Strategies for Bioactive Compound Recovery from Grape and Apple Wastes: Traditional and Emerging Technologies to Reach Zero Waste Discharge

The presence of numerous bioactive compounds in fruit wastes indicates the potential to use them as low-cost resource for generating value-added ingredients. The integral use of residual biomass from wine and cider industries was proposed. Phenolic-rich liquid extracts were produced from grape marc (GM), grape stalk (GS), and apple pomace (AP) by applying hydrothermal (T), ultrasound-assisted (US), and enzymatic (EZ) extractions, and the remaining solid fraction (RSF) was then converted into fiber concentrated powders by air-drying. Additionally, the direct conversion of the whole initial wastes into integral powders rich in both fiber and phenolic compounds was proposed. An integral physicochemical study was performed: phenolic compounds, content and techno-functional properties of dietary fiber, and physical and stability properties. The T extraction conducted for 75 min at 75 °C demonstrated superior efficacy in terms of total polyphenol content (TPC) and antioxidant capacity (AC). Under these optimal conditions, the extracts exhibited a TPC content of 251 ± 2 mg GAE/100 g from GM, 1420 ± 5 mg GAE/100 g from GS, and 23 ± 2 mg GAE/100 g from AP. GS extract was mainly composed of procyanidins, catechin, and epicatechin, while in AP, flavonols (mainly rutin) represented the majority group. The GM extract showed a richer polyphenol profile including condensed tannins, flavonols (mainly rutin), flavan-3-ols (catechin), and a greater variety of phenolic acids. Within the fiber-rich powders produced from the RSF (optimal extraction conditions), those from grape wastes registered higher remaining TPC and total dietary fiber content (47–51