures, as shown in

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Table 3. Samples (about 2 g) were placed in Petri dish inside the different water

activity jars, and then placed in the thermostat at 30, 45 and 60 ºC (Noshad et al.,

2012). Samples were weighed until constant weight (±0.001 g) over three weeks. The

moisture contents of samples were determined by the oven method at 105 ºC for 24 h

(Rahman et al., 2009). Each experiment was performed in triplicate.

2.7. Modeling of adsorption isotherms

The six sorption models fit the experimental data of untreated and pretreated fried

PSP slices at 30, 45 and 60 ºC, as shown in Table 4. The fittings were performed using

the software Matlab R2009a (MathWorks Inc., Natick, MA, USA). Fitting goodness

of models was evaluated by the coefficient of determination (R2, Eq. (4)), root mean

square error (RMSE, Eq. (5)) chi-square(χ2, Eq. (6)) (Kadam et al., 2015).

N

i

eei

N

i

piei

XX

XX

R

1

2

1

2

2 1 (4)

N

i

piei XXN

RMSE1

21 (5)

nN

XXN

i

piei

1

2

2 (6)

where Xei, Xpi, eX N and n are the experimental value, predicted value, average

experimental value, observation number, and constant number in the model,

respectively. The model was the best with the low RMSE and χ2 values and high R2

value.

2.8. Net isosteric heat of sorption

The net isosteric heat of sorption (ΔH) is used to reflect different water binding

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energies at different temperatures (Fang et al., 2013). The value of ΔH was calculated

by using Clausius-Clapeyron equation as follows:

(7)

where aw1, aw2, T1, T2, R and ΔH are water activity, absolute temperature (K), gas

constant (8.314 J/mol K) and net isosteric heat (kJ/mol), respectively.

2.9. LF-NMR relaxation measurements

A low-field nuclear magnetic resonance (LF-NMR) analyzer (PQ001, Niumag

Electric Corporation, Shanghai, China) with 100 kHz was used to determine the state

of absorbed water in untreated and pretreated fried PSP slices. The temperature of the

magnet chamber was 32 ºC. The fried sample (about 2 g) was placed in a glass tube

with the diameter of 10 mm and then put into the magnet chamber.

Carre-Purcelle-Meiboome-Gill (CPMG) sequence was used to measure transverse

relaxation time (T2). The parameters of CPMG was 16 scans, 6000 echoes, 4 s

between scans, and 300 μs between pulses of 90° and 180°.

3. Results and discussion

3.1. Osmotic-ultrasound dehydration pretreatment

The effects of three variables on responses are presented in Table 2. The

mathematical models of WL and SG for osmotic-ultrasound dehydration of PSP are

shown in the following Eqs. (8) and (9).

(8)

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(9)

As shown in Table 5, ANOVA results show that the experimental data can be well

indicated by the second-order polynomial models with high coefficient of

determination (R2=0.9964 for WL and R2=0.9763 for SG). The high adjusted

determination coefficient (adjusted R2=0.99 for WL and adjusted R2=0.9337 for SG)

presented a good fitting of the models. Regarding WL model, the linear terms of three

variables were significant (p < 0.05). The relative magnitude of b values (Eq. (8))

showed the great positive effect of sucrose concentration (b=5.8), followed by

osmotic time (b=0.9) and ultrasound time (b=0.69) on WL. The interaction effect of

sucrose concentration and osmotic time was significant for WL (p < 0.05). Fig. 2

shows that the WL increased with increasing sucrose concentration and osmotic time.

The quadratic terms effects of ultrasound time and sucrose concentration were

significant for WL. Regarding SG model, the linear terms effects of three variables

were significant for SG (p < 0.05). The relative magnitude of b values (Eq. (9))

showed the great positive effect of sucrose concentration (b=2.68), followed by

ultrasound time (b=0.9) and osmotic time (b=0.87) on SG. Only quadratic term effect

of sucrose concentration was significant for SG (p < 0.05).

