Viability of lactic acid microflora in different types of yoghurt

Abstract

The suitability of three culture media (Skim milk agar, M17 agar and MRS agar, incubated under aerobic and anaerobic conditions) to obtain total and differential counts of yoghurt bacteria, was assayed. A total of 25 samples of different yoghurt types (set, skimmed set, drinking, and set with 'dulce de leche') obtained with the culture IA, were used. The lactic microflora viability was studied at 6°C and 12°C. Skim milk agar (aerobiosis) was the most useful medium for total and differential counts of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Cell viability depended on the yoghurt type and the storage temperature. On the basis of a minimum value of 107 CFU g-1, the shelf life of yoghurts at 6°C was longer than 60 days, with the exception of the drinking type (45 days). At 12°C, shelf life was identical for the skimmed drinking yoghurt but lower for the other yoghurt types. Both the storage temperature and the yoghurt type should be taken into account when shelf life is specified on the basis of the lactic microflora content.