Effect of Different Drying Treatments and Sieving on Royal Gala Apple Pomace, a Thickening Agent with Antioxidant Properties

Currently, there is an increasing interest in the search of natural derived materials as valuable substitutes for microplastics. One of the categories investigated, represented by thickening agents deriving from agri-food waste and apple pomace (AP), was considered of interest. In this study AP was submitted to three different treatments and drying conditions (oven drying at 55 °C for 12 h; homogenization and oven drying at 55 °C for 12 h; homogenization and freeze-drying), and then grinded and sieved obtaining three different dimensional fractions (>400 µm, 250-400 µm and <250 µm). The hydroalcoholic extracts of these fractions, obtained by ultrasound-assisted extraction, were analyzed to compare their total phenol content (TPC), antioxidant properties, and phenol profile. Correlation studies between the above-indicated parameters were also carried out. The highest values of TPC, antioxidant capacity, and phenol content (determined by liquid chromatography) were found for oven dried AP (250-400 μm) or homogenized and freeze-dried (>400 μm) samples. Both samples were most suitable to form stable hydrogels and the sample obtained after drying at 55 °C showed the best performances in terms of ability to form a stable hydrogel. Among the studied treatments and drying conditions, the oven dried AP was demonstrated to be an interesting stabilizing material with potential applications in many fields (such as food, cosmetics, and nutraceuticals) showing both antioxidant activity and thickening capacity.