Influences of combined microwave-vacuum frying on the quality of durian chips

Abstract

This study aimed to determine the effects of process conditions on the drying kinetics of vacuum-fried durian chips. Additionally, the purpose of this research was to determine the most appropriate thin layer equation for predicting product drying kinetics. At an absolute chamber pressure of 8 kPa and frying temperatures of 90, 100, 110, and 120°C, studies on frying were conducted. The influence of microwave power levels (400, 480, 560, and 640W) and ripening levels (1 Day, 2 Days, 3 Days, and 4 Days after harvesting) at the optimal frying temperature was also investigated. The durian was fried until the final moisture content of durian reached 6.0% (w.b.). The fried products were then analyzed for the physical characteristics, as well as the sensory quality. These characteristics were taken into consideration when determining the optimal conditions for microwave-assisted vacuum frying (MWVF) of products. According to this study, MWVF significantly reduced the frying time by 20% compared to VF (P < 0.05) with the effective moisture diffusivity for VF, pre-treatment before vacuum frying (P+VF) and MWVF at 110°C determined to be 4.59 × 10−09, 5.62 × 10−09 and 6.11 × 10−09 m2 s−1, respectively. Additionally, eight mathematical models (Lewis, Page, Logarithmic, Demir et al., Midilli, Approximation of Diffusion, Logistic, and Henderson and Pabis) describing thin-layer drying were investigated. It was found that the Midilli model was considered adequate for describing the thin-layer drying behavior of VF, P+VF, and MWVF. MWVF produced chips with lower shrinkage, more yellowness and higher crispiness (consistent with the microstructure from scanning electron microscopy (SEM)). In addition, MWVF drastically decreased oil absorption by 10%, while P+VF yielded the lowest specific energy consumption (SEC). MWVF with the highest microwave power produced the quickest frying time, the lowest SEC, and a more porous structure in comparison to those from lower power levels. These findings were obtained at 110°C and absolute pressure of 8 kPa. Moreover, the ripeness of durian led to longer frying times with significantly less shrinkage, higher oil absorption, and more yellowness for durians from Day 3 compared to Day 1. Furthermore, the more ripened durians produced a bigger internal pore size and crispier durian chips. From sensory evaluation, the taste panellists gave a higher rating for the durian chips with higher ripeness, indicating a preference for ripened products compared to conventionally fried un-ripened durian chips.