Milk fermentation by monocultures or co-cultures of Streptococcus thermophilus strains

Abstract

Direct vat-set starter cultures are the key ingredient for the production of fermented dairy products. The characteristics of the strains used for fermentation determine the fermentation time, texture and flavor of the fermented milk products. In this study, a large-scale analysis of the acid production rate, texture, carbon source utilization characteristics of Streptococcus thermophilus strains was conducted. All 100 S. thermophilus strains were divided into six groups according to the acid production rate and into two groups according to the consistency texture. A universal medium, basing on the carbon sources metabolic properties were optimized (0.5% lactose and 3.5% glucose), to culture all of the tested strains. Among them 40 strains were used to test pH-controlled conditions using this universal culture medium. After 5-7 h of fermentation, the optical density (OD) values of all fermented products exceeded 10, suggesting the potential for high-density cultivation of S. thermophilus. Although the OD could be further increased by adding more glucose, this may have hindered subsequent lyophilization because of high residual lactic acid in the fermented product. Next, the application of Streptococcus thermophilus strains in fermented milk was studied. Monocultures and co-cultures of strains were evaluated and compared. The results revealed the existence of symbiotic or competitive relationships between different S. thermophilus strains. Based on the findings, the mixing ratio of three symbiotic S. thermophilus strains was optimized. A co-culture of these three strains yielded fermented milk with high viscosity, low post-acidification, good sensory properties and processability.

Keywords: Streptococcus thermophilus; acidification rate; fermented milk; post-acidification; viscosity.

FIGURE 1

Fermentation characteristics of 100 S. thermophilus strains. (A) Correlation analysis between lactic acid production in fresh milk (fermentation time 6 h) and OD. (B) Correlation analysis between lactic acid production in UHT milk (fermentation time 5 h) and OD. (C) Classification of 100 S. thermophilus strains according to their acid production characteristics in fresh milk (fermentation time 6 h) and UHT milk (fermentation time 5 h). (D) The viscosity and texture of 100 S. thermophilus strains in fresh milk for 8 h fermentation.

FIGURE 2

Analysis of 100 S. thermophilus strains by carbon sources use ability. (A) PCA analysis results, the value of coordinate axis PC1/2 is the explanation rate of the overall difference, the arrow represents the original variable, where the direction represents the correlation between the original variable and the principal component, and the length represents the contribution of the original data to the principal component. (B) Strains were divided into three groups according to their utilization of carbon sources by cluster analysis.

FIGURE 3

pH controlled fermentation of 5 glucose-non-fermented (A) and 5 glucose-fermented (B) S. thermophilus strains.

FIGURE 4

Fed-batch fermentation of strain ST20 and callapsed cakes left in drying trays in different fermentation time.

FIGURE 5

Post-acidification of different S. thermophilus strains after the titration acidity reached 70°T (A) and titration acidity of ST49 and ST53 fermentation mixed with other strains for 6 h (B).

FIGURE 6

Acid production curve of two strains (Mean of 3 measurements).

FIGURE 7

The viscosity, post-acidification (A) and sensory evaluation (B) of fermented milk produced by different mixed strains compounds. A ST53:ST49:ST50 = 1:1:1 B ST53:ST49:ST50 = 1:3:2 C ST53:ST49:ST50 = 3:1:2 D ST53:ST49:ST50 = 2:2:1 E ST53:ST49:ST50 = 1:2:2.

FIGURE 8

Rheological properties of the fermented yogurt. (A) Flow ramp of yogurt in linear (B) Flow ramp of yogurt in Log (C) Linear viscoelastic region test (D) Oscillation frequency test.