Yogurt is the fermentation product of milk. In India, it is part of food consumed by the village people. Buttermilk is made from it and in summers it gives refreshment and energy. It has a cooling effect and promotes digestion because it contains probiotic-the “good bacteria” in addition to Calcium, proteins.
As in most of transformations, the process of converting milk into yogurt is carried out by bacteria. Yogurt forms when bacteria ferment the sugar lactose (C12H22O11) into Lactic acid (C3H6O3).
Although Lactic acid is a weak acid, it is strong enough to lower the pH (or make more acid) causing the proteins in milk to coagulate. The main protein in dairy milk is casein. The acidity gives yogurt its tangy flavor, while the coagulated proteins result in a thickened, creamy texture. Several types of bacteria can ferment lactose. Yogurt cultures may contain Lactobacillus delbrueckii subsp. bulgaricus, other Lactobacillus strains, Streptococcus thermophilus, and bifidobacteria.
You can make yogurt from any type of milk. Although most yogurt is made from bovine milk (e.g., cow, sheep, goat), the fermentation process works on other types of “milk”, as long as they contain a sugar for the bacteria to ferment and protein that can be coagulated. Yogurt can be made from soy milk, coconut milk, and almond milk.
The first time you make yogurt, you need a starter culture as a source of the bacteria. You can use ordinary store-bought yogurt with active culture or you can use freeze-dried yogurt starter. Milk is inoculated with the stored bacteria culture which begin to multiply and convert sugars into Lactic acid. The bacteria work optimally at 100°F (38°C). So the temperature should be maintained as close as possible to this temperature and milk undisturbed.