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

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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 This article is protected by copyright. All rights reserved. ABSTRACT BACKGROUND: In order to investigate ultrasound dielectric pretreatment effect on 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 This article is protected by copyright. All rights reserved. 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 This article is protected by copyright. All rights reserved. channel formation due to the acoustic ‘cavitation’. The sponge effect produces the 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, This article is protected by copyright. All rights reserved. selective extraction, reduced equipment size, faster response to process extraction 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 This article is protected by copyright. All rights reserved. process in reducing the oil uptake in the fried potato chips has also been studied.17 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
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