Probiotics are defined as a live microorganism that when administrated in an adequate amount promote the health status of consumers. The most important challenge in consumption of probiotics is how to keep them alive during food processing and throw the digestion system. The aim of this study is the comparison of the effects of sub-lethal pre-culturing on the viability of Lactobacillus plantarum and Lactobacillus fermentum. in this study the effect of sub-lethal stresses such as low pH (2.5, 3.5, 4.5, 5.5, 6.5), Salt (2, 4 %), bile (0.2, 0.4, 0.8, 1 and 1. %) and Cold (4 °C) on increasing of viability of cells was investigated. The survival rate of treated and untreated cells in yogurt and digestion system using the enumeration of viable cells was also comprised. the results showed that sub-lethal pre-culturing had a significant effect on the viability of cells (P≤0.05). The viability of treated cells was increased in all stresses, moreover, the number viable cells in the stool sample of consumers showed a significant increase after one-month consumption of yogurt. the results demonstrate that sub-lethal pre-culturing was led to increasing the number of viable cells in all mentioned stresses, while the increasing rate of viability was different between two bacteria.
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