The maximum WL and minimum SG responds of pretreated conditions would be

optimized by using Derringer’s desirability function method. The optimum conditions

for osmotic-ultrasound dehydration process of PSP were obtained at ultrasound time

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of 11.21 min, sucrose concentration of 56.99% and osmotic time of 74.84 min with

overall desirability value of 0.64. Under these conditions, the predicted values of WL

and SG were 14.98% and 2.52%, respectively. The experimental values of WL and

SG were 15.83% and 2.72%, respectively. The difference between experimental and

predicted was less than 8% of deviation. Thus, models fitted by RSM can predict WL

and SG under the experimental condition used.

3.2. Effect of pretreatment on adsorption isotherms of fried PSP slices

Effect of osmotic-ultrasound dehydration pretreatment on adsorption isotherms of

fried PSP slices is shown in Fig. 3. Adsorption isotherms of untreated and pretreated

fried PSP slices presented the type II sigmoid shaped curve, which is typical to the

most food. Equilibrium moisture content of pretreated fried PSP slices was lower than

that of untreated fried PSP slices. This may be explained by the fact that the

ultrasound can produce micro-channel leading to sucrose entry (Amami et al., 2017).

The similar results were reported by Singh and Mehta (2010), equilibrium moisture

content of carrot osmotically pretreated with sucrose was lower than that of

un-osmosed ones. They obtained that the presence of sucrose makes the product less

hygroscopic. Noshad et al. (2012) presented that the osmosis and ultrasound

pretreatment can also decrease equilibrium moisture content of dried quince slices.

They obtained that equilibrium moisture contents decreased with increasing solids in

quince slices. Equilibrium moisture content of untreated and pretreated fried PSP

slices slowly changed at low water activity and rapidly increased moisture at high

water activity. This may be due to the dissolution of crystalline sugar at low water

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activity and the conversion of crystalline sugar into amorphous sugar at high water

activity (Rao et al., 2006; Saltmarch and Labuza, 1980). Similarly, Agnieszka and

Andrzej (2010) observed that water content of freeze-dried strawberries had a flatter

increased course at low water activity, and had rapid increase at high water activity.

The amount of water to be absorbed increases because the number of adsorption sites

increased leading to its crystalline sugar dissolves.

3.3. Effect of temperature on adsorption isotherms of fried PSP slices

Fig. 3 shows adsorption isotherms of untreated and pretreated fried PSP slices at

30, 45 and 60 ºC. Fig. 3A shows the effect of temperature on adsorption isotherms of

untreated fried PSP slices. Equilibrium moisture content of untreated fried PSP slices

decreased with the increase in temperature at each water activity. This may be due to

an increase of temperature causing the reduction of active sites for water binding in

the product, thus decreasing the moisture adsorption (Mcminn and Magee, 2003;

Polatoğlu et al., 2011). Similar trend has been reported by Fan et al. (2006), who

obtained that water molecules got higher energy at increased temperatures causing

break away from the water binding sites of fried carrot chips. The increased

temperature causes a decrease in the amount of sorbed water in fried yam chips

(Sobukola et al., 2007). Fig. 3B shows the effect of temperature on adsorption

isotherms of pretreated fried PSP slices. Equilibrium moisture content of pretreated

fried PSP slices also obtained a similar trend at aw below 0.6. However, equilibrium

moisture content increased with the increase in temperature at aw above 0.6. This may

be due to the increase in temperature causing an increase in solubility of sugars in the

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aw above 0.6 (Ayranci et al., 1990; Kumar et al., 2012). The inversion point depends

on the composition of the food and the solubility of sugars (Noshad et al., 2012).

Similar results have been obtained by Agnieszka and Andrzej (2010), they found that

water content increased with the increase in temperature at above water activity 0.648

for osmotic dehydrated strawberries in sucrose solution. Noshad et al. (2012) found

that equilibrium moisture content increased with increasing temperature at above

water activity 0.75 for osmosis-ultrasound pretreated quince slices.

