Inoculum size of co-fermentative culture affects the sensory quality and volatile metabolome of fermented milk over storage.

Lacticaseibacillus paracasei PC-01 is a probiotic candidate isolated from naturally fermented yak milk in Lhasa, Tibet, and it has been shown to possess excellent milk fermentation properties. This study used Lacticaseibacillus paracasei PC-01 as a co-fermentation strain to investigate the effect of inoculum size with a commercial starter in milk fermentation on the product flavor and profile of volatile metabolites over 28 d of cold storage. Lacticaseibacillus paracasei PC-01 was allowed to ferment in pasteurized milk with or without the commercial starter (YF-L904) at 42°C until the pH decreased to 4.5. The finished fermented milks were stored at 10°C for 28 d. Milk samples were taken at hour 0 (before fermentation) and then at d 1, 14, and 28 of cold storage. Different inoculum sizes of Lacticaseibacillus paracasei PC-01 had no significant effect on pH or titratable acidity during storage of fermented milk. Viable counts of strain PC-01 continued to increase during cold storage of the fermented milk. Generally, as storage of fermented milk proceeded, the overall sensory quality score decreased in all groups. However, the overall sensory scores of PC-01-M were generally higher than those of other groups, suggesting that a medium dose of Lacticaseibacillus paracasei PC-01 had the most obvious effect of slowing the decline in sensory quality of fermented milk during storage. Changes in sensory scores and consumer preferences were accompanied by increases in both the quantity and variety of key volatile metabolites in fermented milk during fermentation, post-ripening (d 1), and storage. Major differentially abundant metabolites, including acetaldehyde, methyl ketones, medium-chain and short-chain fatty acids, 2,3-butanedione, and acetoin, were enriched in fermented milks rated highly in the sensory evaluation. Our data confirmed that the inoculum size of co-fermentative culture affected the sensory quality and volatile metabolome of fermented milk over storage, and an optimal range of co-fermentative culture was titrated in this work.