Differentiated assessment of Lactococcus enzymatic activity for the creation of starter cultures of direct application

Representatives of the genus Lactococcus are one of the key starter microorganisms in the dairy industry. They play a leading role in lactic acid fermentation in the production of fermented dairy products and cheeses. The article presents the results of a differentiated assessment of the enzymatic activity of nine strains of Lactococcus lactis subsp. lactis in milk at a cultivation temperature of (30 ± 1) °C, as well as during cooling and storage at a temperature of (6 ± 1) °C for 21 days. All studied parameters of acid formation (an increase in titratable acidity during fermentation and storage) demonstrated statistically significant differences between the strains (ANOVA, P < 0.05). The most pronounced variability of the strains was observed in their ability to acidify milk during storage: a decrease in the actual acidity over 21 days ranged from 0.19 to 0.49 pH units. This may be due to their genetic differences in enzymatic activity and resistance to low temperatures, which requires further study. The highest activity at (30 ± 1) °C was observed in four strains (663–12, 792–7, 618–5 and 549–1), which increased the titratable acidity of milk by 69–70 °T. Three of these strains (792–7, 618–5, and 549–1) slowly accumulated acid during storage, increasing the acidity by 4.7–7.3 °T over 21 days. Strains 637–4 and 429–6 showed a low fermentation rate at optimal temperature (an increase in acidity was 56.7 and 54.3 °T, respectively), but a high one during storage (an increase in acidity was 18.3 and 16.7 °T, respectively). Cluster analysis by the Ward method and the Euclidean distance metric performed in the R4.3.1 software environment allowed us to divide the studied L. lactis cultures into three groups according to the nature of the enzymatic profile at the temperature of active fermentation and under the storage conditions. The isolated clusters represent various metabolic profiles that determine the potential use of strains in technological processes. The results obtained indicate that for a comprehensive assessment of the acid-forming activity of cultures when selecting them for using in the composition of starter cultures of direct application for fermented dairy products and cheeses, it is necessary to study both the rate of acid formation and the post-enzymatic potential.