Osmotic-ultrasound dehydration pretreatment improves moisture adsorption isotherms and water state of microwave-assisted vacuum fried purple-fleshed sweet potato slices

Food and Bioproducts Processing 1 1 5 ( 2 0 1 9 ) 154–164

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Osmotic-ultrasound dehydration pretreatment improves moisture adsorption isotherms and water state of microwave-assisted vacuum fried purple-fleshed sweet potato slices Kai Fan a,b , Min Zhang a,c,∗ , Bhesh Bhandari d a

State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China International Joint Laboratory on Food Safety, Jiangnan University, China c Jiangsu Province Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, China d School of Agriculture and Food Sciences, University of Queensland, Brisbane, QLD, Australia b

a r t i c l e

i n f o

a b s t r a c t

Article history:

Effect of osmotic-ultrasound dehydration pretreatment on the moisture adsorption

Received 22 May 2018

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

Received in revised form 14

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

December 2018

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

Accepted 27 March 2019

nuclear magnetic resonance (LF-NMR). Results indicated that the optimized conditions of

Available online 2 April 2019

osmotic-ultrasound dehydration pretreatment were 11.21 min for ultrasound time, 56.99%

Keywords:

isotherms showed type II sigmoid shape. GAB model had the best fit evaluated by the

Osmotic-ultrasound dehydration

higher values of R2 (>0.9868) and the lower values of RMSE (<0.0074) and 2 (<9.4893 × 10−5 )

sucrose concentration and 74.84 min for osmotic dehydration time. Moisture adsorption

Moisture adsorption isotherms

for untreated and pretreated fried purple-fleshed sweet potato slices. Monolayer mois-

Sorption models

ture content from GAB model decreased from 0.0598 at 30 ◦ C to 0.0452 at 60 ◦ C. The net

Net isosteric heat

isosteric heat of adsorption for untreated and pretreated fried PSP was calculated by the

LF-NMR

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 > 0.9595) and pretreated fried PSP (R2 > 0.9845) by using LF-NMR. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

et al., 2015). PSP is transformed into many forms such as baked, juice, dried, dairy, confection, cooked puree, powdered and fried products (Xu

Purple-fleshed sweet potato (PSP) has a large number of anthocyanins as natural pigment. Anthocyanins from PSP are useful for improving

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

antioxidant capacity, sight acuteness, and memory (Liu et al., 2013). The global production of sweet potato was about 120 million tons in

technology (Su et al., 2015). Microwave as heating source assisted vac-

2003 and China accounts for about 90% (108 million tons) of worldwide sweet potato production (Abegunde et al., 2013). PSP is processed

uum 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

around the world to provide the market demand (de Aguiar Cipriano

∗

Corresponding author at: School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China. E-mail address: [email protected] (M. Zhang). https://doi.org/10.1016/j.fbp.2019.03.011 0960-3085/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Food and Bioproducts Processing 1 1 5 ( 2 0 1 9 ) 154–164

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 “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 mois-

155

samples were stored in a dark place at 4 ◦ C 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 ◦ C for 24 h (Rahman et al., 2009).

ture 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 phys-

2.2.

iochemical 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 (Polatoglu et al., 2011). The knowledge of moisture sorp-

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 ◦ C) (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 sample. The other samples were processed for 10, 20 and 30 min of ultrasound treatment time at a constant temperature (25 ◦ C) using a heat exchanger connected to a thermostatic bath (Fernandes and Rodrigues, 2007). The increase of temperature during the experiments was below 2 ◦ C 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.

tion 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 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

2.3.

Ultrasound treatment

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 ◦ C) 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 (HH4, Jintan Ronghua Instrument Manufacture Co., Changzhou, China) at 50 ◦ C 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 ◦ C 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).

investigate the 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

WL =

wo Xo − wt Xt wo

(1)

SG =

wt Xst − wo Xso wo

(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.

