Yogurt is produced by fermentation of milk with a symbiotic starter culture of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. These thermophilic LAB grow at 42-45°C, producing lactic acid that coagulates casein and causes gel formation. The symbiotic relationship is critical: S. thermophilus produces formic acid and CO₂ that stimulate L. bulgaricus, while L. bulgaricus produces peptides and amino acids via proteolysis that support S. thermophilus growth. Yogurt texture is influenced by total solids content, heat treatment of milk (denaturation of whey proteins increases water-holding capacity), and the presence of exopolysaccharide-producing strains.
Cheese production involves concentration of milk fat and casein by acid or rennet coagulation, followed by acidification, salting, and ripening. Rennet (chymosin) cleaves κ-casein at the Phe105-Met106 bond, destabilizing casein micelles and forming a gel. Starter cultures are classified as mesophilic (Lactococcus lactis subsp. cremoris, L. lactis subsp. lactis, typically used at 20-35°C for cheddar, Gouda) or thermophilic (S. thermophilus, Lactobacillus helveticus, used at 37-50°C for Swiss, Parmesan). Secondary cultures (Penicillium roqueforti in blue cheese, Propionibacterium freudenreichii in Swiss) contribute to ripening and flavor development.
Kefir is a fermented milk beverage originating from the Caucasus region, produced by fermentation with kefir grains. These grains are a complex, gelatinous matrix of polysaccharide (kefiran) containing a stable symbiotic community of LAB (Lactobacillus kefiranofaciens, Lactococcus lactis), yeasts (Kluyveromyces marxianus, Saccharomyces cerevisiae), and acetic acid bacteria. The fermentation yields lactic acid, ethanol, and CO₂, resulting in a slightly effervescent, acidic, and yeasty beverage. Unlike yogurt, kefir can be produced by back-slopping and the grains can be recovered and reused.
Probiotic bacteria, such as Lactobacillus acidophilus, Bifidobacterium bifidum, and Lactobacillus casei, are frequently added to fermented dairy products for their purported health benefits, including modulation of the gut microbiota, immune enhancement, and lactose digestion improvement. Post-acidification during refrigerated storage, caused by continued metabolic activity of starter cultures, is a common quality defect in yogurt that can be controlled by selecting less acidifying strains, rapid cooling, and storage temperature management. Fermented dairy relies on lactic acid bacteria and sometimes yeasts for kefir production. Many fermented dairy products contain probiotics that confer health benefits beyond basic nutrition.
