Yogurt is made by inoculating certain bacteria (starter culture), usually Streptococcus thermophilus and Lactobacillus bulgaricus, into milk. Starter culture may be provided by a pre-packaged powder or an amount of existing yogurt with live bacteria. After inoculation, the milk is incubated at 40 to 46 °C (105 to 115 °F) until firm; the milk is coagulated by bacteria-produced lactic acid