Consumers are more aware of additional health effects of food than ever before due to increased available information and new modern life health concerns e.g oxidative diseases. Yogurt is a known nutritious dairy product. In addition to its starter bacteria ( Lactobacillus delbrueckii su . bulgaricus Ah41 (Lb Ah41 ) and Streptococcus salivarius su .thermophilus (St )) functional yogurt contains probiotic bacteria. This type of yogurt should contain at least 10 7 CFU/ml of probiotic bacteria. Therefore, viabilityof bacteria during cold storage condition i.e post acidification and interaction of probiotic bacterium with other micro-organism in yogurt are of upmost importance. In this study effects of serial homologous stress on viability and behavior of probiotic Lactobacillus plantarum ( Lp ) and a domestic strain of Lb , previously isolated by same group was studied. For this purpose, Lb and Lp were stressed using different levels of salt concentration (1, 2, 3, 4.5, 6.5, 8.5 and 10.5 w/v), pH (5.5, 4.5, 3.5, 2.5 and 1.5) and Oxgall concentration (0.1, 0.2, 0.3, 0.4, 0.8, 1/4 %w/v). At first stage, Natural resistance of both bacteria were determined to each of above mentioned levels of salt, pH, and Oxgall in a parallel manner. Levels of Salt, pH and Oxgall which caused more than one decimal reduction of bacterial population after 24 h were 10.5%,3.5 and 0.4% for Lp and 2%, 3.5 and not 0.2% for Lb. All experiments were repeated, this time by applying stresses for salt, pH and Oxgall from mild to harsh condition in a serial manner but separately. As expected in general resistance of both bacteria seemed to increase due to applying progressively harsher condition. This time levels of Salt. pH, and Oxgall which caused more than one decimal reduction of bacterial population after 24 h were 10.5%,2.5 and 0.4% for Lp 3%, 3.5 and 0.3% for Lb . At last stage, yogurt was made using Lb and Lp which had been previously gone through progressively harsher level of stress as described above from 1% to 3% salt and then from pH=5.5 to pH=2.5 and finally Oxgall concentration of 0.1%w/v to 0.4% w/v as homologous stress in a serial sequences based on chain of events from storage to the consumer gut when a probiobiotic product is consumed. Result showed homologous stress could increase viability of Lb and Lp about 1 log CFU/mL unit in yogurt. Key word : Microbial stress, Probiotic yogurt, Post-acidification, Bacterial survival.
