Sustainable Preservation of Opuntia ficus-indica Peel Waste for Resource Recovery Through Pretreatment and Convective-Drying Processes

Opuntia ficus-indica peel waste (OPW) accounts for approximately 50% of the fresh fruit weight, a relatively higher rate in comparison to other fruits, making it a particularly abundant and underutilized agricultural by-product. This study investigates Tunisian OPW through convective drying (50, 60, 75, and 85 °C) using four different pretreatments: no pretreatment (Fresh OPW), dipped peels in acetic acid solution (AA OPW), frozen and thawed peels (FZ OPW), and dipped–frozen combined pretreatment (AA-FZ OPW). The drying kinetic evaluation clearly demonstrated an increase in drying rate by lowering the required drying time for the various pretreatments and drying temperatures. This drying time decreased by 8.64–17.39%, 23.46–36.70%, and 28.39–39.45% for AA OPW, FZ OPW, and AA-FZ OPW, respectively, as compared to fresh OPW. The total phenolic and betalain contents, found in dried OPW, ranged from 7.87 ± 0.254 g GAE/100 g dw to 12.29 ± 0.421 g GAE/100 g dw and from 22.74 ± 2.34 mg/100 g dw to 59.86 ± 3.24 mg/100 g dw, depending on the drying temperature and pretreatment applied. This study employs thin-layer and artificial neural network (ANN) modelling, finding that both methods produce highly accurate moisture-ratio prediction during the drying process. The peels from Opuntia ficus-indica have considerable potential to be reused as functional ingredients.