Effect of acidification on the activity of probiotics in yoghurt during cold storage

The viability of Lactobacillus acidophilus LAFTI® L10, Bifidobacterium lactis LAFTI® B94, and L. paracasei LAFTI® L26 and their proteolytic activities were assessed in yoghurt at different termination pH of 4.45, 4.50, 4.55, and 4.60 in the presence of L. delbrueckii ssp. bulgaricus Lb1466 and Streptococcus thermophilus St1342 during 28 days of storage at 4 °C. All strains achieved the recommended level of 6.00 log cfu g−1 of the product with L. acidophilus LAFTI® L10 and L. paracasei LAFTI® L26 exceeding the number to 8.00 and 7.00 log cfu g−1, respectively. Lactobacilli strains showed a good cellular stability maintaining constant concentration throughout storage period regardless of termination pH. On the other hand, the cell counts of B. lactis LAFTI® B94 decreased by one log cycle at the end of storage. The presence of probiotic organisms enhanced proteolysis significantly in comparison with the control batch containing L. delbrueckii ssp. bulgaricus Lb1466 and S. thermophilus St1342 only. The proteolytic activity varied due to termination pH, but also appeared to be strain related. The increased proteolysis improved survival of L. delbrueckii ssp. bulgaricus Lb1466 during storage resulting in lowering of pH and production of higher levels of organic acids, which might have caused the low cell counts for B. lactis LAFTI® B94.