Effect of Process Variables on the Probiotic and Starter Culter Growth in Sinbiotic Yogurt Beet

Milk fermented with YF-3331 (containing Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus) and a probiotic strain of L. acidophilus La-5 and the effect of different variables were evaluated on the growth of three bacteria in period of 6 hours of incubation. Beet was added to the yogurt at 15% as prebiotic. The effect of variables including skim milk powder, whey powder, inoculum size, L. acidophilus inoculums, and incubation temperature was evulated by Taguchi approach. The results showed that the growth of L. bulgaricus was much higher than others. L. acidophilus showed the lowest growth after the incubation time. Although milk composition had no significant effect on the bacterial growth, increasing whey powder in the milk base enhanced the growth of L. acidophilus slightly. By increasing L. acidophilus inoculums, L. bulgaricus growth was restricted and yoghurt bacteria restricted the growth of L. acidophius. L. bulgaricus, S. thermophilus and L. acidophilus showed higher growth at 37, 40 and 43°C, respectively. This approach provides some useful information to set the best situation for the probiotic growth during fermented milk production and reach much higher level of cell growth after the incubation.