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Food and Bioproducts Processing 1 1 5 ( 2 0 1 9 ) 154–164

Table 1 – Level of actual and coded values used for osmotic-ultrasound dehydration process. Coded factor

Variables

X1 X2 X3

Coded levels

Ultrasound time (min) Sucrose concentration (%) Osmotic time (min)

−1

0

+1

10 30 60

20 45 90

30 60 120

Table 2 – Box–Behnken design and observed values of responses. X1

X2

X3

WL (%)

SG (%)

10 30 10 30 10 30 10 30 20 20 20 20 20 20 20

30 30 60 60 45 45 45 45 30 60 30 60 45 45 45

90 90 90 90 60 60 120 120 60 60 120 120 90 90 90

3.67 6.34 16.03 16.55 12.01 13.2 13.27 14.44 3.14 13.74 4.21 17.41 12.22 11.69 11.63

−2.55 −1.21 2.09 5.28 3.02 4.77 4.23 5.17 −2.44 2.57 0.42 5.39 3.02 3.15 3.48

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: Y = b0 + b1 X1 + b2 X2 + b3 X3 + b12 X1 X2 + b13 X1 X3 + b23 X2 X3 + b11 X12 + b22 X22 + b33 X32

(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 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.

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. Table 3 – Water activities of saturated salt solutions at 30, 45 and 60 ◦ C. o

Temperature ( C)

Salt

CH3 COOK MgCl2 K2 CO3 NaBr KI NaCl KCl

2.5.

30

45

60

0.216 0.324 0.432 0.560 0.679 0.751 0.836

0.195 0.311 0.432 0.520 0.653 0.745 0.817

0.160 0.293 0.432 0.497 0.631 0.745 0.803

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 ◦ C, 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 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 ◦ C (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 ◦ C for 24 h (Rahman et al., 2009). Each experiment was performed in triplicate.

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Table 4 – Moisture sorption isotherm models fitted to experimental data.

Table 5 – ANOVA evaluations of model terms and regression models.

Model

Mathematical expression

Model terms

GAB

X=

Caurie

X=

Halsey Hendenson

Smith

2.7.

Fonteles et al. (2016)

    X=   X=  

Kumar et al. (2012)

X = A − Bln (1 − aw )

Fang et al. (2013)

X=

Oswin

Xm CKaw (1−Kaw )(1−Kaw +CKaw ) Xm C 2C 1−aw Xm aw 1/B −A lnaw −ln(1−aw ) 1/B A B aw A 1−a w

References

˘ et al. (2011) Polatoglu Noshad et al. (2012) Noshad et al. (2012)

Modeling of adsorption isotherms

The six sorption models fit the experimental data of untreated and pretreated fried PSP slices at 30, 45 and 60 ◦ C, 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(␹2 , Eq. (6)) (Kadam et al., 2015). N  

Xei − Xpi

2

i=1

R2 = 1 −

N  

Xei − Xe

2

Model X1 X2 X3 X1 X2 X1 X3 X2 X3 X1 2 X2 2 X3 2 R2 Adjusted R2

WL

SG

F-value

p-Value

F-value

p-Value

155.07 17.29 1207.11 29.43 5.19 4.491 × 10−4 7.59 24 95.68 0.54 0.9964 0.99

<0.0001 0.0088 <0.0001 0.0029 0.0717 0.9839 0.0401 0.0045 0.0002 0.4955

22.92 13.56 119.24 12.71 1.78 0.34 0.06 0.34 49.02 5.8 0.9763 0.9337

0.0015 0.0143 0.0001 0.0161 0.2396 0.5844 0.8160 0.5829 0.0009 0.0609

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

(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 ␮s between pulses of 90◦ and 180◦ .

(4)

3.

Results and discussion

3.1.

