Improving the energy efficiency and the quality of fried products using a novel vacuum frying assisted by combined ultrasound and microwave technology

Highlights

• The combination of ultrasound and microwave in vacuum frying system was investigated.

• The USMVF increased the moisture evaporation kinetics and effective moisture diffusivity.

• The increase of ultrasound power had positive effect to the energy efficiency and product quality.

• The oil uptake of samples was reduced and the chips became crispier in USMVF compared to the VF.

Abstract

The effectiveness of hybrid frying based on vacuum frying with application of ultrasound (USVF) and combined ultrasound and microwave (USMVF) enhancement was presented in this study. The ultrasound revealed significantly the “vibration effect”, and the combination of ultrasound and microwave had a synergistic effect on the energy efficiency and quality attributes of vacuum fried potato chips, shortening the frying time by 36.4%–54.5% and improving the effective moisture diffusivity by 60.5%–144.5% with the ultrasound power levels ranged 300 W–600 W. The USMVF process was found to consume about 34.9%–48.3% less energy compared to that in VF. The oil uptake of vacuum fried potato chips was reduced by 27.4%–32.3%, the texture (crispness) and the color was greatly improved, and the volume shrinkage and water activity were diminished by the combination of ultrasound and microwave. The moisture loss kinetics and quality parameters were also improved markedly when higher ultrasound power was used. The SEM revealed a more porous microstructure in USMVF samples. The vacuum frying technology that combines both ultrasound and microwave found to be much more effective technology in terms of energy efficiency and product quality.

Industrial relevance

In recent years, the fruit-vegetable crisp produced by vacuum frying (VF) has been widely concerned due to natural color, superior sensory and nutritional quality. However, there are certain defects in the application of VF technology, such as backward heating method, low efficiency especially with low-temperature frying, relatively high oil uptake in products and the quality attributes of fried products need to be improved. One of the recommended methods to overcome the limitation of low-temperature dehydration is to utilize the synergistic effect of different energy, which providing the energy by a combination of several mechanisms, such as vacuum frying with microwave radiation, or with ultrasounds, which leads to a significant improvement in energy efficiency and quality attributes of fried fruits and vegetables. Focused on these problems, this research investigated the effects of low-frequency ultrasound and the synergistic effect of ultrasound and microwave on the efficiency and quality of vacuum frying with potato chips as experimental material. The effect of variable hybrid drying conditions was analyzed with different power levels of ultrasound. The application of ultrasound showed improve the moisture loss rate and quality attributes of vacuum fried products as a non-thermal technology. The combination of ultrasound and microwave in vacuum frying system achieved better energy efficiency, lower oil uptake, crispier, better shaped and colored products compared to the VF produced samples. The storage stability and safety of vacuum fried products also improved by the combined ultrasound and microwave. Thereby, the experimental results will contribute to the development of modern vacuum frying technologies of fruits and vegetables, which reduced the oil uptake and retained their high quality, with reduced process time and lower power consumption at much lower frying temperature.

Introduction

Vacuum frying is used to fry fruits and vegetables under sub-atmospheric pressure, preferably below 50 Torr (6.65 kPa). This lowers the boiling point of frying oil and moisture in food and produces better quality fried foods with less oil content when compared with atmospheric frying (Andrés-Bello, García-Segovia, & Martínez-Monzó, 2011; Da Silva & Rosanag, 2008). However, there are certain defects in the application of VF technology, such as backward heating method, low efficiency especially with low-temperature frying and relatively high oil uptake in products (Albertos et al., 2016). The application of microwave in the vacuum frying had been reported to be an efficient method with faster dehydration and more desirable quality parameters in which the microwave is used as the heating resource (Quan, Zhang, Zhang, & Adhikari, 2014; Su, Zhang, Zhang, Adhikari, & Yang, 2016). The dielectric heating characteristics of microwave make the material heated rapidly and increase the dehydration rate (Chandrasekaran, Ramanathan, & Basak, 2013; Gharachorloo, Ghavami, Mahdiani, & Azizinezhad, 2009). While, the demands of improving efficiency and quality in fried products with the frying occurs under lower frying temperature also increased in food industries.

