NoxE NADH oxidase and the electron transport chain are responsible for the ability of Lactococcus lactis to decrease the redox potential of milk.

The redox potential plays a major role in the microbial and sensorial quality of fermented dairy products. The redox potential of milk (around 400 mV) is mainly due to the presence of oxygen and many other oxidizing compounds. Lactococcus lactis has a strong ability to decrease the redox potential of milk to a negative value (-220 mV), but the molecular mechanisms of milk reduction have never been addressed. In this study, we investigated the impact of inactivation of genes encoding NADH oxidases (noxE and ahpF) and components of the electron transport chain (ETC) (menC and noxAB) on the ability of L. lactis to decrease the redox potential of ultrahigh-temperature (UHT) skim milk during growth under aerobic and anaerobic conditions. Our results revealed that elimination of oxygen is required for milk reduction and that NoxE is mainly responsible for the rapid removal of oxygen from milk before the exponential growth phase. The ETC also contributes slightly to oxygen consumption, especially during the stationary growth phase. We also demonstrated that the ETC is responsible for the decrease in the milk redox potential from 300 mV to -220 mV when the oxygen concentration reaches zero or under anaerobic conditions. This suggests that the ETC is responsible for the reduction of oxidizing compounds other than oxygen. Moreover, we found great diversity in the reducing activities of natural L. lactis strains originating from the dairy environment. This diversity allows selection of specific strains that can be used to modulate the redox potential of fermented dairy products to optimize their microbial and sensorial qualities.