Phenolics and ascorbic acid in pumpkin (Cucurbita maxima) slices: effects of hot air drying and degradation kinetics

This study was aimed to investigate the effects of hot air drying (60–100 ℃, 1.5 m/s, 6–17 h; final products contained 10.50 ± 0.29% moisture) on the levels of total phenolic compounds (TPC) including both soluble phenolic compounds (SPC) and insoluble phenolic compounds (IPC) as well as ascorbic acid in pumpkin slices. IPC accounted for an average of 74.9% of the TPC in fresh pumpkin flesh, but only accounted for 49.4% (100 ℃ drying) to 59.7% (60 ℃ drying) of the TPC in the final dried pumpkin slices. As the drying temperature increased, the retention of IPC (22.8–42.7%), TPC (41.6–50.9%), and ascorbic acid (22.2–51.0%) in the dried products significantly decreased ( p  < 0.05), but the retention of SPC (74.5–92.7%) significantly increased which may be due to more conversion of IPC to SPC at a higher temperature. The degradation of ascorbic acid during hot air drying fitted well to the first-order kinetic function (R 2  = 0.985–0.999), and that of IPC, SPC and TPC fitted well to zero-order reactions (R 2  = 0.887–0.997). Unlike the other three compounds, the reduced level of SPC decreased as the temperature increased ( k  = 0.008–0.029/h, Ea  = 33.7 kJ/mol), corresponding to the increased retention of SPC in the final dried pumpkin slices.