Preservation of Streptococcus Thermophilus Strains after Long-term Storage in Lyophilized State

The production of original Bulgarian yoghurt, as well as, the yoghurt produced elsewhere in the world are a result of milk fermentation by selected strains of Lactobacillus delbruecki subsp. bulgaricus and Strepococcus salivarius subsp. thermophilus. According to Krueder et al [4] the milk acidification by lactic acid bacteria to pH values around 4.8 is due predominantly to thermophilic cocci. Recently, a great attention has been paid to investigations of metabolic properties of S. thermophilus concerning the production process. S. thermophilus strains are thermostable and ferment only few sugars lactose, sucrose and glucose, and sometimes also galactose [3]. The metabolic activity of the strains from S. thermophilus species when inoculated in milk is related to such important strain technological properties like acidification, biosynthesis of aroma components, consistence and viscosity of the obtained fermented milk. The acidification ability of S. thermophilus strains is a function of their carbohydrate metabolism [8]. It is difficult to evaluate objectively the acidification ability of the strains due to the high buffering capacity of milk. According to Accolas et al [1] mistakes in this analysis will be made until new methods for objective evaluation of the strains acidification ability are created. These methods will substitute the official procedure based on measurement of the titrable milk acidity.