Effect of Ultrasound Dielectric Pretreatment on the Oxidation Resistance of Vacuum‐Fried Apple Chips

Zhang Min (Orcid ID: 0000-0003-4625-1262)

Effect of ultrasound dielectric pretreatment on the oxidation

resistance of vacuum fried apple chips

Xu Shena, Min Zhanga,b,*, Bhesh Bhandaric, Zhimei Guod

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

Wuxi, Jiangsu, China bJiangsu Province Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China

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

Australia

dWuxi Delin Boat Equipment Co., Wuxi 214191,China

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

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

E-mail: [email protected]

Tel: 0086-(0)510-85807976; Fax: 0086-(0)510-85917089

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/jsfa.8966

oxidation resistance of vacuum fried apple chips, apple slices were pretreated at ultrasonic power for 150, 250 and 400 W and time for 10, 20 and, 30 min, followed by vacuum frying (VF) . The quality and oxidation resistance of fried apple were evaluated by testing the dielectric properties and comparing moisture content, oil uptake, color, acid value (AV) and peroxide value (PV) of apple chips.

RESULTS: Ultrasonic treatment can significantly change dielectric properties of

apple slices. Moisture and oil contents of apple chips decreased with increasing

ultrasonic power and time. During storage, color retention of fried apple chips

processed by ultrasound was improved. AV and PV values of fried apple chips

processed by ultrasound were lower, which improved antioxidant properties of apple

chips.

CONCLUSION: Results of the present study indicated that ultrasound dielectric

pretreatment not only improved the quality of vacuum fried apple chips, but also

improved its antioxidant properties.

KEYWORDS: Ultrasound dielectric pretreatment; apple chips; moisture content; oil

content; color; peroxide value and acid value

INTRODUCTION

Apples have good flavor, color, texture and are also quite nutritious, thus they have gained worldwide popularity. Dehydration is a traditional pretreatment method to extend the shelf-life of fruits and vegetables, and is also an alternative solution for raw material preservation and transportation. The development of new, attractive, high-quality apple chips is desirable in order to widen product availability and diversify the market, particularly as the fruit and vegetable consumption is so highly recommended in the modern daily diet.

Ultrasound is a mechanical wave with frequency ranging from 20 kHz to 100 MHz. It is called ultrasound because this frequency is beyond the range of human hearing.

Ultrasound is employed in various industrial sectors including chemical, biopretreatment, food pretreatment, pharmaceutical, medical and defense.1,2

According to the frequency range of ultrasound, it is mainly divided into two kinds in the food field. One is MHz class high-frequency, low-energy ultrasound; the other is low-frequency, high-energy ultrasound.3 Within the food industry, high frequency ultrasound is typically used as a non-destructive, non-invasive analytical technique for quality evaluation, process control and monitoring,1 whereas low frequency

This article is protected by copyright. All rights reserved. ultrasound is employed for process intensification such as system homogenization, sterilization, enzyme inactivation, juice degassing, flavor ingredients extraction, auxiliary freezing, osmotic treatment and thawing, etc.4,5 High frequency ultrasound employs very low power levels insufficient to cause acoustic cavitation which therefore produces zero or minimal physical and chemical alterations in the material through which the wave passes. Hence, it can be employed for food analysis and quality control without affecting the product. In contrast, low frequency ultrasound employs power levels high enough to generate cavitation and is capable of producing physical and chemical modifications in numerous applications.3 In general, the size of cavitation bubbles produced by low-frequency ultrasound is larger, and more energy is released when bubbles burst. On the contrary, the higher the frequency is, the weaker the cavitation effect is. Studies have shown that the cavitation effect hardly occurs when the ultrasonic frequency is higher than 2.5 MHz.6 Not only can the ultrasonic treatment greatly reduces the food pretreatment time and decreases drying temperature, it also influences the some physical properties of food like shrinkage, density, porosity, mechanical strength, water content and color change.7 In the food dehydration applications, the direct effects induced by ultrasound are related to the

