Changes in bioactive compounds and antioxidant activity during fermentation and in vitro digestion of melon and pumpkin leaves smoothies

This research investigated the effects of lactobacillus strains (LABs) Lactiplantibacillus plantarum (L75) or Bifidobacterium longum (BF) fermentation and subsequent in vitro digestion on bioactive compounds and antioxidant activity in smoothies made from the combination of watermelon/cantaloupe melons with Cucurbita moschata or C. pepo leaves. This study attempts to fill a knowledge gap regarding strain-specific and matrix-dependent biotransformation in underutilized plants to different LABs. The results revealed that both LABs maintained viability >7 Log CFU/mL during in vitro digestion, thus achieving the colonisation thresholds for probiotic effects. Fermented samples and in vitro digestion revealed clustering of bioactive compounds caused by strain-specific and matrix-dependent biotransformations. In all cantaloupe and pumpkin leaves (Cucurbita moschata or Cucurbita pepo) smoothies fermented with L75 or BF discriminated from all fermented watermelon and pumpkin leaf smoothies based on the increase in trans β-carotene (4322.23 mg/100 mL) and cis β-carotene (186.43 mg/100 mL) in the intestinal phase (IP). In contrast, watermelon and C. moschata or C. pepo leaves smoothies fermented with 75 or BF discriminated from the fermented cantaloupe and pumpkin leaf smoothies by displaying higher levels of epicatechin (0.190 mg/100 mL), hesperidin (0.158 mg/100 mL), gallic acid (0.843 mg/100 mL), and quercetin (0.133 mg/100 mL) in the IP. Antioxidant activity peaked post-digestion in watermelon-C. moschata+BF smoothies. Food matrix influences compound stability, bioaccessibility, and strain efficacy. For functional foods, fermenting cantaloupe-pumpkin leaves with L75 maximises carotenoids, while watermelon-pumpkin leaves with L75 phenolics in the IP. Our study demonstrated that fermentation using strain L75 or BF enhances the bioavailability of active compounds in melon-pumpkin leaf smoothies, offering a novel method for functional food design.