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  • In total strains with relatively rapid

    2022-01-20

    In total, 5 strains with relatively rapid acid-producing ability (MA14, 15M2, DQHXNQ38–12, 5G2, and 6G5) were selected for further analysis. Compared with the strains ATCC19435, ZN8, and D-XJ4–12, which were used as controls, these 5 strains exhibited significantly different growth and titratable acidity when cultured in skim milk (Figure 2A and 2B). The titratable acidity of L. lactis 5G2 and 15M2 obtained from fermented yak milk reached 85°T after 12 h, whereas that of ATCC19435, MA14, ZN8, 6G5, and DQHXNQ38–12 reached 30°T to 50°T and that of D-XJ4–12 changed only slightly. Therefore, strains that had similar growth rates and cell densities when grown in the MRS-lactose broth demonstrated significantly different Methyllycaconitine citrate characteristics when grown in skim milk, likely due to the variation in the nutrients such as intact milk protein (80% casein, 20% whey protein) in skim milk and tryptone in MRS-lactose broth. The results of residual lactose and galactose contents are shown in Figure 2C. Methyllycaconitine citrate Lactococcus lactis 5G2, 15M2, DQHXNQ38–12, and ZN8 consumed approximately 10 to 16% of the lactose after 12 h of fermentation, whereas L. lactis 6G5, MA14, and ATCC19435 consumed only approximately 5%, and D-XJ4–12 did not significantly change the lactose content. In addition, the reduction in galactose levels after fermentation varied among the strains, with the most significant reduction in the galactose content shown by ZN8. It is known that L. lactis NCDO2054 expresses β-galactosidase to catalyze the intracellular lactose (Vaughan et al., 1998), whereas many other strains, such as L. lactis NCDO712, SK11, and UC509.9, exhibited the T6P pathway for lactose utilization (Gasson, 1983; Wu et al., 2015). Hence, the variation in the use of lactose among L. lactis strains was further investigated, and the results are shown in Figure 2D. Lactococcus lactis ATCC19435, DQHXNQ38–12, 6G5, 5G2, 15M2, and MA14 exhibited 6-P-β-galactosidase activity, with the highest activity shown by strain ATCC19435, a type strain isolated from the dairy niche with the complete genes for T6P pathway in the draft genome (Fujii et al., 2015). In a previous study, the 6-P-β-galactosidase activity of 65 LAB strains of 15 species, including L. lactis, was reported to range from to 50 units (Honda et al., 2007). It has been reported that LAB may obtain the phospho-β-galactosidase gene via horizontal gene transfer while co-existing with other bacteria, such as Lactobacillus casei, as a previous study suggested that this enzyme is plasmid-encoded in L. lactis and chromosome encoded in L. casei (Wu and Shah, 2017). In contrast, among the 8 strains (ATCC19435, MA14, 15M2, 5G2, 6G5, DQHXNQ38–12, ZN8, and D-XJ4–12), only ZN8 expressed β-galactosidase. This finding is consistent with that of Aleksandrzak-Piekarczyk (2013), who showed that the lactose permease β-galactosidase system plays a minor role in L. lactis strains. Although the strains exhibiting 6-P-β-galactosidase activity were isolated from various dairy niches, we need to be cautious to draw conclusions regarding the connection between ecological niches because the β-galactosidase and 6-P-β-galactosidase activities were not observed for D-XJ4–12, a strain separated from koumiss. Therefore, classifying the L. lactis strains into domesticated and environmental strains is more appropriate because we do not possess the information of the history of the strains in specific environments (Passerini et al., 2010). In addition, the slow growth of D-XJ4–12 in skim milk may correlate with the lack of enzymatic activities associated with the use of lactose. Taken together, the results indicate variation among L. lactis strains in the ability of lactose utilization, via the T6P pathway, via the Leloir pathway, or both. Figure 3A and 3B presents the concentrations of typical volatile compounds, including lactic acid, acetic acid, diacetyl, acetoin, and acetaldehyde, considered as the main metabolites of lactose metabolism. The highest concentration of lactic acid was present in the sample of 5G2, and that of acetic acid was present in the sample of 15M2. However, diacetyl, acetoin, and acetaldehyde, which are desirable compounds in fermented milk, were present in highest concentrations in the sample of DQHXNQ38–12. Lactic acid, acetic acid, and acetaldehyde are the important metabolites of lactose when metabolized via the pyruvate metabolism pathway, and pyruvate can also be metabolized to flavor compounds such as diacetyl and acetoin, which play important roles in the flavor of dairy products (Stefanovic et al., 2017).