‘sponge effect’ and inertial flux which can keep micro channels and pores unobstructed, facilitating mass transfer, and the indirect effect is related to micro

release of liquid from the inner part of the particle to the solid surface, and the forces

involved in this mechanism can be higher than the surface tension which maintains

the water molecules inside the capillaries of the material, creating microscopic

channels and making the interchanges of matter easier.8 When ultrasound waves travel

through the product, the phenomenon of ‘cavitation’ takes place in the water inside or

outside the product cells, resulting in cell and tissue disruption and the consequent

formation of cavities and micro channels which are believed to be the main effect of

the low-frequency ultrasound technology in food applications.7 Therefore, Ultrasound

is often used to change the structure of fruits and vegetables.1 Low-frequency ultrasound changes the structure of fruits and vegetables through "hole effect" and

"degassing effect", thereby affecting the oil content of fruits and vegetables after vacuum-frying. Some researches show that the use of ultrasound not only reduce the oil content, but also reduce the oil in the chips contact with oxygen, thus reducing the

lipid oxidation rate.2-5 Dielectric property can be changed after ultrasound dielectric

pretreatment by destroying the cell structure, changing physical and chemical

properties and water content. The advantages of using ultrasound for food

pretreatment, includes: more effective mixing and micro-mixing, faster energy and

mass transfer, reduced thermal and concentration gradients, reduced temperature,

control, faster start-up, increased production, and elimination of process steps.

Ultrasound offers a net advantage in term of productivity, yield and selectivity, with

better pretreatment time, enhanced quality, reduced chemical and physical hazards,

and is environmentally friendly.2

Nowadays, consumers’ health concerns have forced the snack foods industry to produce low-fat foods while keeping their traditional flavor and texture characteristics.9 Vacuum frying (VF) is one of the new methods applied to produce

fried fruits and vegetables with low oil content and while still possessing desired

texture and flavor characteristics.10,11 Fruits and vegetables can be made into fried

products with superior sensory and nutritional qualities at low atmospheric pressures

(especially below 6.65 kPa) by vacuum frying which can lower the boiling point of

frying oil and water in food.11 Compared with atmospheric frying, vacuum frying

reduces oil content and acrylamide content in fried products.12,13 Due to the low

temperature and oxygen content of vacuum frying process, it also protects the natural

color and taste of the product.12,14 But it also has the problem of high oil content, so

methods for reducing oil absorption in fried products are being researched. The effects

of pretreatment and post-treatment on the oil uptake of vacuum fried products have

been studied by several researchers.15,16 The effectiveness of a two stage frying

The objective of this study was to investigate the effect of ultrasound dielectric pretreatment of raw chips on the oxidation resistance of vacuum fried apple chips.

The effects of different ultrasonic power (150, 250 and 400 W) and ultrasonic time

(10, 20 and 30 min) were tested in the ultrasonic pretreatment experiment. The difference in the microstructure of apple slices processed under different ultrasonic conditions were evaluated using the parameters of dielectric constant, dielectric loss factor, moisture content and oil content. Changes in parameters such as color

(brightness, redness and yellowness), peroxide value and acid value during the storage period were used to evaluate the effect of ultrasound dielectric pretreatment on the antioxidant properties of vacuum fried apple chips.

EXPERIMENTAL

Materials and methods

Materials

Fresh Red Fuji apples and palm oil were purchased from a local supermarket in Wuxi,

China. The apple samples were stored at 4 oC in a refrigerator until used in the experiments. No apple sample was used after 5 days of storage. Palm oil is commonly used for deep fat frying of foods because it has good oxidative stability during

Preparation of apple slices

Washed, stemmed and peeled apples were cut into 3 ± 0.2 mm thick slices. A circular

cutting mold was used to produce uniform slices of 32 ± 2 mm in diameter. These

slices were blanched in water at 90 oC for 1 min, cooled under running tap water for 1

min and drained on absorbent paper, then immersed in a mixed solution of 1.5% NaCl

and 0.5% citric acid for 10 min, washed, drained and immersed in 4% Trehalose

solution and 5% NaCl solution for 30 min, respectively. These slices were frozen in a

refrigerator at ﹣20 oC overnight.

Ultrasound treatment

JY98-ⅢN ultrasonic cell disruptor (Ningbo Xinzhi Biotechnology Co., Ltd. zhejiang,

China) was used in this experiment. The schematic diagram of this instrument is

shown in Fig. 1. The probe diameter (5) was 15 mm. The maximum output power of

the ultrasonic source was 2000W. Ultrasonic pulse value was 50% (5s + 5s).

