| Abstract [eng] |
In assessing the impact of the protective medium composition and storage temperature on the vitality of cells, the cultures of Lactococcus lactis subsp. lactis, Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, Salmonella typhimurium (ATCC 14028), Staphylococcus aureus (ATCC 25923), and Bacillus cereus (ATCC 10876) were studied. Samples of protective medium were prepared, i. e. reconstituted dried skim milk with 20, 30 and 40 % glycerin and brain-heart infusion with 20, 30 and 40 % glycerin. The samples were tested at -18 ºC and -72 ºC. It is more appropriate to store cultures at -72 ºC, however, the resistance of different microorganism species to storage conditions varies. At both temperatures major cells of S. typhimutrium survive in all the media samples. The resistance of S. aureus to storage conditions is average. The resistance of B. cereus is the weakest: at the end of the experiment only percentiles of the culture remain in the samples. The resistance of lactic acid bacteria cultures (S. thermophilus, L. bulgaricus, and L. lactis) was found to be better; the vitality of the cultures was ensured by both temperatures (-18 ºC and -72 ºC) in the different samples of protective medium. The strains of Lactococcus lactis subsp. lactis biovar. diacetylactis ir Lactococcus lactis subsp. lactis biovar. paracitrovorus frozen in reconstituted skim milk with 10 % of glycerin were stored at -18 ºC for three-five years. During this period 22 strains of Lactococcus lactis subsp. lactis biovar. diacetylactis retained their vitality, the number of cells in the defrosted medium varied between ten thousands to millions. The 10 and 16 frozen strains of Lactococcus lactis subsp. lactis biovar. paracitrovorus remained active, the number of vital cells varying from hundreds thousands to millions, the cultures retained sufficient vitality and promoted milk fermentation within 24 days. [...]. |
