Aims:  To investigate the production of antioxidant activity during fermentation with commonly used dairy starter cultures. Moreover, to study the development of antioxidant activity during fermentation, and the connection to proteolysis and bacterial growth. Methods and Results:  Antioxidant activity was measured by analysing the radical scavenging activity using a spectrophotometric decolorization assay and lipid peroxidation inhibition was assayed using liposomal model system with a fluorescence method. Milk was fermented with 25 lactic acid bacterial (LAB) strains, and from these six strains, exhibiting the highest radical scavenging activity was selected for further investigation. Leuconostoc mesenteroides ssp. cremoris strains, Lactobacillus jensenii (ATCC 25258) and Lactobacillus acidophilus (ATCC 4356) showed the highest activity with both the methods used. However, the radical scavenging activity was stronger than lipid peroxidation inhibition activity. The development of radical scavenging activity was connected to proteolysis with four strains. Molecular distribution profiles showed that fermentates with high scavenging activity also possessed a higher proportion of peptides in the molecular mass range of 4–20 kDa, while others had mostly large polypeptides and compounds below 4 kDa. In addition, the amount of hydrophobic amino acids was higher in these fermentates. Conclusions:  The development of antioxidant activity was strain‐specific characteristic. The development of radical scavengers was more connected to the simultaneous development of proteolysis whereas, lipid peroxidation inhibitory activity was related to bacterial growth. However, high radical scavenging activity was not directly connected to the high degree of proteolysis Significance and Impact of the Study:  To the best of our knowledge, this seems to be the first report, which screens possible antioxidant activity among most common dairy LAB strains. Use of such strains improve nutritional value of fermented dairy products.

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