3.4. Fitting of sorption models

Table 6 shows the parameters and R2, RMSE and χ2 of the six models. The

goodness of the fit was presented for the higher values of R2 (> 0.9868) and the lower

values of RMSE (< 0.0074) and χ2 (< 9.4893×10-5) for GAB model at 30, 45 and 60

ºC. From Table 6, GAB model is the best fit to the experimental data at the

experimental conditions for untreated and pretreated fried PSP slices. Fig. 4 shows

that the experimental and predicted equilibrium moisture contents from GAB model

are compared at the experimental conditions. The R2 value (0.9942) of linear

regression model for the experimental and predicted equilibrium moisture contents

was very high. Accordingly, GAB model is suitable to describe the adsorption

behavior of fried purple-fleshed sweet potato slices. GAB model has been reported by

other authors to give a good fit for adsorption behavior of other materials. For

example, Sawhney et al. (2011) reported that the GAB model was the best fit equation

for dried acid casein at 25-45 ºC for the aw range of 0.11-0.97. Similarly, Lago et al.

(2013) found that the GAB model can also well describe adsorption behaviors of

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potato and sweet potato flakes at 15-30 ºC for the aw range of 0.1-0.9. Therefore,

GAB could be applied to the sorption data for the fried PSP over a wide water activity

range.

The monolayer moisture content (Xm) values from the GAB model for untreated

and pretreated fried PSP slices at 30, 45 and 60 ºC are presented in Table 6. Results

showed that Xm of pretreated fried PSP slices was lower than that of untreated fried

PSP slices because of the addition of sugar causing decreasing monolayer moisture

content. Similar results have been reported by Noshad et al. (2012), who found that

the monolayer moisture content from the GAB model decreased with increasing

temperature for pretreated quince slices in sucrose solution. Xm values of untreated

and pretreated fried PSP slices decreased with the increase in temperature. Xm values

of untreated fried PSP slices decreased from 0.0598 at 30 ºC to 0.0452 at 60 ºC. The

Xm values of pretreated fried PSP slices decreased from 0.0449 at 30 ºC to 0.0367 at

60 ºC. These results were related to the decrease in active sites at high temperature

(Polatoğlu et al., 2011). The results have a good agreement with those reported by

Choudhury et al. (2011), they found that monolayer moisture content decreased with

increasing temperature.

3.5. Net isosteric heat of sorption

The net isosteric heat (ΔH) of adsorption for fried PSP slices was calculated by

using the Eq. (7) to the experimental data from GAB model. The ΔH values of

adsorption for untreated and pretreated fried PSP slices as a function of moisture

contents are presented in Fig. 5. ΔH values of adsorption decreased with increasing

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moisture content. The ΔH values of adsorption at low moisture content were high

indicating high binding energy. This may be due to adsorbed water molecules

constituting the monomolecular layer (Fang et al., 2013). Water first adsorbs

preferentially on the most active sites with great interaction energy, and sites were

predominantly occupied causing further adsorption generation on less active site with

lower heats of adsorption. Similar results have been reported by Sobukola et al.

(2007), they indicated that the high interaction energy between the water molecules

and fried yam chips was observed at low moisture content. The increase of the net

isosteric heat of adsorption at low moisture contents was an indication of strong

water-surface interactions in the dried sucuk (Polatoğlu et al., 2011). Sawhney et al.

(2011) indicated that the strong binding sites and the great water solid interaction

were obtained for dried acid casein. Low ΔH values of adsorption were due to the

increase amounts of adsorbed water. The ΔH values of adsorption for untreated fried

PSP slices were higher than that of adsorption for pretreated fried PSP slices. This

indicated that the energy required for untreated fried PSP slices was higher than

pretreated fried PSP slices because pretreatment reduced the monolayer moisture

content causing the low energy adsorption. Similar results have been reported by

Noshad et al. (2012), who observed that net isosteric heat of adsorption for pretreated

quince slices was lower than that of untreated quince slices. Therefore, the

information on the isosteric heat of sorption for fried PSP could be used in calculating

the cumulative energy requirement for dehydration.

