Dahi is made by bacterial fermentation of milk. In this process lactose in milk is converted into lactic acid by several probiotic microorganisms. The species involved in the fermentation depends on the temperature and humidity of the environment, and may include Lactococcus lactis, Streptococcus diacetylactis, Streptococcus cremoris, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus.
Dahi starter is made with dried red chillies (or their stems) in hot milk. Milk is boiled and then allowed to cool for a while. When lukewarm, dried chili peppers or their stems are added. The reason for this tradition is that dried chillies are rich in a type of lactobacilli, the bacteria that helps in fermentation of milk to form dahi. The bowl is then kept undisturbed in a warm place for 5 to 10 hours.
After the starter is made, or saved from previous batch of dahi, milk is boiled and cooled. In a separate bowl dahi is mixed with its whey, and then mixed together with the milk. It is then left to sit undisturbed for 5 to 10 hour