Kinetics of cashew apple drying through mechanistic models and analysis of the effects of drying conditions on the retention of bioactive compounds

Abstract

Global cashew nut production is nearly 4 million tons per year, valued at 7 billion US dollars. Remarkably, almost the entire cashew apple crop, amounting to 20 million tons annually, goes to waste. However, the cashew apple contains valuable nutraceutical compounds, including tannins, polyphenols, and carotenoids, estimated to be worth 150 million US dollars annually. Due to the highly perishable nature of cashew apples, degradation is a significant issue. In response, the current work has established drying as an effective preservation technique for these bioactive components. The effect of drying temperature on bioactive compounds has been thoroughly investigated. The non-random two liquid (NRTL) activity coefficient model effectively captures the thermodynamics of the drying process. To facilitate the selection and design of drying equipment, two mechanistic mass transfer models were developed. The first model employs the Maxwell-Stefan framework to account for internal diffusion, with external mass transfer resistance appearing as a boundary condition. While this model works well for products like grapes, it proved inadequate for explaining the drying behaviour of cashew apples. Consequently, a second model was developed, postulating rapid moisture transport by capillary action within the cashew apple. This model effectively captures the effects of a wide range of operating conditions, using only external mass transfer resistance as the tuneable kinetic parameter. This mechanistic model is more suitable for dryer design compared to conventional phenomenological models like the logarithmic model and the two-term exponential model.