3.6. Water state in fried PSP slices by LF-NMR

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According to proton signals intensity, transverse relaxation time (T2) spectra of

fried PSP slices were derived from water and oil proton signal. The first and second

peaks in fried PSP slices were signals peaks from water by LF-NMR analysis. The

third and fourth peaks in fried PSP slices were signals peaks from oil by LF-NMR

analysis. Four relaxation populations of fried PSP slices were centered at

approximately 0.01-1 ms (T21), 4-20 ms (T22), 40-200 ms (T23) and 200-700 ms (T24),

respectively. T21 represents bound water in the fried PSP slices. The bound water

mainly included chemical bound water and adsorbed bound water. The chemical

bound water is bound to interactions between water and macromolecules. The

adsorbed bound water is bound to the surface of the material and inside the colloidal

particles. T22 represents free water in the fried PSP slices, which mainly depend on

surface adhesion, water adhesion and capillary force in the material. T23 and T24

represent two fatty acids of different chain lengths in fried food, respectively (Chen et

al., 2017).

T21 signals peaks area have obvious changes for fried PSP slices with different

water activities at 30, 45 and 60 ºC. T22, T23 and T24 signal peaks markedly had no

changes. Table 7 present that T21 of untreated fried PSP slices increased with the

increase in water activity of saturated salt solutions at each temperature. T21 peaks

area of untreated fried PSP slices decreased with the increase in temperature at each

water activity. Table 7 present that T21 of pretreated fried PSP slices increased with the

increase in water activity of saturated salt solutions at each temperature. At water

activity below 0.6, T21 peaks area of pretreated fried PSP slices decreased with the

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increase in temperature at each water activity. However, at water activity above 0.6,

T21 peaks area of pretreated fried PSP slices increased with the increase in

temperature at each water activity. This may be due to dissolution of sugar (Kumar et

al., 2012). T21 peaks area of pretreated fried PSP slices were lower than that of

untreated fried PSP slices indicating that pretreated fried PSP slices were easier than

untreated fried PSP slices to achieve low moisture content, thus improving stability

during storage.

A plot of T21 area and equilibrium moisture content of fried PSP slices at 30, 45

and 60 ºC is shown in Fig. 6. The figure indicated that a change in T21 peaks area is

related to equilibrium moisture content of untreated and pretreated fried PSP slices.

Fig. 6A, B and C show that the R2 values of linear regression models for untreated

fried PSP slices were above 0.9595 at 30, 45 and 60 ºC. Fig. 6D, E and F show that

the R2 values of linear regression models for pretreated fried PSP slices were above

0.9845 at 30, 45 and 60 ºC. These results suggested that there was a very high

correlation between the T21 peak areas and equilibrium moisture content. Similar

results have been reported by Chen et al. (2017). They found that there was a very

high correlation (R2 > 0.9999) between the peak areas of relaxation spectra and the

water content in the fried starch. Therefore, LF-NMR can be used for prediction of

moisture related measurements in the fried food.

4. Conclusions

Moisture adsorption isotherm of fried purple-fleshed sweet potato was type II

sigmoid shaped curve. Osmotic-ultrasound dehydration pretreatment decreased

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equilibrium moisture content of fried purple-fleshed sweet potato. Equilibrium

moisture content of osmotic-ultrasound dehydration pretreated fried purple-fleshed

sweet potato increased with the increased in temperature at above 0.6. Monolayer

moisture content from the best fitted GAB model for fried purple-fleshed sweet potato

by osmotic-ultrasound pretreatment was reduced. Osmotic-ultrasound pretreatment

reduced net isosteric heat values. LF-NMR results showed that osmotic-ultrasound

pretreated fried purple-fleshed sweet potato was easier to achieve low moisture

content compared to untreated fried purple-fleshed sweet potato. High correlation

between the T21 peak areas and equilibrium moisture content was obtained. Therefore,

osmotic-ultrasound dehydration pretreatment improves moisture adsorption isotherms

and water state of fried products during storage.

Acknowledgments

We acknowledge the financial support from the following sources:

National Natural Science Foundation Program of China (Contract No. 31671864),

China Key Research Program (Contract No. 2016YFD0400704-5), National

First-class Discipline Program of Food Science and Technology (No.

