Yogurt forms when bacteria ferment the sugar lactose (C12H22O11) into lactic acid (C3H6O3). The lactic acid makes the milk more acidic (lower the pH), 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. There is no simple chemical equation for yogurt production since multiple reactions occur. Several types of bacteria can ferment lactose. Yogurt cultures may contain Lactobacillus delbrueckii subsp. bulgaricus, other Lactobacillus strains, Streptococcus thermophilus, and bifidobacteria.
Simple Homemade Yogurt Recipe
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.
If you use commercial yogurt starter, follow the packaging directions, since activating the culture varies depending on the product. Once you make your first batch of yogurt, you can use a couple of tablespoons of it to start future batches. While it may seem like you would want to add more active culture to a recipe, adding too much bacteria produces a sour yogurt rather than a pleasantly tangy yogurt.