Yogurt and Fermented Dairy

Fermented dairy products — yogurt, sour cream, crème fraîche, kefir — are milk or cream transformed by lactic acid bacteria. The bacteria convert lactose to lactic acid, which drops the pH, coagulates casein proteins, thickens the liquid, and generates tangy flavor. The result is more digestible (lower lactose), longer-lasting (acid inhibits pathogens), and more flavorful than the starting milk.

The fundamental reaction

Lactic acid bacteria (LAB) — primarily Lactobacillus and Streptococcus species — consume lactose and excrete lactic acid. As pH drops from milk’s neutral 6.6 toward 4.5, casein micelles lose their electrical charge, clump together, and form a gel that traps water. This is the same acid coagulation used in fresh cheese making, just stopped at a different point.