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 celcius, 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 celcius drying) to 59.7% (60 celcius 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.

Automated summary

The effects of hot air drying on phenolic compounds and ascorbic acid in pumpkin slices were investigated in this study. Results showed that as the drying temperature increased, the retention of total phenolic compounds and ascorbic acid significantly decreased, while the retention of soluble phenolic compounds significantly increased. The degradation of ascorbic acid followed a first-order kinetics while phenolic compounds degradation followed zero-order reactions.