Dry cultures for making yogurt can be purchased in some health food stores, but they are usually expensive. Dry cultures also may be purchased directly from a manufacturer such as: Chr. Hansen's Laboratory, Inc., 9015 West Maple Street, Milwaukee, Wisconsin 53214.
The easiest and least expensive way of obtaining a starter culture is to purchase plain yogurt at a grocery store. It should be plain--no fruit added. Fruit may contribute undesirable yeasts and bacteria to the yogurt, making it a poor starter culture.
You must use a brand of plain yogurt whose label indicates that the product contains alive culture; some brands of plain yogurt do not contain a live culture because the yogurt has been pasteurized.
To maintain a culture, save a small portion of yogurt (1 c is enough for a 1-gal batch) to use as a starter culture for the next batch. Be sure to refrigerate the starter culture in a clean, air-tight container.
From time-to-time a culture may become contaminated, and a new culture is needed. By using a newculture, the original flavor and a minimal coagulation time are retained.
Accurate temperature control helps assure rapid coagulation and a good-tasting yogurt. A thermometer that measures temperature in the range of 90°F to 120°F should be adequate. A good stainless steel thermometer (Model 2292) is available from: Weston Instruments, Inc., 614 Frelinghuysen Avenue, Newark, New Jersey 07114. A glass thermometer can be used, but may break easily. Thermometers are not needed with special yogurt-making equipment.
Yogurt can be made by using only nonfat dry milk (NFDM) and water, or by adding NFDM to skim milk, 2% milk, or regular milk. Nonfat dry milk is commonly available in two forms, instant and regular. Ideally, the milk powder should be weighed to obtain the desired solids content (15 percent on a weight basis). Because weighing might not be possible in all home kitchens, measurements both by weight and volume are provided in the following recipes (Table I). For each recipe, the quantity of ingredients necessary for making either 1 qt or 1 gal of yogurt is given. G79-449-A (Reprint March 1990)
Making Yogurt at Home
Information on and instructions for making yogurt are included here.
Susan Sumner, Extension Food Microbiologist, and Robert Hutkins, Research Food Scientist*
Yogurt is a tangy, nutritionally excellent dairy product that can be made at home. The milk used contains a higher concentration of solids than normal milk. By increasing the solids content of the milk, a firm, rather than soft, end product results. Addition of nonfat dry milk (NFDM) is the easiest at-home method for doing this.
Yogurt is made by inoculating certain bacteria (starter culture), usually Streptococcus thermophilus and Lactobacillus bulgaricus, into milk. After inoculation, the milk is incubated at approximately 110°F ± 5°F until firm; the milk is coagulated by bacteria-produced lactic acid.
Making yogurt at home is fun and less expensive than buying it. It can be made with ordinary kitchen utensils. The materials and directions necessary for making yogurt follow.
Table I - Yogurt Recipes
Add Liquid Ingredient + Dry Ingredient (Non Fat Dry Milk) (both instant or regular given)
1 gal water + 22.2 NFDM oz = 8 1/3 c Instant or 4 3/4 c Regular Non Fat Dry Milk
1 qt water + 5.6 oz NFDM = 2 c Instant or 1 1/4 c Regular Non Fat Dry Milk
1 gal skim milk + 10.4 NFDM oz = 4 c Instant or 2 1/4 c Regular Non Fat Dry Milk
1 qt skim milk + 2.6 oz NFDM = 1 c Instant or 1/2 c Regular Non Fat Dry Milk
1 gal 2% milk + 7.2 oz NFDM = 2 3/4 c Instant or 1 1/2 c Regular Non Fat Dry Milk
1 qt 2% milk + 1.8 oz NFDM = 3/4 c Instant or 1/3 c Regular Non Fat Dry Milk
1 gal regular milk + 4.8 NFDM oz = 1 3/4 c Instant or 1 c Regular Non Fat Dry Milk
1 qt regular milk + 1.2 oz NFDM = 1/2 c Instant or 1/4 c Regular Non Fat Dry Milk
*NFDM = Nonfat dry milk, gal = gallon, oz = ounce, c = cup, Regular Non Fat Dry Milk
Method for making yogurt
1. Mix the appropriate quantities of liquid and dry ingredients given in Table I.
2. Heat this milk in a saucepan or double boiler to boiling and cool immediately to 110°F. Discard any "skin" that may have formed on the milk. Sugar may be added to the milk before boiling, if desired. Heating the milk to boiling kills any undesirable bacteria that might be pre-sent and also changes the properties of the milk protein so that it gives the yogurt a firmer body and texture.
