Sensitivity of bifidobacteria to oxygen and redox potential in non-fermented pasteurized milk

The effect of deaeration, cysteine addition and electroreduction of milk on the viability of various Bifidobacterium strains in pasteurized milk during refrigerated storage at +7 °C for 4 weeks was assessed. Preliminary assays in deaerated milks showed considerable variability to oxygen sensitivity among the eight strains examined during refrigerated storage. Three strains with different sensitivity were selected for subsequent assays. Assays on the effect of oxygen during growth at 37 °C were included in the screening and results suggest that the growth inhibition observed could serve as an indicator of the negative effect of oxygen on stability of bifidobacteria during refrigerated storage in similar oxygen conditions. The electroreduction treatment had a positive impact on the survival of two of the three cultures tested during the 4 week storage period at 7 °C. Addition of cysteine and deaeration alone had similar effects on cell viability during storage, suggesting that the benefit of electroreduction was mainly linked to its action of lowering the level of dissolved oxygen in the samples. Data from this study suggest that electrochemical reduction of milk, as well as deaeration or addition of reducing agents could be applied to enhance the survival of bifidobacteria during extended storage at 7 °C in milk.