Abstract

Twenty two strains of lactic acid bacteria (LAB) were isolated from 30 samples of commercial Nigerian bottled yoghurt (produced on a small scale). Isolates were phenotypically characterized and their technological properties such as acidification, exopolysaccharides (EPS) production, enzymatic activities, biogenic amine production and bacteriocin production were studied following standard procedures. The population of LAB in the yoghurt samples varied between 0.00 and 6.63 log cfu ml^-1. The pH and titratable acidity ranged from 3.80 to 4.48 and 0.73 to 1.78% lactic acid respectively. The isolated LAB strains belonged to Lactobacillus, Lactococcus, Leuconostoc, Streptococcus and Enterococcus genera. Thirteen strains showed proteolytic activity while four strains showed high lipolytic activity. The LAB strains exhibited medium to low acidification activities. About 77% of the LAB strains produced EPS. Six strains produced biogenic amines. Seven strains showed antagonistic properties due to organic acid against indicator organisms (Staphylococcus aureus, Klebsiella pneumoniae, Escherichia coli and Proteus vulgaris) while two strains (Enterococcus faecalis and Lactococcus cremoris) produced additional inhibitory substances againstK. pneumoniae. The results revealed that the assayed commercial Nigerian bottled yoghurts contain several LAB species and genera different from Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. The production of biogenic amine by some of the LAB strains is undesirable in yoghurt. The technological properties of the LAB associated with the fermented milk produced on a small scale will help in the selection and development of beneficial strains as starter or adjunct cultures.

Key words: Yoghurt, lactic acid bacteria, technological properties, strain selection.