JUFSTR20180205), Jiangsu Province Key Laboratory Project of Advanced Food

Manufacturing Equipment and Technology (No. FMZ201803).

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Fig. 1. Schematic diagram of microwave-assisted vacuum frying equipment.

1. Frying chamber; 2. Vacuum chamber; 3. Oil tank; 4. Temperature sensor; 5.

Controller; 6. Microwave system; 7. Vacuum pump; 8. Valve for breaking

vacuum; 9. Circulation pump; 10. Conveyor; 11. Electric motor.

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Fig. 2. Response surface plots for water loss of purple-fleshed sweet potato

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Fig. 3. Effect of temperature on adsorption isotherms of untreated (A) and pretreated

(B) fried purple-fleshed sweet potato slices.

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Fig. 4. Comparison of experimental and predicted equilibrium moisture contents from

GAB model.

Fig. 5. Net isosteric heat values of adsorption of fried purple-fleshed sweet potato as a

function of moisture contents.

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Fig. 6. Relationship between T21 area and equilibrium moisture content of untreated

(A, B and C) and pretreated (D, E and F) fried purple-fleshed sweet potato slices at 30,

45 and 60 ºC.

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Table 1 Level of actual and coded values used for osmotic-ultrasound dehydration

process.

Coded factor Variables Coded levels

-1 0 +1

X1 Ultrasound time (min) 10 20 30

X2 Sucrose concentration (%) 30 45 60

X3 Osmotic time (min) 60 90 120

Table 2 Box-Behnken design and observed values of responses.

X1 X2 X3 WL (%) SG (%)

10 30 90 3.67 -2.55

30 30 90 6.34 -1.21

10 60 90 16.03 2.09

30 60 90 16.55 5.28

10 45 60 12.01 3.02

30 45 60 13.2 4.77

10 45 120 13.27 4.23

30 45 120 14.44 5.17

20 30 60 3.14 -2.44

20 60 60 13.74 2.57

20 30 120 4.21 0.42

20 60 120 17.41 5.39

20 45 90 12.22 3.02

20 45 90 11.69 3.15

20 45 90 11.63 3.48

Table 3 Water activities of saturated salt solutions at 30, 45 and 60 ºC.

Salt Temperature (ºC)

30 45 60

CH3COOK 0.216 0.195 0.160

MgCl2 0.324 0.311 0.293

K2CO3 0.432 0.432 0.432

NaBr 0.560 0.520 0.497

KI 0.679 0.653 0.631

NaCl 0.751 0.745 0.745

KCl 0.836 0.817 0.803

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Table 4 Moisture sorption isotherm models fitted to experimental data.

Model Mathematical expression Reference

GAB

Fonteles et al. 2016

Caurie

Kumar et al. 2012

Halsey

Polatoğlu et al. 2011

Hendenson

Noshad et al. 2012

Oswin

Noshad et al. 2012

Smith Fang et al. 2013

Table 5 ANOVA evaluations of model terms and regression models.

Model terms WL SG

F-value p-value F-value p-value

Model 155.07 < 0.0001 22.92 0.0015

X1 17.29 0.0088 13.56 0.0143

X2 1207.11 < 0.0001 119.24 0.0001

X3 29.43 0.0029 12.71 0.0161

X1X2 5.19 0.0717 1.78 0.2396

X1X3 4.491×10-4 0.9839 0.34 0.5844

X2X3 7.59 0.0401 0.06 0.8160

X12 24 0.0045 0.34 0.5829

X22 95.68 0.0002 49.02 0.0009

X32 0.54 0.4955 5.8 0.0609

R2 0.9964 0.9763

Adjusted R2 0.99 0.9337

Note: p < 0.05 is significant; p > 0.05 is not significant.

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Table 6 Evaluated parameters and statistical coefficients of several models for adsorption isotherms of untreated and pretreated fried

purple-fleshed sweet potato at 30, 45 and 60 ºC.