Experiments were carried out at different power levels (150, 250 and 400 W) below

30 oC for 20 min and at a fixed power level (400W) below 30 oC for 10, 20, 30 min.

Triplicate runs were carried out.

Vacuum frying

described earlier by Fan et al. and Song et al.20-22 The vacuum fryer (Fig. 2) was

equipped with a centrifuge (Nan Feng Company, Wuxi, China). The capacity of

vacuum chamber was 15 L. The maximum attainable temperature and vacuum degree

of this equipment were 150 oC and 0.095 MPa, respectively. The heating power of VF

was 3000 W. A batch of 50 g apple slices was fried in 5L of palm oil under vacuum

(0.085±0.002 MPa) at 90 oC for 20 min. After frying, the slices were centrifuged in

situ at 300 rpm for 5 min without breaking the vacuum in order to remove excess

frying oil. Triplicate runs were carried out and The oil was changed after each of the

four experiments.

Storage of finished product

Finished product was stored in a brown dry dish at room temperature. During storage the packets were inspected to identify packets with leaks. The vacuum fried apple slices were sampled and analyzed through the storage period. The storage of finished product was conducted in triplicate.

Analysis

Dielectric properties

After the pretreatment with ultrasound, the water in the surface of apple slices was

an Agilent 85070E dielectric probe kit. The probe was linked to vector network

analyzer (E5062A, Shenzhen Shengteng instrument Co., Ltd, Guangdong, China).

The network analyzer detected the reflectance at the interface between probe and

samples. Treated and untreated samples were tested and every chip was tested in

duplicate after testing and the values were recorded. The average of 21 values of

dielectric properties (ε', ε'') around frequency 2450 kHz were taken as the final value

of dielectric constant (ε') and dielectric loss factor (ε''). The tests were carried out in

duplicate.

Moisture content

Moisture content of apple chips was measured in an oven by drying at 102 ± 3 oC.

Approximately 3 g of samples were placed in the oven, and the cap is inclined to the

edge of the bottle. After drying 2h ~ 4h, the samples were taken out, and then put into

the desiccator to cool for 0.5 h and weighed. Then put it into the drying oven for

about 1h, remove and put it into the desiccator and cool it for 0.5h before weighing

again. Repeat the above operations until the weight difference between two adjacent

weighing does not exceed 2 mg, then the mass was stabilized.23 Each sample was

tested in triplicate.

Oil content

This article is protected by copyright. All rights reserved. The oil content was determined by Soxhlet extraction method with petroleum.23 Each extraction run was carried out for 8 h. Samples were crushed and oven dried at 102 ±

3 oC to a constant mass. Then, the dried samples were transferred to a single walled

cellulose extraction thimble and the oil was extracted gravimetrically using a Soxhlet

extraction system for 8 h.23 The flask containing oil was dried to constant mass in a

vacuum dryer at 55 oC. The oil uptake in the sample was expressed as g oil/100 g dry

mass of the sample. Analyses were carried out in duplicate.

Color

The color values of samples were measured using a chromaticity instrument (CR-400,

Konica Minolta Sensing, Inc., Osaka, Japan) on days 1, 4, 8 and 12. The samples were

placed in a standard light and the Hunter L* (lightness), a* (redness), and b*

(yellowness) values were measured. The colorimeter was standardized utilizing a

white calibration plate. Three slices or chips obtained at each condition were tested

and three readings were taken at different positions for an apple chip and all the data

points were averaged. Results are expressed in L*, Hue [H* = tan-1 (a*/b*)] and

Chroma [C* = (a*2 + b*2)1/2].32

Determination of peroxide value and acid value

The vacuum fried apple slices were analyzed on days 2, 4, 6, 8, 9, 10 and 11. Oil was

extracted from the sample according to the oil extraction method described above.

Union of Pure and Applied Chemistry.24 Oil samples were dissolved in

isopropanol/toluene (1:1) and the FFA value expressed as acid value (AV). Peroxide

value (PV) was determined according to the American Oil Chemists’ Society Standard

methods.25 All analyses were carried out in duplicate.

Statistical analysis

All data were analyzed using SPSS software for Windows (V 11.5.1 SPSS Inc.,

Chicago, IL). Difference among the mean values of test samples was determined using one-way analysis of variance (ANOVA). A 95% confidence level (p < 0.05) was used for this purpose.

