Lactobacillus and Streptococcus acidify milk by fermenting lactose
Yogurt is created by inoculating milk with a starter culture containing Lactobacillus bulgaricus and Streptococcus thermophilus. These bacteria consume lactose (milk sugar) and produce lactic acid as a metabolic byproduct, gradually lowering milk pH from neutral (6.8) to acidic (4.6). At this lower pH, casein protein unfolds and aggregates, forming a gel that traps whey and transforms liquid milk into solid yogurt.
The two bacteria work synergistically. Streptococcus thermophilus grows quickly and produces acid and CO2, making the environment favorable for Lactobacillus bulgaricus. Lactobacillus bulgaricus then dominates, producing additional acid and the characteristic tangy flavor. The fermentation typically takes 4-12 hours depending on temperature and starter strength.
Temperature drives fermentation speed and final taste
Incubation at 42-46C (warm yogurt setting) accelerates fermentation to 4-6 hours. Lower temperatures (30-35C, Greek yogurt style) slow fermentation to 8-12 hours, resulting in a less acidic (higher pH) and milder flavor. Fermentation beyond coagulation (past pH 4.6) continues to pH 3.5-4.0, producing increasingly sour yogurt with more whey separation. Overfermented yogurt tastes sharp and may show excessive whey drainage. Stopping fermentation at pH 4.6 (by cooling) produces mild, creamy yogurt.