Probiotic viability and storage stability of yogurts and fermented milks prepared with several mixtures of lactic acid bacteria.

Discipline: probiotics: Key words: yogurt, fermented milk, probiotic viability, storage stability, sensory preference.                                                                  

Probiotics offer the food industry ample opportunity to develop products positive to health. Thus, the range of probiotic yogurts on the market can be increased using a variety of probiotic bacteria as each offers different and specific health benefits. However, they also bring other characteristics into the yogurt, some of which have not been well-studied. As little information exists on the influence of probiotic strains on the physicochemical properties and sensory characteristics of yogurts and fermented milks, this study reports on the Probiotic viability and storage stability of yogurts and fermented milks prepared with several mixtures of lactic acid bacteria. The paper was published in the Journal of Dairy Science, Volume 97 of May 2014, pages 2578 to 2590 by Dr E. Mani-López and co-workers.

Six probiotic yogurts or fermented milks and one control each were prepared. Several physicochemical properties including pH, titratable acidity, texture, colour and syneresis were evaluated. In addition, the microbial viability of the starter cultures Lactobacillus delbrueckii spp. bulgaricus and Streptococcus thermophilus and the probiotics Lactobacillus acidophilus, Lactobacillus casei and Lactobacillus reuteri during fermentation and storage for 35 days at 5 ^oC, as well as the sensory preference among them, were monitored.

The results showed decreases in pH of 0.17 to 0.50 units and increases in titratable acidity of 0.09 to 0.29% during storage. Only the yogurt with S. thermophilus, L. delbrueckii spp. bulgaricus and L. reuteri differed in firmness. No differences in adhesiveness were found among the tested yogurts, fermented milks and the control. Syneresis was in the range of 45 to 58%. No changes in colour during storage were observed and no colour differences were detected among the evaluated fermented milk products. Counts of S. thermophilus decreased from 1.8 to 3.5 log units during storage. Counts of L. delbrueckii spp. bulgaricus also decreased in the probiotic yogurts, varying from 30 to 50% of the initial population. The probiotic bacteria also lost viability throughout storage, although the three probiotic fermented milks maintained counts of equal or more than 10⁷ cfu/mL for three weeks. The probiotic bacteria showed variable viability in the yogurts, maintaining counts of L. acidophilus equal or more than 10⁷ cfu/mL for 35 days, of L. casei for 7 days and of L. reuteri for 14 days. The authors could, however, found no significant sensory preferences among the six probiotic yogurts and fermented milks or the control, although the yogurt and fermented milk made with L.casei tended to be better accepted.

It is evident that the study provided valuable information on physicochemical, sensory and microbial properties of probiotic yogurts and fermented milks, which should guide the dairy industry in developing new and more forceful probiotic products.