RESULTS AND DISCUSSION

Ultrasound dielectric pretreatment

Dielectric properties are the response characteristics of bound charge in biological

molecules to external electric field.26 Through the study of the dielectric properties of

food, we can better analyze and monitor the composition, organization and status of

food, and also can effectively use the electrical properties of materials for food

pretreatment. The dielectric constant (ε') is related to the capacity of the material to

store electrical energy, and the dielectric loss factor (ε'') is related to the dissipation

pretreatment time on the dielectric properties of apple slices immersed in different

solutions are shown in Fig. 3. It can be seen from Figure. 3 (a) that the dielectric

constant (ε') of apple slices soaked with trehalose solution increases with the increase of ultrasonic power and ultrasonic treatment time. As shown in Fig. 3 (b), the dielectric loss factor (ε'') of apple slices immersed in trehalose solution has little

change with the increase of ultrasonic power and ultrasonic treatment time. The effect

of ultrasound in liquid media can be attributed to the cavitation effect of ultrasound,

which involves the growth, contraction, rupture and a series of physical disturbances

of bubbles in the sound field. The bubble burst at once, producing high temperature

(about 5000 K) and high pressure (about 100 MPa) around the bubble, this extreme

environment will make the cell membrane rupture and solid surface particles fall off.27

Low-frequency ultrasound has the effect of degassing, which can make the air in

fruits and vegetables discharge from the capillary of the surface of fruits and

vegetables, and provide the channel for the permeation of medium and the transfer of

water.8 Therefore, the cause of the above phenomenon (increase of ε’ and no change

of ε”) may be the effect of ultrasonic degassing. The micro pores formed on the

surface of the treated apple slices are helpful to the absorption of water, and the

dielectric constant will increase with the increase of moisture content. Trehalose

This article is protected by copyright. All rights reserved. belongs to non-electrolyte and has little influence on dielectric loss factor. As can be seen in Fig. 3 (c), the dielectric constant of the sample immersed in salt solution has little change with the increase of ultrasonic power and ultrasonic treatment time. As shown in Fig. 3 (d), the dielectric loss factor of the sample immersed in salt solution

increases with the increase of ultrasonic power and ultrasonic treatment time. This is

because the cellular structure of the food material after sonication is destroyed, the

cell sap, organic acid, and soluble sugar etc. may flow out of the cell and the salt may

flow into the it, which contribute to cause an increase in ion loss thus increasing the

dielectric dissipation factor (ε'') . These observations show that ultrasonic treatment

has an important effect on the dielectric properties of apple slices(P<0.05).

Moisture content

Fig. 4 shows the moisture content of apple chips treated under different ultrasonic

conditions at a fixed frying temperature (90 oC) and different frying time. Loss of

moisture during frying presented a classical drying profile. The initial rapid decrease

of water content is mainly due to the loss of surface and unbound inner water,

followed by a gradually decreasing gradient due to crust formation. At 0.085±0.002

MPa, the moisture content of untreated apple chips was above 8% and the treated

ones were below 8% after frying for 5 minutes. After 15 minutes of frying, the

moisture content of untreated samples decreased to about 3%, while the treated apple

This article is protected by copyright. All rights reserved. chips decreased to about 2% and then decreased only slightly afterwards. During this stage, the bound moisture in material is slowly removed by capillary action.28 In a

vacuum frying operation, the food is heated in a closed system under reduced pressure to reduce the boiling point of the frying oil and water in the food. When the oil temperature reaches the boiling point of the water, the unbound water in the fried food can be removed quickly. At the beginning of frying, the moisture content of apple slices is very high and the loss of water is mainly through the cell wall membrane and intercellular channels. It can be seen in the Fig. 4 (a) and (b) that the moisture content

in treated sample decreased faster (P<0.05) than that in the untreated samples due to

the formation of micro pores on the surface of apple chips and the destruction of cell

structure after ultrasonic treatment. As assumed, with the increase of ultrasonic power

or ultrasonic time, the dehydration rate in VF was faster.

