Upcycling Grape Pomace in a Plant-Based Yogurt Alternative: Starter Selection, Phenolic Profiling, and Antioxidant Efficacy on Human Keratinocytes

Due to its appealing composition, grape pomace (GP), the major by-product of the wine industry, could be considered an ideal candidate for innovative functional foods development. In this study, a rice/GP-yogurt alternative, also known as gurt, fermented with selected lactic acid bacteria, was designed. An extensive characterization of the gurts led to the selection of the one fermented with <i>Lactiplantibacillus plantarum</i> T0A10. The strains showed good pro-technological performances (fast acidification and growth up to 9 log cfu/g in the specific plant-based composite substrate), as well as the ability to increase DPPH radical scavenging activity compared to the unfermented control (57% against 40%). Then, an in-depth focus on the effect of fermentation on phenolic compounds and their related antioxidant efficacy on human keratinocytes was provided, elucidating a compound/function relationship. Fermentation significantly modified the phenolic profile of the gurt, reducing glycosylated forms of flavonols and phenolic acids and increasing the content of catechin and pyrogallol (more than 100 mg/kg combined). Such modification was responsible for significantly up-regulating (<i>p</i> < 0.05) the expression of the antioxidant enzyme superoxide dismutase 2, thus protecting NCTC 2544 cells against oxidative stress. Overall, these findings provide a foundation for developing value-added products from GP, supporting both circular economy initiatives and functional ingredient innovation.