Yogurt gets its sour taste from lactic acid, which acts as a natural preservative against harmful bacteria.

The production of yogurt relies on a process called thermophilic fermentation, primarily driven by two specific bacterial strains: Lactobacillus bulgaricus and Streptococcus thermophilus. These microbes consume lactose, the primary sugar found in milk, and convert it into lactic acid through anaerobic respiration. This biochemical transformation is what gives yogurt its distinctively sharp and tangy flavor profile.The accumulation of lactic acid causes the pH level of the milk to drop from around 6.7 to approximately 4.5 or lower. This acidification causes milk proteins, specifically casein, to denature and coagulate, creating the thick and creamy texture we associate with yogurt. More importantly, most pathogenic bacteria, such as E. coli or Salmonella, cannot survive in environments with a pH below 4.6.Historically, this preservation method dates back to at least 5000 BCE in Mesopotamia, where herdsmen discovered that milk stored in animal stomachs curdled and remained edible longer. Modern food science confirms that the competitive exclusion provided by these 'good' bacteria prevents the growth of molds and yeasts. Today, the FDA requires yogurt to have a minimum acidity level to ensure safety and shelf stability. This natural preservation remains one of the oldest and most effective examples of biotechnology in human history.