Oil content

Oil content is an important index to evaluate the quality of fried products. High oil

retention in the fried product can affect the flavor of the product, and the fried

products with high oil content are also considered unhealthy. The effects of ultrasonic

power level and ultrasonic pretreatment time on the oil content of apple chips are

shown in Fig. 5. As can be seen in the Fig. 5 (a), the oil content of apple chips

increased with the frying time. However, the oil content of apples chips processed was

This article is protected by copyright. All rights reserved. lower (P<0.05) than that of the untreated ones at the same frying time. For example, the oil content of the treated apple chips ranged from 31.40 to 34.43 g oil / 100 g dry solid and the oil content in untreated apple chips was 35.07 g / 100 g dry solid at 10 min of frying. The oil content of apple chips decreased with the increase of ultrasonic treatment time as shown in the Fig. 5 (b). For example, the oil content of apple chips treated with ultrasonic treatment for 10 min and 30 min was 35.82 g / 100 g dry solid and 34.32 g / 100g dry solid as showed in the figure, respectively. The above results indicate that the oil content of apple chips decreased significantly (P<0.05) with the increase of ultrasonic power or ultrasonic treatment time.

When apple slices were placed in hot oil, moisture content fell rapidly. The outer surface of the slice was dried, providing a diffusion gradient, and the moisture inside the slice was converted to steam, causing a pressure gradient.29 As the moisture content decreases the path required for diffusion increases and the increased solid and/or oil content greatly increase resistant to outward diffusion of moisture.30 When water evaporates, the surrounding area dry out, forming porous area, and lose their hydrophilicity. The oil then adheres to the potato chips, occupying the porous area left by evaporated water. However, apple chips form micro pores on the surface after ultrasonic treatment, so water diffusion requires less pathways. The moisture pathway in treated apple slices will make less pore areas for the connection between the dry

discharge during deoiling. As Pieczywek et al investigated the effect of ultrasonic

treatment on the hardness and microstructure of apple cell wall, the result

demonstrated a multiscale effect of ultrasound treatment on fruit tissue.31 At a

microscale level, it was confirmed that the ultrasound creates intercellular spaces. At a

nanoscale level (cell wall), the ultrasounds increase pectin solubilization and decrease

stiffness. The new spaces facilitate water migration through the tissue. Pectin

degradation and an eventual cellulose/hemicellulose network slackening may also

result in the formation of new pores in cell walls that facilitate water migration. As a

consequence, the oil uptake became less in treated apple chips in contrast with the

untreated.

Color

The color changes of fried apple chips were observed by analyzing L *, H* and C*

coordinates. The effect of ultrasonic treatment on the color of apple chips after frying

20 min in VF as shown in Fig. 6. L* is a critical parameter in the frying of food, and

is usually used as a quality control parameter.32 L* values about 60 are acceptable for

these products.33 Lower values indicate the deterioration of total color, which is not

desirable for fried products. In this study, fried apple chips exceeded this value. The main mechanism of color formation in fried products is Maillard reaction, which

This article is protected by copyright. All rights reserved. requires the presence of reducing sugars and amino acids and is highly dependent on temperature.34 As can be seen from Fig. 6 (a, d) the L * value of the apple samples just after ultrasonic treatment was significantly (P<0.05) higher than that of the untreated samples. It can also be seen from the pictures that the L * value of samples of both the control group and the ultrasound group showed a downward trend during the whole storage period. However, with the prolongation of storage period, the L * value of the ultrasound group samples increased with the increase of sonication time and ultrasonic power, and the L * values of ultrasound group were significantly (P<0.05) larger than that of the control group. This may be due to the fact that ultrasonic treatment destroys the cell structure, which leads to the outflow of polyphenols, preventing non-enzymatic browning reactions (including Maillard reaction, caramelization and chemical oxidation), thereby reducing color deterioration. As can be seen in the Fig. 6 (b, e), different ultrasonic power levels and ultrasonic time promoted the reduction in the Hue angle after frying. Chroma parameter indicates the degree of saturation, and it is proportional to the strength of color.35 The evaluation of color revealed that fried samples sonicated for 10 minutes presented a higher incidence of browning during storage, which showed the lower values as shown in Fig.

6 (c, f).

