Where do the bacteria that make yogurt usually grow first?

The bacteria used to make yogurt are typically cultivated in controlled dairy starter cultures before being added to milk. The two classic yogurt bacteria are Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. They are grown in warm, sanitized production tanks where they rapidly acidify milk and help create yogurt’s texture and tang. Fermented milk products have been made for thousands of years, but modern yogurt production uses carefully selected cultures to ensure consistent taste and safety. The process is one of the oldest and most widespread examples of food microbiology.