Abstract: To explore the effect of mixed-culture fermentations on antioxidant properties of fermented milk, yogurts fermented with Bifidobacterium animalis subsp. lactis (Ba) and/or Lactobacillus plantarum (Lp) combined with a commercial direct?vat?set?(DVS)?yogurt?starter culture containing Sterptococcus thermophilus and L. bulgaricus (1:1) were evaluated for acidification characteristics (pH and titratable acidity), proteolytic activity, exopolysaccharide (EPS) content, peptide content, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, hydroxyl radical scavenging activity, Fe2+ chelating activity and reducing power. The results showed that the acidification activity and proteolytic activity of fermented milk increased (P < 0.05) during refrigerated storage. EPS was mainly produced by the commercial starter culture, and was not responsible for the difference in the antioxidant capacity of fermented milks made with different starter cultures. The peptide content and reducing power showed an upward and then downward trend, reaching maximum values on the 7th day. Y in conjunction with Ba and/or Lp enhanced the proteolytic activity, peptide content and antioxidant activity when compared with Y alone (P < 0.05). Yogurt fermented with Y-Ba/Lp showed the highest DPPH radical scavenging activity, hydroxyl radical scavenging activity, Fe2+ chelating activity and reducing power (P < 0.05). Yogurt fermented with Y-Lp showed higher hydroxyl radical scavenging activity, but lower DPPH radical scavenging activity, Fe2+ chelating activity and reducing power than that fermented with Y-Ba (P < 0.05). This study suggests that co-fermentation with probiotics improves the antioxidant activity of fermented milks, and this effect is mainly due to the higher proteolytic activity of probiotics.

Key words: fermented milk; co-fermentation; antioxidant activity; peptide; proteolysis