Evaluation of GABA-Producing Fermented Whey Formulations: From Strain Selection to Raspberry-Enriched Beverages with Psychobiotic Potential

Certain probiotic strains have been proposed to alleviate mental health conditions, such as anxiety and stress, by modulating the gut-microbiota-brain axis through the production of metabolites like gamma-aminobutyric acid (GABA). This study evaluated kefir-derived microbial strains for their GABA-producing capacity in mono- and co-culture systems using whey as the growth substrate. Based on the screening results, two microbial consortia were selected to develop fermented whey beverages with raspberry (FWF-R1 and FWF-R2). These beverages were characterized for their technological and functional properties over 21 days of refrigerated storage and following gastrointestinal digestion. Both formulations maintained stable acidity and showed a slight increase in viscosity during storage. The microbial counts remained above 8.5 log colony-forming units/mL, with high post-digestion viability, confirming their probiotic potential. The GABA levels increased progressively during storage, reaching 2.67 mM in FWF-R1 and 4.65 mM in FWF-R2, with recovery rates of 40-45% after digestion. The total phenolic content decreased moderately during storage but increased ~5-fold after digestion; the total anthocyanins declined by up to 70%. FWF-R2 achieved higher sensory acceptability and was preferred by 58% of consumers, emerging as the most promising formulation. These findings highlight the psychobiotic potential of these beverages and support the sustainable valorization of dairy and fruit by-products.