The effect of co-fermentation of Lactobacillus paracasei subsp. paracasei and Streptococcus thermophilus on functional components, aroma compounds and non-volatile metabolites in ‘Summer Black’ grape juice

This study screened and identified suitable strains for ‘Summer Black’ grape juice (SBGJ) fermentation, and further analyzed the functional ingredients, volatile organic compounds (VOCs), and non-volatile metabolites (NVMs) in SBGJ produced through single-inoculated fermentation and co-inoculated fermentation. The experiment determined LP as the optimal strain for single-inoculated fermentation and LS as the optimal mixed-inoculated, with the best mixing ratio of LP:LS = 2:1 (LPS). The results demonstrated that both the individual fermentation of LP and its co-fermentation with LS enhanced the functional components of SBGJ. Specifically, LP fermentation increased citric acid by 17.9% and LPS by 3.9%, while also producing gallic acid. In addition, LPS produced more floral and fruity aroma compounds (e.g., linalool, citral, geraniol) than LP. Non-targeted metabolomics analysis indicated that the related metabolic pathways were mainly enriched in flavonoid biosynthesis, amino acid biosynthesis and carbohydrate metabolism. Among them, the key metabolites closely related to flavonoid biosynthesis and branched-chain amino acid metabolism significantly increased, further enhancing the nutritional value of SBGJ. Overall, mixed fermentation demonstrated greater advantages in quality and flavor improvement. This study provides a theoretical basis for enhancing SBGJ through lactic acid bacteria (LAB) fermentation and offers insights into processing technology improvements. • LS and LP were selected based on sensory evaluation and viable cell count. • Mixed bacterial fermentation significantly enhances the antioxidant capacity of SBGJ. • Single-strain fermentation promotes the generation of acidic VOCs. • Mixed bacterial fermentation enriches the flavor of SBGJ. • Metabolic analysis shows that mixed fermentation has advantages.