Influence of Pretreatments on the Hot-Air-Drying Kinetics and Bioactive Compounds of Pumpkin and Its By-Products

Hot air drying of pumpkin (Cucurbita moschata) and its by-products, mainly the peel and placenta with seeds, has been investigated, analysing the influence of pretreatments on drying kinetics and bioactive compound content. Pumpkin flours not only stand out for their microbiological stability (low water activity), but also for their bioactive compounds important for health, including phenolic compounds and other antioxidants. Pretreatments prior to drying may improve both the quality and the drying efficiency, although their optimization has not been studied in pumpkin by-product flours. Hence, we studied different pretreatments, such as steam blanching (SB) and freezing (F) (−18 °C), to investigate their effect on the pumpkin by-product flour quality (color and water activity) after drying and compared to flours made with the edible part (pulp). In addition, different drying kinetics models were evaluated. SB pulp and peel achieved lower water activity (0.25/0.20) than F (0.35/0.36), compared to untreated pumpkin by-product flour (0.40/0.45). The SB placenta with seeds achieved a lower water activity (0.19) than F (0.55). The total phenolic content (TPC) increased up to 41.7, 60.2 and 40.9% in pre-treated and dried pulp, peel and placenta with seeds, respectively, compared to control (CTRL). A similar result was obtained for total flavonoid content (TFC), where increases of up to 86.4, 36.4, and 32.2% were observed in pre-treated pulp, peel and placenta with seeds, respectively. For total antioxidant capacity (TAC), results showed an increase of up to 33.1% in pre-treated peel, with slight reductions in pulp (20.1%) and placenta with seeds (39.1%), compared to CTRL. These results are promising for obtaining new dehydrated plant products from industrial processing by-products, while maintaining their beneficial health characteristics. These powdered products can be used in future research on the formulation of new fortified foods, such as bakery products, or the development of natural additives for beverages or soups.