Influence of water activity on fermentation, organic acids production and viability of yogurt and probiotic bacteria.

Five one-litre batches of probiotc yogurts were made using reconstituted skim milk supplemented with 0, 4, 8, 12 or 16% sucrose. The incubation time to decrease the pH to 4.5 and changes in organic acids production and counts of yogurt bacteria (Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus) and probiotic bacteria (Lactobacillus acidophilus and Bifidobacterium spp.) in yogurt were assessed for up to 30 days. As the sugar level increased, the water activity decreased and the incubation time to reach pH of 4.5 increased. The organisms produced pyruvic acid, acetic acid, lactic acid and propionic acid at varying levels. There were no major differences in the initial counts of yogurt and probiotic bacteria in all the five batches. However, the counts of all the four groups of organisms declined during storage, in particular in the batches containing 12% and 16% sugar. Thus, sugar addition may be deleterious to the growth of yogurt and probiotic bacteria, especially in products such as fermented frozen dairy desserts, which contain approximately 16% sugar.