Osmotic-ultrasound dehydration pretreatment

i=1

  N  1  2 RMSE = Xei − Xpi N

N  

2 =

(5)

i=1

Xei − Xpi

2

i=1

(6)

N−n

Net isosteric heat of sorption

The net isosteric heat of sorption (H) is used to reflect different water binding energies at different temperatures (Fang et al., 2013). The value of H was calculated by using Clausius–Clapeyron equation as follows: ln

˛w2 H = ˛w1 R

1 T1

−

1 T2

 (7)

where aw1 , aw2 , T1 , T2 , R and H are water activity, absolute temperature (K), gas constant (8.314 J/mol K) and net isosteric heat (kJ/mol), respectively.

2.9.

WL = 11.85 + 0.69X1 + 5.8X2 + 0.9X3 − 0.54X1 X2 − 0.005X1 X3 +0.65X2 X3 + 1.2X12 − 2.4X22 + 0.18X32

where Xei , Xpi , Xe 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 2 values and high R2 value.

2.8.

The effects of three variables on responses are presented in Table 2. The mathematical models of WL and SG for osmoticultrasound dehydration of PSP are shown in the following Eqs. (8) and (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 ◦ C. The fried sample

(8)

SG = 3.22 + 0.9X1 + 2.68X2 + 0.87X3 + 0.46X1 X2 − 0.2X1 X3 − 0.085X2 X3 + 0.21X12 − 2.53X22 + 0.87X32

(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

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Fig. 2 – Response surface plots for water loss of purple-fleshed sweet potato. 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 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 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 ◦ C. 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 ˘ et al., 2011). Similar trend has and Magee, 2003; Polatoglu 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 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 2 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 2 (<9.4893 × 10−5 ) for GAB model at 30, 45 and 60 ◦ C. 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.

Food and Bioproducts Processing 1 1 5 ( 2 0 1 9 ) 154–164

159

Fig. 3 – Effect of temperature on adsorption isotherms of untreated (A) and pretreated (B) fried purple-fleshed sweet potato slices. 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 ◦ C 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 potato and sweet potato flakes at 15–30 ◦ C 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 ◦ C 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 ◦ C

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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 ◦ C. Model

