Effects of Probiotic Addition on Fermented Milk Quality: Mechanisms and Applications of Lactobacillus and Bifidobacterium—A Review

Probiotic strains of Lactobacillus and Bifidobacterium are increasingly incorporated into fermented milk to improve product quality and deliver health benefits. Their effects, however, are strongly strain-specific and depend on complex interactions with traditional starters (Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus). This review synthesizes recent advances on how these two genera modulate the key quality attributes of yogurt. It examines their divergent influences on acidification and post-acidification control, gel coagulation and texture (through exopolysaccharide production and proteolytic systems), and the formation of characteristic flavor compounds. Lactobacilli generally drive rapid acidification and synthesize exopolysaccharides that reinforce gel structure. Their proteolytic systems release bioactive peptides, and they directly generate aroma compounds such as acetaldehyde and diacetyl. Many lactobacilli also suppress post-acidification by impairing the viability of Lactobacillus delbrueckii subsp. bulgaricus during refrigerated storage. Bifidobacteria instead accelerate early acidification mainly by cross-feeding Streptococcus thermophilus, but contribute limited exopolysaccharide-based texture improvement. They reshape flavor profiles indirectly through quorum-sensing signals (autoinducer-2) and short-chain fatty acid accumulation. As strict anaerobes, bifidobacteria frequently require co-cultivation or prebiotic support to maintain viability and functional activity in milk. This review further proposes a multidimensional screening framework spanning acidification, textural, and flavor traits to identify yogurt-suitable strains. The synthesis offers practical guidance for the rational design of stable, sensory-acceptable, and functionally enhanced probiotic fermented milk.