Yogurt ChemistryYogurt 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 RecipeYou 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.
- 1 quart milk (any kind)
- 1/4 to 1/2 cup non-fat dry milk (optional)
- 2 tablespoons plain yogurt with live cultures (or you can use freeze-dried bacteria instead)
- Set the starter yogurt out at room temperature while you prepare the milk. This warms the yogurt so that it won't chill your recipe too much when you add it later.
- Heat the milk to 185°F (85°C). The purpose of this step is to re-pasteurize the yogurt, preventing any unwanted bacteria from growing, and to denature the proteins so that they will be able to coalesce and thicken the yogurt. The easiest way to do this is to use a double boiler or set your container of milk inside a pan of water. Heat the water to near-boiling. Don't worry -- the milk can't boil using this technique. If you have to heat the milk directly, stir it constantly and watch the temperature to make sure it doesn't boil or burn. If you do not have a thermometer, the milk will start to froth at 185°F (85°C).
- Once the milk reaches the temperature or starts to froth, remove it from heat and allow the milk to cool 110°F (43°C). One way to do this is to place the container of milk in a cold water bath. Otherwise, you can leave the milk on the counter and allow it to cool. Either way, stir the milk occasionally so that the temperature is uniform. Don't proceed to the next step until the temperature of the milk is below 120°F(49°C), but don't let the milk cool below 90°F (32°C). 110°F (43°C) is the optimal temperature.
- At this point, you can add nonfat dry milk. This is an optional step that helps the yogurt thicken more readily, plus it add nutritional content to the yogurt. It's purely a matter of preference, whether you add the dry milk or not.
- Stir in the starter yogurt.
- Put the yogurt into clean, sterile containers. Containers can be sterilized by boiling them. The reason for sterilizing the containers is to prevent unwanted mold or bacteria from growing in your yogurt. Cover each container with plastic wrap or a lid.
- Keep the yogurt as close to 100°F (38°C) as possible and undisturbed, to bacterial growth. Some ovens have a "proof" setting that you can use. Other ideas include setting the yogurt on a heating mat (being sure to check the temperature) or placing the containers in a warm water bath. You'll have a custard-like yogurt after about 7 hours. It won't resemble store-bought yogurt, because that has thickeners and additional ingredients. Your yogurt should have a yellowish or greenish liquid on top, a creamy custard texture, and may have a cheesy odor. The thin yellowish liquid is whey. You can pour it off or mix it in, whichever you prefer. It's completely edible, though you may add fruit, flavorings, or herbs, according to your taste. If you leave the yogurt at this temperature longer than 7 hours, it will thicken and become more tangy.
- When the yogurt is the thickness and flavor you want, refrigerate it. Homemade yogurt will keep for 1-2 weeks.
- You can use yogurt from this batch as starter for the next batch. If you are going to use yogurt as a starter, use unflavored yogurt, within 5-7 days.