Parameter

Untreated

Pretreated

30 ◦ C

45 ◦ C

60 ◦ C

30 ◦ C

45 ◦ C

60 ◦ C

GAB

Xm C K R2 RMSE 2

0.0598 0.9024 7.0309 0.9941 0.0047 3.9048 × 10−5

0.0549 0.8956 7.4648 0.9953 0.0035 2.1960 × 10−5

0.0452 0.9346 12.5427 0.9893 0.0049 4.2040 × 10−5

0.0449 0.9024 27.1017 0.9868 0.0051 4.4675 × 10−5

0.0403 0.9969 20.2562 0.9978 0.0027 1.2862 × 10−5

0.0367 1.0571 14.6082 0.9889 0.0074 9.4893 × 10−5

Caurie

A B R2 RMSE 2

−3.7001 2.6412 0.9871 0.0070 6.8553 × 10−5

−3.7302 2.5352 0.9961 0.0032 1.4389 × 10−5

−3.8019 2.5366 0.9867 0.0055 4.1730 × 10−5

−3.6448 2.2520 0.9660 0.0081 9.2331 × 10−5

−4.0416 2.9948 0.9763 0.0089 1.1157 × 10−4

−4.3933 3.6134 0.9652 0.0130 2.3757 × 10−4

Halsey

A B R2 RMSE 2

0.0228 1.4290 0.9915 0.0057 4.4934 × 10−5

0.0199 1.4299 0.9924 0.0045 2.8316 × 10−5

0.0200 1.3852 0.9897 0.0048 3.2419 × 10−5

0.0111 1.6290 0.9880 0.0048 3.2587 × 10−5

0.0306 1.2192 0.9967 0.0033 1.5349 × 10−5

0.0514 1.0006 0.9851 0.0085 1.0196 × 10−4

Hendenson

A B R2 RMSE 2

9.6960 1.1253 0.9849 0.0076 8.0406 × 10−5

12.0269 1.1759 0.9869 0.0059 4.8661 × 10−5

13.2528 1.1830 0.9731 0.0078 8.4500 × 10−5

18.8807 1.3294 0.9621 0.0086 1.0295 × 10−4

8.1642 0.9827 0.9659 0.0107 1.6034 × 10−4

5.3571 0.7995 0.9565 0.0146 2.9701 × 10−4

Oswin

A B R2 RMSE 2

0.0937 0.5574 0.9946 0.0045 2.8534 × 10−5

0.0859 0.5470 0.9949 0.0037 1.9086 × 10−5

0.0799 0.5548 0.9870 0.0054 4.1015 × 10−5

0.0809 0.4830 0.9838 0.0056 4.4067 × 10−5

0.0794 0.6465 0.9881 0.0063 5.5775 × 10−5

0.0762 0.7882 0.9759 0.0108 1.6429 × 10−4

Smith

A B R2 RMSE 2

0.0145 0.1168 0.9899 0.0062 5.3710 × 10−5

0.0160 0.1027 0.9941 0.0040 2.1853 × 10−5

0.0155 0.0954 0.9841 0.0060 5.0134 × 10−5

0.0239 0.0830 0.9773 0.0066 6.1664 × 10−5

0.0053 0.1139 0.9678 0.0104 1.5109 × 10−4

−0.0086 0.1374 0.9400 0.0171 4.0956 × 10−4

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

to 0.0452 at 60 ◦ C. The Xm values of pretreated fried PSP slices decreased from 0.0449 at 30 ◦ C to 0.0367 at 60 ◦ C. These results were related to the decrease in active sites at high tempera˘ et al., 2011). The results have a good agreement ture (Polatoglu with those reported by Choudhury et al. (2011), they found that monolayer moisture content decreased with increasing temperature.

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

3.5.

Net isosteric heat of sorption

The net isosteric heat (H) of adsorption for fried PSP slices was calculated by using the Eq. (7) to the experimental data from GAB model. The H values of adsorption for untreated and pretreated fried PSP slices as a function of moisture contents are presented in Fig. 5. H values of adsorption decreased with increasing moisture content. The H values of

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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 ◦ C. Untreated T21 peak areas

Salt

CH3 COOK MgCl2 K2 CO3 NaBr KI NaCl KCl

Pretreated T21 peak areas

30 ◦ C

45 ◦ C

60 ◦ C

30 ◦ C

45 ◦ C

60 ◦ C

48.48 106.13 265.81 753.09 1030.99 1699.52 1917.96

22.23 66.49 183.11 624.28 971.39 1330.15 1504.12

11.99 44.99 109.18 479.11 851.28 1149.87 1469.28

25.56 90.57 195.23 657.40 707.65 966.69 1474.01

18.09 55.10 177.34 511.46 970.96 1316.34 1844.97

4.06 37.76 97.19 370.13 995.18 1346.77 2138.46

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 ˘ et al., 2011). Sawhney et al. (2011) in the dried sucuk (Polatoglu indicated that the strong binding sites and the great water solid interaction were obtained for dried acid casein. Low H values of adsorption were due to the increase amounts of adsorbed water. The H 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

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 ◦ C. 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 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 ◦ C 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–C show that the R2 values of linear regression models for untreated fried PSP slices were above 0.9595 at 30, 45 and 60 ◦ C. Fig. 6D–F show that the R2 values of linear regression models for pretreated fried PSP slices were above 0.9845 at 30, 45 and 60 ◦ C. 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 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 purplefleshed sweet potato by osmotic-ultrasound pretreatment was reduced. Osmotic-ultrasound pretreatment reduced net isosteric heat values. LF-NMR results showed that osmoticultrasound pretreated fried purple-fleshed sweet potato was easier to achieve low moisture content compared to untreated fried purple-fleshed sweet potato. High correlation between

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Fig. 6 – Relationship between T21 area and equilibrium moisture content of untreated (A–C) and pretreated (D–F) fried purple-fleshed sweet potato slices at 30, 45 and 60 ◦ C. 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. 31,671,864), 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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