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Among Streptococcus thermophilus cultures, the principle component of yoghurt and cheese starters, a minority of strains forms the group of ‘H’-strains which show an unusually high acidification rate, grow faster and coagulate milk 3–5 hours earlier than the typical S. thermophilus cultures. A large-scale screening study was performed to select ‘H’-strains of S. thermophilus from more than 100 samples of home-made yoghurt, industrial yoghurt starters and single cultures, maintained in the LBB culture collection. Only four strains – LBB.TN1, LBB.M23, LBB.M34 and LBB.M60 – were isolated/selected due to their ability to form large yellowish colonies on milk agar, supplemented with beta-glycerophosphate and bromocresol purple. While in general S. thermophilus is described as a species with limited proteolytic capacity and in contrast to all other tested S. thermophilus cultures, the four selected strains invariably gave positive amplification product with the polymerase chain reaction when primers, specific for the membrane proteinase-coding gene prtS were used. The macrorestriction profiles of the genomic DNA of the four strains confirmed that they are non-isogenic and not related to each other. When grown in milk and compared to the control industrial strain LBB.A, the four strains showed a dramatically faster acidification, coagulating milk within four hours. The application of strain TN1 or M23 as adjunct culture to industrial yoghurt starter LBB.BY5-12 resulted in shortening the fermentation time with more than 30 min.

Three-component starters for yogurt were obtained on the base of starter LBB.BY 5-12 for traditional Bulgarian yogurt, containing strains Lactobacillus delbrueckii ssp. bulgaricus B5 and Streptococcus thermophilus A with the addition of either an exopolysaccharide-producing S. thermophilus strain 6V or the fast acidifying S. thermophilus strain N1. To differentiate between the three strains in the starter cultures, randomly amplified polymorphic DNA (RAPD) technique was applied to develop strainspecific probes. Southern hybridization against dot-blots of chromosomal DNA from the three S. thermophilus strains confirmed that two probes, derived from a 770 bp RAPD product obtained with primer RAPD-4 and a 290 bp sequence obtained with primer OPP-7 were specific for S. thermophilus 6V and S. thermophilus A, respectively, while no hybridization to S. thermophilus N1 DNA was observed. The selected probes were used to differentiate between S. thermophilus colonies on a solid agar medium by colony hybridization. The evaluation of the viable cell counts revealed that the populations of S. thermophilus A and the added S. thermophilus strains 6V or N1 in the three-component starters and in yogurt had nearly equal proportion allowing each strain to contribute to the enriched properties of starter and product.