Lactic acid bacteria have a long history in the production of fermented dairy products. They are of great importance in the production of bioactive peptides and increasing the health effects due to their proteolytic ability. In the present study, the ability of two strains of Lactic acid bacteria were investigated after isolation from Iranian dairy products and were compared in terms of fermentation ability, proteolytic properties, and production of fermented milk with antioxidant and acceptable sensory properties.
Two isolated strains, including Lactobacillus fermentum (PTCC 1929) and Lactobacillus helveticus (PTCC1930)  were identified based on the ability of sugar fermentation and 16S rRNA sequence analysis. The results showed that in spite of the better growth of Lactobacillus fermentumbacteria in the first 24 hours, the initial acidification was higher in L. helveticus.  The pH value in the Lactobacillus helveticussample was less than 4 in the early 24 hours, while this value was achieved after 72 hours in L. fermentum. In the co-culture, the pH reduction steps reached at a minimum level of 3.63 with the initial and secondary acidifications of Lactobacillus helveticusand L. fermentum, respectively. The results showed that, despite the higher proteolysis ability of Lactobacillus helveticus, the antioxidant activity of the sample containing Lactobacillus fermentumwas 1.5 to 1.7 times higher. The highest antioxidant activity was observed in all samples at the first 24 hours. Increase in proteolysis did not cause an increase in the production of antioxidant peptides. Milk fermented by Lactobacillus fermentum for 24 hours was significantly lower in taste and acceptability score compared to the other samples (P<0.05). In general, the results showed that two strains co-culture has a better ability to produce antioxidant bioactive peptides with a suitable overall acceptance and has the potential for the production of fermented dairy products with healthy properties.