Effect of sublethal preculturing on the survival of probiotics and metabolite formation in set-yoghurt

• Lactobacillus rhamnosus GG and Bifidobacterium lactis BB12 were stressed prior to inoculation in yoghurt.
• Sublethal preculturing was performed under elevated NaCl and low pH conditions.
• Sublethal preculturing did not improve the growth of LGG and BB12 in milk.
• Sublethal preculturing improved the survival of LGG and BB12 in yoghurt.
• Stress-adapted LGG and BB12 influenced certain aroma volatiles of yoghurt.

The objective of this study was to investigate the effect of preculturing of Lactobacillus rhamnosus GG and Bifidobacterium animalis subsp. lactis BB12 under sublethal stress conditions on their survival and metabolite formation in set-yoghurt. Prior to co-cultivation with yoghurt starters in milk, the two probiotic strains were precultured under sublethal stress conditions (combinations of elevated NaCl and low pH) in a batch fermentor. The activity of sublethally precultured probiotics was evaluated during fermentation and refrigerated storage by monitoring bacterial population dynamics, milk acidification and changes in volatile and non-volatile metabolite profiles of set-yoghurt. The results demonstrated adaptive stress responses of the two probiotic strains resulting in their viability improvement without adverse influence on milk acidification. A complementary metabolomic approach using SPME-GC/MS and 1H-NMR resulted in the identification of 35 volatiles and 43 non-volatile polar metabolites, respectively. Principal component analysis revealed substantial impact of the activity of sublethally precultured probiotics on metabolite formation demonstrated by distinctive volatile and non-volatile metabolite profiles of set-yoghurt. Changes in relative abundance of various aroma compounds suggest that incorporation of stress-adapted probiotics considerably influences the organoleptic quality of product. This study provides new information on the application of stress-adapted probiotics in an actual food-carrier environment.