Yogurt contains beneficial bacteria known as probiotics. Typically, probiotics found in yogurt are from the species Lactobacillus delbrueckii subsp. bulgaricus or Streptococcus thermophilus. These are both bacterial species that thrive in the human digestive tract and promote healthy gut flora.
During the production process, yogurt is usually heated to at least 80 degrees Fahrenheit to kill any harmful bacteria in the yogurt culture. The probiotics are added after the yogurt mixture has been heated and re-chilled. Fermentation, curdling and thickening takes place over the next 4 to 7 hours after chilling, at which point the yogurt is ready to eat.