Microbiological changes in modified yoghurts during manufacture and storage

The live cells of yoghurt cultures were enumerated after milk inoculation and after fermentation at the temperatures: 30, 37 and 42 degreesC as well as during 28 days of storage at 6 degreesC. Active or preserved bacteria cultures were used for milk inoculation at the level 10(5)-10(7) cfu/g. During fermentation up to pH 4.6-4.8 Streptococcusthelmophilus and Lactobacillus delbrueckii subsp. bulgaricus multiplied, their population numbers increased 1-2 log cycles and reached the level of 10(8)-10(9) cfu/g, whilst the numbers of probiotic bacteria Lactobacillus acidophilus and Bifidobacterium animalis increased only 2-4 times independently of the inoculum level, form of bacteria cultures and temperature of fermentation. Time of fermentation was in negative correlation with temperature and ranged from 2 h 55 min - 3 h 40 min at 42 degreesC to 4 h 35 min - 7 h 30 min at 30 degreesC. Duration of the fermentation was also affected by the form of bacteria cultures with one exception at the temperature of 37 degreesC where the time of fermentation, ranging from 4 h to 4 h 10 min, was not influenced by the bacteria culture composition and form. The population of every species decreased slowly in limited range during storage of the yoghurts, therefore after 28 days the live cell numbers were few times lower than after fermentation. The results indicate the necessity of inoculation of milk during bio-yoghurt manufacture with the number of probiotic bacteria greater than or equal to 10(6) live cells/g which is recommended in the final product.