“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.