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.