“Technological properties and genotyping of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus strains isolated from home-made yogurt” , K. Pashova-Baltova, V. Andreeva, Z. Urshev, M. Michaylova, S. Minkova, M. Fukui, K. Isawa The development of industrial LBB starters for yogurt relies on the abundance of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus strains isolated from natural sources. As yogurt in Bulgaria has been traditionally prepared long before any industrialization, home-made yogurt proved to be an excellent source for isolation of such strains. In an integrated study yogurt bacteria were isolated from 28 samples of home made yogurt, collected from mountain regions of Bulgaria. Of all isolates 73 were defined as L. delb. ssp. bulgaricus and 117 as S. thermophilus. Parallel studies on their basic technological properties and genotype were performed. The counts of viable cells in fermented milk (16h) for L. delb. ssp. bulgaricus strains were in the range of 3.5 x 107 – 8.4 x 108 cfu/ml (mean 2.9 x 108, n = 66) and for S. thermophilus strains varied from 1.4 x 108 cfu/ml to 1.1 x 10 9 cfu/ml (mean 4.8 x 108 cfu/ml, n = 111). The acidity of milk fermented at 37oC with L. delb. ssp. bulgaricus strains varied from 0.90 % lactic acid (%LA) to 1.54 %LA (16h) and from 1.01 %LA to 1.77 %LA (40h). The same parameters for S. thermophilus strains were in the range of 0.91-1.24 % LA (16h) and 0.96-1.33 % LA (40h). Acidity of fermented milk in the presence of 15% sugar syrup was reduced to 19.5-63.2% (mean 30.9 %, n = 66) for L. delb. ssp. bulgaricus strains and to 16.2-86.3 % (mean 58.5% n = 111) for S. thermophilus strains compared to the control. The kinematic viscosity of filtered whey ranged from 0.842 cSt to 1.204 cSt and from 0.890 cSt to 1.489 cSt when milk was fermented with L. delb. ssp. bulgaricus or S. thermophilus, respectively. Genotyping of the strains with PFGE resulted in the identification of 29 types of L. delb. ssp. bulgaricus and 42 types of S. thermophilus. Interestingly the majority of home-made yogurt samples contained more than one strain of L. delb. ssp. bulgaricus and two or more S. thermophilus strains. Both the study on the basic technological properties of these cultures and genotyping revealed extreme diversity of the phenotype and genotype of yogurt bacteria strains isolated from the home-made product. No correlation could be made between genetic typing data and the results from the technological studies. Notably, even isolates which were grouped in one genetic type demonstrated considerable variation in their technological properties. The diversity of the properties of the isolated strains proves to be a good prerequisite to the development of starters for different fermented milk products, while genetic identification makes the application of the cultures more reliable.