Effects of fermentation and ultrasound pretreatment followed by fermentation on nutritional, phytochemical and in vitro bio-accessibility of veldt grape (Cissus quadrangularis L.) stem pulp

Normal fermentation (NF) and ultrasound pretreatment followed by fermentation (UPFF) using Saccharomyces cerevisiae (active dry yeast) were comparatively evaluated as bioprocessing strategies to enhance the nutritional quality, bioactive profile, and in vitro bio-accessibility of veldt grape (Cissus quadrangularis L.) stem pulp, an underutilized edible plant resource. Fermentation duration (2–5 days) was initially assessed, and the selected 3-day fermentation period was subsequently combined with ultrasound pretreatment (20–50 min). Both strategies increased phytochemical content and antioxidant activity while reducing antinutritional factors, with pronounced effects observed under UPFF. FTIR analysis indicated changes in molecular interactions without substantial alterations in major functional groups, whereas XRD and HR-SEM confirmed treatment-dependent modifications in crystallinity and structural characteristics. Based on an integrated evaluation of physicochemical, phytochemical, antinutritional and mineral profiles, NF3 and UPFF-40 were selected for simulated gastrointestinal digestion studies. UPFF-40 demonstrated higher bio-accessibility of polyphenols (66 %), flavonoids (52 %), antioxidants (73 %) and minerals (57–90 %), while the bio-accessibility of antinutritional factors remained comparatively low (19–41 %). These findings highlight that UPFF is an effective bioprocessing strategy for improving the nutritional quality and gastrointestinal bio-accessibility of veldt grape stem pulp, supporting its potential application as a sustainable and nutrient-dense functional food ingredient.