Peroxide value and acid value

This article is protected by copyright. All rights reserved. The apple chips were sealed in light and gas impermeable bags and stored at ambient temperature. The oil in the apple chips was extracted regularly to analyze the changes in acid value (AV) and peroxide value (PV) of the oil. The effect of different ultrasonic treatment conditions on the AV and PV of the oil in the apple chips during storage are shown in Fig. 7. As shown in Figure. 7 (a, c), AV gradually increased as the storage time increased. In the storage period of 0 to 8 days, oil AV of unsaturated fried apple chips rose slowly. AV changed significantly on the 9th day. This is because the initial stage of storage is the induction period of the reaction, and impurities and moisture in the oil are very low. The rate of oxidation, hydrolysis, polymerization and pyrolysis of the oil is very slow at a certain temperature.36 With the extension of time, polar substances in oils are also increased, which promotes the hydrolysis and pyrolysis reaction. AV increases rapidly as reaction intensifies. However, the change in AV of the oil in the samples processed by ultrasound was delayed by 2 days in contrast with the untreated one, and the rising degree of AV slowed down with the increase of ultrasonic power or ultrasonic time. PV is the main index to judge the quality of oil. Oil is exposed to air and will in contact with oxygen, which is bound to have varying degrees of oxidation. The PV changes of the oil in apple chips are shown in Fig. 7 (b, d). PV gradually increased with the increase of storage time.

During the storage of 0 to 5 days, PV of the oil in the apple chips without ultrasonic

This article is protected by copyright. All rights reserved. treatment changed slowly. The PV mutation occurred at 6th day, but it did not reach the value specified by AOCS. PV exceeded the 19.7 meq/kg standard for fatty peroxide value specified in AOCS at 8th day. The PV increased sharply from 23.5 meq/kg to 146.1 meq/kg at 9th day. This may be because small molecular substances produced in the early stages of oxidation have a strong ability to oxidize, which makes oil oxidation intensified. However, the change in PV of the oil in ultrasonic treated samples was delayed by 2 days in contrast with the untreated one, and the rising degree of PV slowed down with the increase of ultrasonic power or ultrasonic time. The AV and PV of oil in apple chips processed by ultrasound were significantly improved (P<0.05). This may be because Low-frequency ultrasound changes the intercellular space of the apple tissue through the "hole effect", and the microporous channel is formed on the surface of the apple slices because of the "degassing action".

When the apple chips are deoiled, changes in organizational structure facilitate the discharge of oil, thereby reducing the oil content of the chips. And reduce the oil in the chip in contact with oxygen because of structural changes, thus reducing the lipid oxidation reaction rate. The decrease of oil content and the rate of lipid oxidation can improve the antioxidant activity of apple chips. Therefore, ultrasound dielectric pretreatment can improve the oxidation resistance of vacuum fried apple chips.

CONCLUSIONS

found that ultrasound dielectric pretreatment improved the quality of apple chips

better. The use of ultrasound significantly reduced oil uptake and moisture content,

and retained the natural color of apple chips better. During storage, ultrasound

pretreatment delayed the color deterioration of apple chips better. What's more, AV

and PV values of fried apple chips processed by ultrasound were lower than those of

untreated ones during the storage period of 12 days. This indicated that ultrasound dielectric pretreatment can significantly improve the antioxidant activity of fried apple chips. This was because low-frequency ultrasound changed the cell space of apple tissue through "hole effect", and formed microporous channels on the surface of apple slices through the "degassing effect", which significantly changed the dielectric properties of apple slices. When apple chips were deoiled, the changes in the tissue structure are beneficial to the oil emission, thus reducing the oil content of fried chips.

At the same time, it reduced the contact between oil and oxygen in apple chips due to structural changes, thus reducing the rate of lipid oxidation. The conclusion of this work is that ultrasound dielectric pretreatment can improve the quality and antioxidant properties of vacuum fried apple chips.

Acknowledgments We acknowledge the financial support from the following sources:

This article is protected by copyright. All rights reserved. National Natural Science Foundation Program of China (Contract No. 31671864), China Key Research Program (Contract No. 2016YFD0400704-5), Jiangsu Province (China) “Collaborative Innovation Center for Food Safety and Quality Control” Industry Development Program, Jiangsu Province (China) Infrastructure Project (Contract No. BM2014051).

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36 Kita A, LisiDska G and Gołubowska G, The effects of oils and frying temperatures

(2007).

