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Isolation, Identification and Characterization of Lactic Acid Bacteria from Milk and Yoghurts

Md. Ibrahim Khalil, Md. Nural Anwar

Nineteen samples of freshly drawn raw milk, pasteurized milk, locally and commercially manufactured yoghurts samples were evaluated for total viable count of Lactic Acid Bacteria (LAB) using selective media under aerobic and anaerobic conditions. Yoghurt samples showed higher LAB count ranged from 1.0 × 106 to 5.6 × 107 cfu/mL and 2.2 × 107 to 5.4 × 108 cfu/mL in comparison to milk samples varied from 1.1 × 105 to 7.2 × 106 cfu/mL and 6.1 × 105 to 5.3 × 107 cfu/mL respectively both at aerobic and anaerobic condition. In general, anaerobic LAB growth (6.1 × 105 to 5.4 × 108 cfu/mL) counted higher than the aerobic growth (1.1 × 105 to 5.6 × 107 cfu/ mL). One hundred ninety-one (191) distinct colonies of LAB were isolated and categorized into13 groups on the basis of their colony morphology and microscopic features. These isolated and selected 13 LAB strains were identified on the basis of phenotypic, physiological and biochemical properties. Seven isolates (53.8%) were found to belong to the genus Lactobacillus while the remaining six isolates (46.2%) were under the genus Streptococcus. Identified Lactobacillus species includes L. cellobiosus, L. delbrueckii, L. hilgardii, L. coryniformis subsp. coryniformis, L. salivarius, L. leichmanni and L. plantarum. On the other hand, Streptococcus species were identified as S. faecalis, S. lactis, S. thermophilus, S. faecium, S. sanguis and S. uberis. Individual LAB were characterized for their ability to produce the high quality yoghurt in consideration to color, consistency, amount of curd, whey as well as presence or absence of gas. Considering every aspect, mixture of S. lactis and S. uberis were found to produce the better organoleptic characteristic yoghurt. Isolated LAB were also screened for the top acid producer. Among the identified all isolates, S. uberis produced the highest acid (w/v) 0.18% and L. cellobiosus and L. delbrueckii among Lactobacillus species produced highest quantity 0.15% after 6 hours of incubation period. While after prolong incubation period of 72 hours, S. lactis and S. uberis were the utmost producer of acid (w/v) 0.46% and 0.47% equivalent to 360 mg and 370 mg of lactate respectively. These two species in combination and with other all the species produced 0.50% and 0.60% acid equivalent to 394 mg and 472 mg of lactate respectively.