Innovative Drying Techniques for Purple Prickly Pear Peels: Drying Kinetics and Bioactive Compounds

The present study investigated the effect of two techniques for refractance window (RW) drying on its drying kinetics and bioactive compounds of red prickly pear peels (Opuntia ficus indica), such as antioxidant capacity, phenolics and betalains. First, it assessed the effect of ethanol applications before RW drying at 50°C, 60°C, 70°C, and 80°C, finding that these treatments did not significantly accelerate drying processes. Later, three drying methods were evaluated: RW, ultrasound-assisted refractance window (RW US) and convective air drying (AC), at 50°C and 70°C. The results showed that ultrasound accelerates RW drying by reducing around 40% of drying time at 50°C to reduce moisture to approximately 10% (w.b.). Further, RW US was the best method to preserve antioxidant activity. Although the samples dried at 70°C significantly had more betalains than the samples dried at 50°C, the betalains extracted from the samples dried at 50°C were more stable for 21 days at 4°C, especially using RW US. In conclusion, RW US showed to be an efficient and promising alternative to stabilize the bioactive compounds of red prickly pear peels, strengthening its potential for further applications in foods, beverages, and other functional products.