The differences in metabolites fermentation characteristics
The differences in metabolites, fermentation characteristics in skim milk, and the use of lactose revealed by principal component analysis are shown in Figure 3C. Notably, D-XJ4–12 appeared to be isolated from the rest of the strains mainly due to its lack of ability to use lactose. Lactococcus lactis 5G2 was clustered with 15M2 because they share similar acetic ID-8 and lactic acid production abilities. In contrast, DQHXNQ38–12 showed the highest ketone-producing activity among the strains; it produced the highest levels of acetaldehyde, diacetyl, and acetoin.
Interestingly, 5G2 and DQHXNQ38–12, which showed roughly similar residual lactose contents in skim milk, showed considerably different phenotypic characteristics. Although these 2 strains showed similar lactose metabolism abilities, variations in their metabolic patterns could produce different flavor profiles in fermented dairy products. Our findings highlight the variations in lactose metabolism abilities among 16 strains of L. lactis, and half of those strains exhibited different activities of galactosidases with 3 strains (15M2, 5G2, and DQHXNQ38–12) having potential in application in dairy fermentation. It also suggests the requirement of further whole-genome sequence analysis to reveal the genetic basis of different phenotypic characteristics among the 8 strains that had significance variation in galactosidase activities. Evaluation of subsequent metabolic fluxes is also warranted to explain the variations in volatile compound profiles and determine the pathways involved in lactose metabolism, such as the analysis of end products and intermediate metabolites, before application in dairy fermentation.
In summary, this study presents insights into the enzymatic and phenotypic diversities of the 16 strains of L. lactis, and provides evidence to support the importance of lactose metabolism in the adaptation to dairy niche with 8 strains exhibiting a significant difference in galactosidase activities. Analysis of the flavor compounds from lactose metabolism revealed 3 strains that have the potential to be applied in the dairy industry. However, a combined investigation of the genomic and phenotypic traits of those strains could be an efficient strategy to explain the variations and recommend them strongly as starter cultures that can be applied in dairy fermentation.
Introduction Galactooligosaccharides (GOS) are naturally occurring functional oligosaccharides composed of one to seven galactose molecules linked to a terminal galactose or glucose molecule (Tzortzis & Vulevic, 2009). GOS constitute one type of prebiotic; as such, they improve intestinal flora and immunity, reduce blood fat, and help the body resist tumors and aging (Silk et al., 2009, Vulevic et al., 2013). GOS are produced from lactose through a transglycosylation process that can be catalyzed by a number of enzymes. Recently, the enzyme β-galactosidase (β-gal) has become widely used in the synthesis of GOS. β-Gals from different sources exhibit different abilities to catalyze the production of GOS from lactose. The β-gals from Aspergillus oryzae (Aoβ-gal) (Albayrak and Yang, 2002, Guerrero et al., 2014, Vera, 2011) Bacillus circulans (BgaC) (Gosling, Stevens, Barber, Kentish, & Gras, 2011), and Kluyveromyces (Martínez-Villaluenga, Cardelle-Cobas, Corzo, Olano, & Villamiel, 2008) are the most widely studied. The primary GOS produced by Aoβ-gal, 6′-O-β-galactosyl-lactose, exhibits an excellent probiotic effect (Urrutia et al., 2013). The optimal temperature for Aoβ-gal activity ranges from 45 to 55 °C (Guidini et al., 2010, Güleç et al., 2010) with an optimal pH for lactose transglycosylation of 4.8 (Tanaka, Kagamiishi, Kiuchi, & Horiuchi, 1975). These optima are consistent the use of Aoβ-gal for the industrial production of GOS. However, the low GOS yield of Aoβ-gal greatly limits its industrial application (Yin, Bultema, Dijkhuizen, & van Leeuwen, 2017). Therefore, increasing Aoβ-gal’s GOS yield has become the focus of a great deal of research.