Ultrasound assisted dehydration has been a topic of interest for many years. Ultrasound is a mechanical wave with a frequency range of 20 kHz to 10 MHz (Bantle & Eikevik, 2011; Ozuna, Cárcel, Walde, & Garcia-Perez, 2014). Power ultrasound in the kHz range is commonly employed to assist the dehydration process (Başlar, Kılıçlı, Toker, Sağdıç, & Arici, 2014; Chen, Guo, & Wu, 2016). The cavitation produced by high intensity acoustic waves causes heating and micro-vibration effects (Kowalski & Mierzwa, 2015), and facilitates the removal of strongly bound moisture more effectively during dehydration (Fernandes, Gallão, & Rodrigues, 2008). Other effects associated with ultrasound such as sponge effect (Stojanovic & Silva, 2007), microstreaming (Mason, Paniwnyk, & Lorimer, 1996), and formation of microscopic channels (Cárcel, Benedito, Rosselló, & Mulet, 2007) also help enhance the mass transfer and alters the physicomechanical and/or chemical properties of biomaterials (Cárcel et al., 2007). For example, Santacatalina et al. (2014) reported that the application of power ultrasound in the convection drying increased the drying kinetics and shortened the drying time when apple samples were dried at low temperature. An application of ultrasound at 75 W power level at −10 °C shortened the drying time by 80.3% (Santacatalina, Contreras, Simal, Carcel, & Garcia-Perez, 2016). Similarly, Rodríguez, Santacatalina, et al. (2014) showed that the application of ultrasound in a convective drying process decreased the loss of total polyphenol and flavonoid content in apples when compared with air dried samples. The above mentioned improvements in quality of the dried products can be attributed to the lowering of the drying temperature and/or shortening the drying time when the ultrasound was used in the drying process (Chou & Chua, 2001). Dehghannya, Naghavi, and Ghanbarzadeh (2016) reported that both ultrasound and pre-drying pretreatment reduced the oil uptake of fried potato strips. Qiu, Zhang, Wang, and Bhandari (2018) indicated that the frying time and oil uptake of the vacuum fried sweet potato chips were reduced constantly by ultrasound pretreatments. Then, the application of ultrasound in the vacuum frying may hand in promising results in improving the effectiveness with low frying temperature and reducing oil uptake.

One of the recommended methods to overcome the limitation of low-temperature dehydration is to utilize the synergistic effect of different energy (Awad, Moharram, Shaltout, Asker, & Youssef, 2012; García-Pérez, Carcel, Benedito, & Mulet, 2007). A number of recent publications have reported the combination of microwaves and ultrasound to enhance the dehydration process and positively affects the quality properties of dried fruits and vegetables (Kowalski, Pawłowski, Szadzińska, Łechtańska, & Stasiak, 2016; Szadzińska, Kowalski, & Stasiak, 2016). Szadzińska, Łechtańska, Kowalski, and Stasiak (2017) reported that the combination of ultrasound and microwave in a convective drying process significantly shortened the drying time, reduced the energy consumption and improved the quality parameters. The combination of ultrasound and microwave in a convective drying process significantly increased the heat and mass transfer processes, thus improving the efficiency of convective drying of strawberries (Szadzińska et al., 2016). Kowalski et al. (2016) also reported an improvement of the drying kinetics as well as the energy utilization during air drying of raspberries when ultrasound and microwave were combined. The combination of ultrasound and microwave also significantly lowered the degree of shrinkage, better retained the fresh-like color, aroma and bioactive compounds (Awad et al., 2012; Santacatalina et al., 2016). Devi, Zhang, and Law (2018) determine the effectiveness of ultrasound on the microwave-assisted vacuum fried mushroom at different microwave power levels (800, 900 and 1000 W) and different frying temperature (80, 85 and 90 °C), which result with the higher moisture loss rate and lower oil content in fried products compared to VF and MVF.

However, the effect of ultrasound on the vacuum frying and the synergistic effect of ultrasound and microwave had not been discussed in the vacuum frying system. Thus, this work was aimed at combining the ultrasound and microwave to increase the energy or process efficiency and improve the quality of vacuum fried foods. The effect of low-frequency ultrasound on vacuum frying process is discussed both in the independent form and the synergism with microwave at different power levels. It was hoped that the combination of these two heat and mass transfer intensifying systems would enable frying to occur at low temperature at the same time would hasten the removal of moisture. The distribution of ultrasound power density in the frying chamber at defined height levels of oil and across the oil column (bottom to top) was comprehensively measured and discussed. The surface temperature of samples during frying was determined for better understand of the effect of ultrasound and microwave on the moisture loss and quality attributes. The enhancement of vacuum frying process by the application of combined ultrasound and microwave was evaluated using moisture loss kinetics, energy consumption, oil uptake, crispness, shrinkage, color and water activity of fried products. The microstructure of fried potato chips was comparatively observed using the scanning electron microscope.