Characterization and classification of Streptococcus thermophilus and Lactobacillus delbrueckii subsp bulgaricus strains isolated from traditional Greek yogurts

Streptococcus thermophilus (74 strains) and Lactobacillus delbrueckii subsp . bulgaricus (75 strains) isolates from traditional Greek yogurts were test ed on the basis of their acidification kinetics, acidity, viscosity, and ar oma compounds formation in milk pure cultures. Strains of Lb. bulgaricus pr esented greater viscosity values, higher acidification activity and produce d more acetaldehyde at culture pH 4.6 than S. thermophilus strains. Acetoin was formed only by the latter strains, while diacetyl was not detected in either species. Application of cluster and discriminant analysis permitted definition of distinct groups of strains and of the most significant variab les to discriminate among them. Finally, classification and regression tree analysis allowed clusters characterization by critical values of the most discriminant variables. Evaluation of results as above could be a useful to ol to select, through a large number of screened strains, the suitable star ters for specific purposes.