Acid production by bifidobacteria and yoghurt bacteria during fermentation and storage of milk

Three species of bifidobacteria, namely Bifidobacterium bifidum, Bifidobacterium longum and Bifidobacterium adolescentis were used in pure culture and in combination with yoghurt bacteria (B3 and SBI cultures) for the production of fermented milks. The number of bacteria during fermentation and the level of acid produced during fermentation and storage were assessed using Rogosa's modified selective agar and high performance liquid chromatography (HPLC). It was found that during fermentation all bifidobacteria exhibited growth uncoupled from acid production. Two of the species examined produced only low levels of acids when grown individually and only B. adolescentis produced appreciable amounts. In mixed cultures, the level of acid was a reflection of the combination of yoghurt culture and species of Bifidobacterium, and this, observation suggests that there is a degree of influence between the cultures. During storage, the acid concentration remained quite stable in most samples. The prevention of post-production acidification that normally occurs during storage of yoghurt can be attributed to the presence of bifidobacteria, and it could be that acetic acid has a marginally inhibitory effect on the Lactobacillus and Streptococcus spp. (C) 1996 Academic Press Limited