Consequences of oxygen on Streptococcus thermophilus physiology and effects on milk fermentation

Streptococcus thermophilus (ST) is essential for acidifying milk in food production, including yogurt and mozzarella. Oxygen is not controlled in dairy production and varies across scales and steps, partially due to ingress from the headspace and varying diffusion rates in large-scale vessels, where mixing is often minimized to preserve curd integrity. This review summarizes known consequences of oxygen variability on ST growth, acidification, exopolysaccharide formation, and flavor formation in milk, which are important for starter selection and process design that minimize performance variation caused by different oxygen levels. Oxygen conditions in laboratory setups also differ from those in industrial settings, making the understanding of oxygen important for strain screening as well. Current knowledge indicates that oxygen is a strain-dependent regulator of physiology, metabolism, and stress, but large knowledge gaps remain. To aid future research in filling these gaps, we highlight recent approaches to study ST metabolism and stress, as well as recent reviews of the general antioxidant properties of lactic acid bacteria. To include perspectives on future foods, we also include recent studies suggesting the use of ST for future plant-based products, as well as discuss the contribution to oxygen sensitivity of certain compounds, which may differ in plant-based substrates. Finally, we discuss beneficial traits for food production that potentially overlap with different oxygen-related mechanisms. We conclude that oxygen conditions should be incorporated into strain selection strategies, both to better predict industrial performance and to possibly exploit beneficial traits associated with oxidative adaptation, consequently enabling more robust and functionally optimized milk fermentations.