*1.12.1 Role of oxygen in viability of yogurt probiotics*

The potential reduction of yogurt bifidobacteria after storage in different regions of the world has been documented as oxygen toxicity [36, 37]. Expenditure of probiotics to dissolved oxygen leads to a build-up intracellular of harmful oxygenic metabolites, like superoxide anion [38]. A high level of oxygen is unavoidably included in the product by several processing processes involved in the processing of milk products (e.g., agitation and mixing procedures). In addition, some packing materials during the storage period enable the transmission of ambient oxygen into the food, such as high impact polystyrene packaging—a commonly used packaging material for yogurt worldwide because of its vision, strength, and hygiene levels [39]. The use of oxygen-impermeable containers, process of two-stage fermentation, acid and bile salt resistant strains, probiotics microencapsulation and prebiotics are significant preconditions for enhancing stability and viability of yogurt-based probiotics [40].