This article is protected by copyright. All rights reserved. Figure captions Fig. 1. Schematic diagram of the JY98-ⅢN ultrasonic cell disruptor. Fig. 2. Schematic diagram of the vacuum frying instrument. Fig. 3. Effect of different ultrasonic power and ultrasonic time on the dielectric properties of apple slices immersed in different solutions. Fig. 4. Effect of ultrasound dielectric pretreatment on moisture content of apple chips. Fig. 5. Effect of ultrasound dielectric pretreatment on oil content of apple chips. Fig. 6. Effect of ultrasound dielectric pretreatment on the L*, Hue and Chroma of apple chips during during storage. Fig. 7. Effect of ultrasound dielectric pretreatment on the AV and PV of the oil in the apple chips during.

Fig. 1. Schematic diagram of the JY98-ⅢN ultrasonic cell disruptor. 1. thermostatic water bath 2. conduit 3. Jacketed beaker 4. ultrasonic transducer 5. ultrasound probe 6. samples 7. temperature probe 8. ultrasonic generator 9. data logger 10. computer.

Fig. 2. Schematic diagram of the vacuum frying instrument. 1. Frying basket 2. Frying chamber 3. Vacuum chamber 4. Temperature controller 5. Vacuum pump 6. Pressure gauge 7. Valve for breaking 8. Lifting shaft 9. Electric motor.

10min 70 20min 30min

ε' 55

40 Control 150 250 400 Ultrasonic power/W

(a)

10min 12 20min 30min

ε'' 6

0 Control 150 250 400 Ultrasonic power/W

(b)

10min 70 20min 30min

ε' 55

40 Control 150 250 400 Ultrasonic power/W

(c)

10min 20min 30 30min

ε'' 15

0 Control 150 250 400 Ultrasonic power/W

(d)

This article is protected by copyright. All rights reserved. Fig. 3. Effects of different ultrasonic power and ultrasonic time on the dielectric properties of apple slices immersed in different solutions. (ε'=dielectric constant and ε''=dielectric loss factor).

Control 100 10min 95 20min 90 30min 85 80 75 70 65 60 55 50 45 40 35 30 25 Moisture content (%, wb) 20 15 10 5 0 0 5 10 15 20 Frying time (min)

(a)

Control 100 150W 95 250W 90 400W 85 80 75 70 65 60 55 50 45 40 35 30 25 Moisture content (%, wb) 20 15 10 5 0 0 5 10 15 20 Frying time (min)

(b) Fig. 4. Effect of ultrasound dielectric pretreatment on moisture content of vacuum

30 Control 150W 25 250W 400W 20

15 Oil content (%, db) 10

0 0 5 10 15 20

(a)

30 Control 10min 25 20min 30min 20

15 Oil content (%, db) 10

0 0 5 10 15 20 Frying time (min)

(b) Fig. 5. Effect of ultrasound dielectric pretreatment on oil content of vacuum fried

Control 10min 80 20min 30min

L* 60

40 0 4 8 12 Storage time (days)

(a)

Control 10min 105 20min 30min

100

95 Hue

85 0 4 8 12 Storage time (days)

(b)

25 Chroma

15 0 4 8 12 Storage time (days)

(c)

Control 80 150W 250W 400W

L* 60

40 0 4 8 12 Storage time (days)

(d)

100

95 Hue

85 0 4 8 12 Storage time (days)

(e)

Control 35 150W 250W 400W

25 Chroma

15 0 4 8 12 Storage time (days)

(f) Fig. 6. Effect of ultrasound dielectric pretreatment on the L*, Hue and Chroma of vacuum fried apple chips during storage.

1.2

1.0

0.8

0.6 Control 10min 0.4 20min

Acid value (mg KOH/g) 30min

0.2

0.0 0 1 2 3 4 5 6 7 8 9 10 11 12 Storage time (days)

(a)

150 Control 10min 120 20min 30min

60 Peroxide value (meq/kg) 30

0 0 1 2 3 4 5 6 7 8 9 10 11 12 Storage time (days)

(b)

1.2

1.0

0.8

0.6 Control 150W 250W 0.4 400W Acid value (mg KOH/g)

0.2

0.0 0 1 2 3 4 5 6 7 8 9 10 11 12

(c)

150 Control 150W 120 250W 400W

60 Peroxide value (meq/kg) 30

0 0 1 2 3 4 5 6 7 8 9 10 11 12 Storage time (days)

(d) Fig. 7. Effect of ultrasound dielectric pretreatment on the AV and PV of the oil in the vacuum fried apple chips during storage for 12 days.