3. To 1 gal of milk, add 1 cup of warm 110°F starter culture. Mix well but gently. Do not incorporate too much air. If too much air is mixed in, the starter culture will grow slowly.
4. Sanitize yogurt containers by rinsing with boiling water.
5. Pour milk into clean container(s) and cover with lid. If fruit is to be added to the yogurt, put in the bottom of the cup before adding the inoculated milk. The fruit should be at a temperature of 110°F.
(Omit fruit from a small portion of the recipe and save it to use as a starter culture in the next batch.) Incubate filled containers at 110°F. Do not stir the yogurt during this period. There are several ways to control temperature during incubation:
a. Special yogurt-making equipment allows for careful temperature control without a thermometer and reduces the chances of failure.
b. Yogurt containers can be kept warm in a gas oven with pilot light and electric bulb, or an electric oven with light bulb of sufficient wattage (approximately 100 watts).
c. A Styrofoam box with light bulb may be used as an incubator.
d. Another good way to control temperature is to place yogurt containers into pans of 110°F water in an oven or an electric frying pan. Set oven temperature at lowest point to maintain water temperature at 110°F.
e. Wide-mouth thermos bottles, heating pads, and sunny windows also have been used. Regardless of the method of temperature control used, determine ahead of time that the proper temperature can be maintained. To do this, place water or a container of water in the incubator and monitor its temperature with a thermometer.
6. Maintain 110°F temperature until the milk coagulates with a firm custard-like consistency (3-6 hrs). Check by gently tilting cup. Then refrigerate. It will keep for two to three weeks in the refrigerator.
1. Problem: Yogurt does not have a custard-like body but rather is soft and not smoothly solidified.
a. Addition of starter culture to the milk before it has cooled down may kill the culture and prevent coagulation. Solution: Wait until the milk cools down to 110°F before inoculating.
b. Both high and low incubation temperatures slow down culture growth and increase the amount of time necessary for coagulation. Solution: Use a thermometer to control temperature.
c. Extended storage of the starter culture reduces the number of live bacteria in the culture. Solution: Use more starter culture in the recipe or obtain a new culture.
d. Contamination of the culture with undesirable bacteria. Solution: Get a new culture. Also clean and sanitize yogurt containers each time yogurt is made.
e. Omitted or added an insufficient amount of nonfat dry milk to the milk. Solution:Accurately measure or weigh the nonfat dry milk.
f. Over-agitation before incubation may slow down starter activity. Solution: Combine starter culture and milk by mixing gently.
2. Problem: Yogurt tastes bad.
a.Starter culture is contaminated. Solution: Obtain new culture.
b.Yogurt has over-set or incubated too long. Solution: Refrigerate yogurt immediately after
a firm coagulum has formed.
c.Overheating of the milk causes an off-flavor. Solution: Do not overheat the milk.
3. Problem: Whey collects on the surface of the yogurt.
a. Yogurt was over-set or incubated too long. Solution: Refrigerate yogurt immediately after a firm coagulum has formed.
b. Yogurt was bumped, moved or stirred during incubation. Solution: Place yogurt in a quiet location where it will not be disturbed.
*This WebGuide was originally prepared by Stan Wallen, former Extension Food Scientist.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kenneth R. Bolen, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources. University of Nebraska Cooperative Extension educational programs abide with the non-discriminationpolicies of the University of Nebraska-Lincoln and the United States Department of Agriculture.
Temperature G79-449-A (Reprint March 1990) Making Yogurt at Home