Milk plus live cultures
Yogurt is milk fermented by lactic acid bacteria, mainly Streptococcus thermophilus and Lactobacillus bulgaricus. Kept warm for several hours, they turn lactose into lactic acid, which makes milk proteins set into a gel. All you need is milk, a little plain yogurt with live cultures as a starter, and a way to keep it warm.
The method
- Heat the milk to about 180°F (82°C) and hold it a few minutes, stirring. This makes a thicker yogurt. Ultra-pasteurized milk can skip this step.
- Cool it to about 110°F (43°C).
- Stir in the starter: about 2 tablespoons per quart (about 30 g per liter), first whisked with a little warm milk.
- Incubate at 108 to 112°F for 6 to 12 hours, undisturbed: in a yogurt maker, an electric pressure cooker on the yogurt setting, or an oven with the light on.
- Chill for at least 4 hours before eating.
The calculation
- Starter = milk in liters × starter per liter (30 g per liter is about 2 tablespoons per quart)
- Milk powder, optional, for a firmer yogurt = milk weight × percentage (2 to 4% is typical)
- Jars = total volume ÷ jar size, rounded down
- After straining: about half the volume for Greek yogurt, about a third for labneh
Example
Half a gallon (2 quarts) of whole milk with 2 tablespoons of starter per quart, in 8-ounce jars:
- Starter: about 57 g, or 4 tablespoons
- Jars filled: 8
- Strained into Greek yogurt: about 1 quart of yogurt and 1 quart of whey
Tips
- More starter is not better: crowded bacteria can make yogurt sour, thin or grainy.
- Whole milk gives the creamiest result; adding milk powder thickens low-fat milk.
- Keep some of each batch as the starter for the next one, though the cultures may weaken after several generations; start fresh with store-bought yogurt from time to time.
- Whey from straining can be used in baking, smoothies or to cook grains.
- If the yogurt has not set, the starter may have been too old or the milk too hot when it was added; try again with fresh starter.