What We've Done So Far:
- Sterilized all of our equipment
- Heated milk to 185°F, quickly and evenly by using a hot water jacket
- Cooled it quickly and evenly to 110°F, using a cold water bath
- Added or "pitched" 2-3 Tbs of fresh yogurt and stirred
- Covered and kept warm to wait for seven hours
The reason for sterilization and heating of the milk to 185°F is to remove the possibility of contamination, and to prepare the milk proteins for bacterial reproduction. Doing it quickly and evenly helps achieve predictable results. Cooling the milk to 110°F is required so that the yogurt cultures can work their magic.
Yogurt is created using "helpful" bacteria (usually lactobacillus bulgaricus or streptococcus thermophilus, or both), which cause the milk to ferment. These micro organisms are alive in your yogurt starter that you pitched in the previous step. When they are added to milk at 110°F, they consume the sugar found in milk, called lactose. As a result, the milk thickens or curdles, and lactic acid is produced as a byproduct. The lactic acid gives yogurt its "tangy" taste, and preserves the milk from spoiling. Before pasteurization and refrigeration, this was the only way to make milk last for long periods of time.
The end result is a creamy, tangy, milk-based product, with millions of active bacteria cultures which aid in digestion. Further, the lactic acid allows some people who otherwise cannot tolerate lactose to enjoy this dairy treat. Seven hours at a warm temperature seems to produce a yogurt that is sufficiently thick, and just tangy enough. The longer you let it ferment, the thicker it will get, but the more tangy it will become. Make your first batch at exactly seven hours, and then adjust according to your taste preferences.