Yogurt is made by adding Streptococcus thermophilus and Lactobacillus bulgaricus into heated milk. After this inoculation the milk is held at 110°F until firm. The milk is coagulated (thickened) by an increase in acidity from lactic acid produced by the bacteria. With its slightly sour taste, creamy texture, and good nutrient content, skim or whole milk yogurt remains a healthy food itself and one that can be used in recipes from appetizers to desserts.
Yogurt is thought to have originated many centuries ago among the nomadic tribes of Eastern Europe and Western Asia. Milk stored in animal skins would acidify and coagulate. The acid helped preserve the milk from further spoilage and from the growth of pathogens (disease-causing microorganisms).
Ingredients (to make 4-5 cups of yogurt) :
- 1-quart milk (cream, whole, low fat, or skim) – In general the higher the milk fat level in the yogurt the creamier and smother it will taste. Note: If you use home-produced milk it must be pasteurized before preparing yogurt.
- Nonfat dry milk powder – Use 1/3-cup powder when using whole or low fat milk, or use 2/3-cup powder when using skim milk. The higher the milk solids the firmer the yogurt will be. For even more firmness add gelatin (directions below).
- Commercial, unflavored, cultured yogurt – Use 1/4 cup. Be sure the product label indicates that it contains a live culture. Also note the content of the culture. L. bulgaricus and S. thermophilus are required in yogurt, but some manufacturers may in addition add L. acidophilus and/or B. bifidum. The latter two are used for slight variations in flavor, but more commonly for health reasons attributed to these organisms. All culture variations will make a successful yogurt.
- (Optional) 2 to 4 tablespoons sugar or honey.
- (Optional) For a thick, firm yogurt swell 1 teaspoon unflavored gelatin in a little milk for 5 minutes. Add this to the milk and non-fat dry milk mixture before cooking.
- Double Boiler, preferred or regular saucepan 1-2 quarts in capacity larger than the volume of yogurt you wish to make.
- Cooking or Jelly Thermometer. A thermometer that can clip to the side of the saucepan and remain in the milk works best. Accurate temperatures are critical for successful processing.
- Mixing spoon
- Yogurt containers, e.g. cups with lids or canning jars with lids.
- Incubator: a yogurt-maker, oven, heating pad, or warm spot in your kitchen. To use your oven, place yogurt containers into deep pans of 110°F water. Water should come at least halfway up the containers. Set oven temperature at lowest point to maintain water temperature at 110°F. Monitor temperature throughout incubation making adjustments as necessary.
1. Pasteurization for any non-commercial milk.
Heat water in the bottom section of a double boiler and pour milk into the top section. Cover the milk and heat to 165Â°F while stirring constantly for uniform heating. Cool immediately by setting the top section of the double boiler in ice water or cold running water. Store milk in the refrigerator in clean containers until ready for making yogurt.
2. Combine ingredients and heat.
Heating the milk is a necessary step to change the milk proteins so that they set together rather than to form curds and whey. Do not substitute this heating step for pasteurization. Place cold, pasteurized milk in top of a double boiler and stir in nonfat dry milk powder. Adding non-fat dry milk to heated milk will cause some milk proteins to coagulate and form strings. Add sugar or honey if a sweeter, less tart yogurt is desired. Heat milk to 200Â°F, stirring gently and (a) hold for 10 minutes for thinner yogurt or (b) hold 20 minutes for thicker yogurt. Do not boil. Be careful and stir constantly to avoid scorching if not using a double boiler.
3. Cool and inoculate.
Place the top of the double boiler in cold water to cool milk rapidly to 112-115°F. Remove one cup of the warm milk and blend it with the yogurt starter culture. Add this to the rest of the warm milk. The temperature of the mixture should now be 110-112°F.
Pour immediately into clean, warm container(s); cover and place in prepared incubator. Close the incubator and incubate about 4 – 7 hours at 110°F. Yogurt should set firm when the proper acid level is achieved (pH 4.6). Incubating yogurt for several hours past the time after the yogurt has set will produce more acidity. This will result in a more tart or acidic flavor and eventually cause the whey to separate.
Rapid cooling stops the development of acid. Yogurt will keep for about 10-21 days if held in the refrigerator at 40Â°F or lower.
- Set yogurt: A solid set where the yogurt firms in a container and not disturbed.
- Stirred yogurt: Yogurt made in a large container then spooned or otherwise dispensed into secondary serving containers. The consistency of the â€œsetâ€ is broken and the texture is less firm than set yogurt. This is the most popular form of commercial yogurt.
- Drinking yogurt: Stirred yogurt to which additional milk and flavors are mixed in. Add fruit or fruit syrups to taste. Mix in milk to achieve the desired thickness. The shelf life of this product is 4-10 days, since the pH is raised by fresh milk addition. Some whey separation will occur and is natural. Commercial products recommend a thorough shaking before consumption.
- Fruit yogurt: Fruit, fruit syrups, or pie filling can be added to the yogurt. They are placed on top, on bottom, or stirred into the yogurt.
- Yogurt cheese: Line a large strainer or colander with cheesecloth. Place this over a bowl and then pour in the yogurt. Do not use yogurt made with the addition of gelatin. Gelatin will inhibit whey separation. Let it drain overnight covered with plastic wrap. Empty the whey from the bowl. Fill a strong plastic storage bag with some water, seal and place over the cheese to weigh it down. Let the cheese stand another 8 hours after which it is ready to use. The flavor is similar to a sour cream with a texture of a soft cream cheese. A pint of yogurt will yield approximately 1/4 lb. of cheese. The yogurt cheese has a shelf life of approximately 7-14 days when wrapped and placed in the refrigerator and kept at less than 40Â°F. For uses, recipes, and more information on yogurt cheese see the “Resources”; section below.
- Frozen yogurt: Follow directions given with most home ice cream makers.
source: www.uga.edu, photo from biology.clc.uc.edu