Model Parameter Untreated Pretreated

30 ºC 45 ºC 60 ºC 30 ºC 45 ºC 60 ºC

GAB Xm 0.0598 0.0549 0.0452 0.0449 0.0403 0.0367

C 0.9024 0.8956 0.9346 0.9024 0.9969 1.0571

K 7.0309 7.4648 12.5427 27.1017 20.2562 14.6082

R2 0.9941 0.9953 0.9893 0.9868 0.9978 0.9889

RMSE 0.0047 0.0035 0.0049 0.0051 0.0027 0.0074

χ2 3.9048×10-

5

2.1960×10-

5

4.2040×10-

5

4.4675×10-

5

1.2862×10-

5

9.4893×10-

5

Caurie A -3.7001 -3.7302 -3.8019 -3.6448 -4.0416 -4.3933

B 2.6412 2.5352 2.5366 2.2520 2.9948 3.6134

R2 0.9871 0.9961 0.9867 0.9660 0.9763 0.9652

RMSE 0.0070 0.0032 0.0055 0.0081 0.0089 0.0130

χ2 6.8553×10-

5

1.4389×10-

5

4.1730×10-

5

9.2331×10-

5

1.1157×10-

4

2.3757×10-

4

Halsey A 0.0228 0.0199 0.0200 0.0111 0.0306 0.0514

B 1.4290 1.4299 1.3852 1.6290 1.2192 1.0006

R2 0.9915 0.9924 0.9897 0.9880 0.9967 0.9851

RMSE 0.0057 0.0045 0.0048 0.0048 0.0033 0.0085

χ2 4.4934×10-

5

2.8316×10-

5

3.2419×10-

5

3.2587×10-

5

1.5349×10-

5

1.0196×10-

4

Hendenson A 9.6960 12.0269 13.2528 18.8807 8.1642 5.3571

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B 1.1253 1.1759 1.1830 1.3294 0.9827 0.7995

R2 0.9849 0.9869 0.9731 0.9621 0.9659 0.9565

RMSE 0.0076 0.0059 0.0078 0.0086 0.0107 0.0146

χ2 8.0406×10-

5

4.8661×10-

5

8.4500×10-

5

1.0295×10-

4

1.6034×10-

4

2.9701×10-

4

Oswin A 0.0937 0.0859 0.0799 0.0809 0.0794 0.0762

B 0.5574 0.5470 0.5548 0.4830 0.6465 0.7882

R2 0.9946 0.9949 0.9870 0.9838 0.9881 0.9759

RMSE 0.0045 0.0037 0.0054 0.0056 0.0063 0.0108

χ2 2.8534×10-

5

1.9086×10-

5

4.1015×10-

5

4.4067×10-

5

5.5775×10-

5

1.6429×10-

4

Smith A 0.0145 0.0160 0.0155 0.0239 0.0053 -0.0086

B 0.1168 0.1027 0.0954 0.0830 0.1139 0.1374

R2 0.9899 0.9941 0.9841 0.9773 0.9678 0.9400

RMSE 0.0062 0.0040 0.0060 0.0066 0.0104 0.0171

χ2 5.3710×10-

5

2.1853×10-

5

5.0134×10-

5

6.1664×10-

5

1.5109×10-

4

4.0956×10-

4

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Table 7. T21 peak areas of untreated and pretreated fried purple-fleshed sweet potato

slices with different water activities of saturated salt solutions at 30, 45 and 60 ºC.

Salt Untreated T21 peak areas Pretreated T21 peak areas

30 ºC 45 ºC 60 ºC 30 ºC 45 ºC 60 ºC

CH3COOK 48.48 22.23 11.99 25.56 18.09 4.06

MgCl2 106.13 66.49 44.99 90.57 55.10 37.76

K2CO3 265.81 183.11 109.18 195.23 177.34 97.19

NaBr 753.09 624.28 479.11 657.40 511.46 370.13

KI 1030.99 971.39 851.28 707.65 970.96 995.18

NaCl 1699.52 1330.15 1149.87 966.69 1316.34 1346.77

KCl 1917.96 1504.12 1469.28 1474.01 1844.97 2138.46

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