Yogurt is a coagulated, sour dairy food. Human beings have probably consumed fermented milks, such as yogurt, since they first milked cows and other animals. Before modern times, however, people did not know of the need for sanitation, adequate refridgeration, and heat treatment of milk to prevent spoilage. Milk fermented naturally (or spoiled) if it was left unconsumed. The quality of the product depended on the microorganisms present in the original milk. Since the discovery, about 100 years ago, of the microscopic organisms that cause fermentation and spoilage of milk and other foods, we have learned how to control fermentation to produce uniform, high-quality products.
Milk quickly spoils if it is not handled under strict sanitary conditions and is not properly refridgerated. The type of fermentation that occurs under such conditions may differ widely from one time to the next. Cultured dairy foods, however, are produced by controlled fermentation, using known types of harmless, active bacteria to create the desired flavor and body. The fermentation in most cultured dairy foods are:
- Lactic acid fermentation. Some bacteria break down the milk sugar (lactose) to produce lactic acid, a sour-tasting compound. When enough acid has developed, it coagulates (clabbers) the milk protein.
Citric-acid fermentation. Some bacteria produce armoa and flavor compounds from the small amounts of citric acid in milk. These compounds contribute to the distinctive flavor of many cultured dairy foods.
Pure cultures of bacteria are necessary to develop the desired flavor, armoa, and body in cultured dairy foods. Dairy-food processing plants maintain these pure cultures under strict sanitary conditions. The cultures are added to milk or cream as starters to make commercially processed buttermilk, sour cream, yogurt, cottage cheese, and cheeses. Because commercial cultured dairy foods contain pure cultures of the right kind of bacteria, they are ideal starters for homemade products. Purchase the freshest products to use as starters, because bacteria become less active with age. Before adding the starter, it is essential to heat-treat (pasteurize) the raw milk or cream to destroy harmful bacteria and other microorganisms that may intefere with a proper fermentation and the development of a good-flavored, wholesome product. To pasteurize the milk or cream, heat it to 145 F (62.8 C) for 30 minutes.
Making Yogurt at Home
Yogurt is a cultured dairy product that can be made from whole, lowfat, or nonfat (skim) milk, with added milk solids from either nonfat dry milk or evaporated milk. Any type of milk can be used; cow's milk is traditionally used in the United states, but other countries have been known to make yogurt from milk of water buffalo, yak, goat, horses, and sheep.
The bacterial culture used in making yogurt differs from those used in other cultured dairy foods. Yogurt bacteria grow best between 100 F and 105 F (37.8 C and 40.6 C).
Yogurt may be served: as is; as a custard with berries, peaches, or apple pie; seasoned with salt and chives or chopped green onions; as a topping for fruits; or with sweets, such as honey, maple syrup, jelly, or molasses. Anyone who likes buttermilk should like yogurt.
The nutritive value of yogurt is about the same as the milk from which it is made. The protein and calcium content may be slightly higher than that of whole milk because of the added nonfat dry milk solids.
Add nonfat dry milk or evaporated milk to the quart of fresh milk.
Optional If you prefer a firmer yogurt, dissolve the unflavored gelatin in the cold milk. You could also heat the milk mixture to 180 F (82.2 C) in the double boiler and hold the milk at that temperature for up to 45 minutes. The longer the time, the firmer the finished yogurt will be. Keep the double boiler covered during the heat treatment. Cool the milk to 120 F (48.9 C).
Double boiler that holds 5 cups
Food thermometer with a range of at least 100 to 200 F (37.8 to 93.3 C)
Sterilized container that holds at least 5 cups (glass jar, crockery, or stainless steel bowl)
Yields 4 to 5 cups
1 quart pasteurized milk (whole, lowfat, or nonfat)
1/3 cup nonfat dry milk or 13-ounce can evaporated milk
1/3 to 1/2 cup commercial, unflavored yogurt
Optional: 1/2 package unflavored gelatin
- Heat the milk to 120 F (48.9 C). If you add gelatin be sure it is completely dissolved. Pour it into the sterilized container. Cool it to 110 F (43.3 C).
- Remove 1/2 cup of the warm milk (110 F or 43.3 C) and throughly blend it into the commercial, unflavored yogurt. Blend this mixture into the remaining warm milk. You can leave the milk in the large container and cover it. Or put the milk in clean 1/4 pint bottles or other small containers and cover with loose fitting caps. (Clean custard cups or jelly glasses are satisfactory for home use, because the yogurt can be eaten directly from the container in which it is made.)
- Place the container(s) in a pan of water at 110 F (43.3 C). Let the milk ripen at that temperature until it has thickened and has a tart, acid flavor. The ripening period usually takes 3 to 6 hours. If you prefer a sharper flavor, lengthen the incubation time.
- Remove the ripened yogurt from the water bath and chill it in the refridgerator. Refridgerate the yogurt until it is used. Yogurt should keep for a week or longer if held at 45 F (7.2 C), or lower (normal refridgerator temperature).