Functional Watermelon Jelly Fortified with Potato By-Product-Derived Single-Cell Protein: Antioxidant and Enzyme Inhibitory Potential with Possible Health-Promoting Properties

Watermelon (Citrullus lanatus) is a widely consumed fruit rich in carotenoids, phenolic compounds, vitamins, and other bioactive constituents associated with antioxidant and health-promoting properties. The present study aimed to develop novel low-sugar watermelon jelly formulations enriched with pectin and single-cell protein (SCP), and to evaluate their bioactive and functional properties, including the antioxidant and enzyme inhibitory activities of their 70% ethanol (70% EtOH) extracts. Six jelly formulations (W1–W6) differing in pectin and SCP supplementation were prepared using concentrated watermelon pulp processed under low-temperature vacuum conditions to preserve thermolabile bioactive compounds. The total phenolic content (TPC) and total flavonoid content (TFC) were determined in the 70% EtOH extracts using spectrophotometric methods. TPC remained relatively stable among the investigated formulations, with the highest value recorded for sample W6, which contained 0.58 g pectin and 0.215 g SCP (1.37 mg gallic acid equivalents (GAE)/mL extract). The highest TFC was observed for sample W5, corresponding to the formulation supplemented with 0.5 g pectin and 0.300 g SCP (5.87 mg quercetin equivalents (QE)/mL extract). Antioxidant activity, evaluated using ABTS, DPPH, and FRAP assays, demonstrated considerable antioxidant potential in all 70% EtOH extracts. The strongest antioxidant activity was observed in pectin-rich samples, with ABTS and FRAP values reaching 89.06 mg Trolox equivalents (TE)/mL extract and 8.99 mg ascorbic acid equivalents (AAE)/mL extract, respectively. The potential carbohydrate-hydrolyzing enzyme inhibitory activity of the 70% EtOH extracts was assessed through α-amylase and α-glucosidase inhibition assays. The formulation containing both pectin and SCP exhibited the strongest α-amylase inhibitory activity (58.93%), whereas the highest α-glucosidase inhibition (60.54%) was observed in the pectin-containing formulation without SCP supplementation. These findings indicate that both pectin and SCP significantly influenced the nutritional and bioactive properties of the developed watermelon jelly formulations. Overall, the results demonstrate that enrichment with pectin and SCP significantly enhances the nutritional and bioactive properties of watermelon jelly formulations. These findings highlight indicate that pectin and SCP were associated with differences in the bioactive and functional properties of the developed watermelon jelly formulations. Overall, the results demonstrate the potential of pectin- and SCP-enriched watermelon jellies as sustainable functional food products with promising antioxidant and enzyme inhibitory properties.