Properties of exopolysaccharide producer Streptococcus thermophilus ST8.01 isolated from homemade yoghurt

A total of 38 presumptive Streptococcus thermophilus strains were isolated from homemade yoghurts from Turkey. Isolated strains were analysed for exopolysaccharide (EPS) production by evaluating the colour of the colonies grown in ruthenium red milk agar. Only one strain, ST8.01, produced EPS and was selected for further analyses. As a result of the identification tests, EPS producer strain was identified as Strep. salivarus subsp. thermophilus ST8.01. The presence of epsCD genes in Strep. thermophilus ST8.01 was determined and epsCD fragments were sequenced. The epsCD fragment showed from 89% to 94% similarity with EPS synthesis gene clusters of different Strep. thermophilus strains. The amount of EPS produced in M17 broth at 42 degrees C for 24 h was found to be (132 +/- 3.0) mgl(-1). The viscosity of the culture of strain ST8.01 in skim milk was determined to be (142 +/- 5.0) mPa.s. The acid production experiment showed that ST8.01 strain was a fast acidifying strain. The proteolytic activity of ST8.01 strain was determined to be (0.15 +/- 0.01) mg of tyrosine per 1 ml. Acetaldehyde was detected as a major flavour compound produced by Strep. thermophilus ST8.01. The results of this study suggest that Strep. thermophilus ST8.01 strain may be used as commercial starter culture to manufacturing dairy products.