Fermenting Yogurt at Home
Brian A. Nummer, Ph.D.
National Center for Home Food Preservation
October 2002
Introduction
Yogurt is made by adding Streptococcus thermophilus and
Lactobacillus bulgaricus into heated milk. After this inoculation
the milk is held at 110°F ± 5°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.
History
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 ¼-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.
Tools
- 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.
Processing
- 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 165°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.
- 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 200°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.
- 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.
- Incubate.
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 ± 5°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.
- Refrigerate.
Rapid cooling stops the development of acid. Yogurt will keep for
about 10-21 days if held in the refrigerator at 40°F or lower.
Yogurt Types
- 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 “set” 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 40°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.
Trouble-shooting
If your:
- Milk forms some clumps or strings during the heating step. Some
milk proteins may have jelled. Take the solids out with
a slotted spoon or in difficult cases after cooking pour the milk
mixture through a clean colander or cheesecloth before inoculation.
- Yogurt fails to coagulate (set) properly. Milk proteins will
coagulate when the pH has dropped to 4.6. This is done by the culture
growing and producing acids.
- Adding culture to very hot milk (+115°F) can kill bacteria--Use
a thermometer to carefully control temperature.
- Too hot or too cold of an incubation temperature can slow down
culture growth--Use a thermometer to carefully control
temperature.
- The starter culture was of poor quality--Use a fresh,
recently purchased culture from the grocery store each time
you make yogurt.
- Yogurt tastes or smells bad.
- Starter culture is contaminated--Obtain new culture
for the next batch.
- Yogurt has over-set or incubated too long--Refrigerate
yogurt immediately after a firm coagulum has formed.
- Overheating or boiling of the milk causes an off-flavor--Use
a thermometer to carefully control temperature.
- Whey collects on the surface of the yogurt. This is called syneresis.
Some syneresis is natural. Excessive separation of whey, however,
can be caused by incubating yogurt too long or by agitating the
yogurt while it is setting.
Food safety, spoilage and shelf life
Yogurt provides two significant barriers to pathogen growth: (a)
heat and (b) acidity (low pH). Both are necessary to ensure a safe
product. Acidity alone has been questioned by recent outbreaks of
food poisoning by E. coli O157:H7 that is acid-tolerant.
E. coli O157:H7 is easily destroyed by pasteurization (heating).
Therefore, always pasteurize milk or use commercially pasteurized
milk to make yogurt.
Discard batches that fail to set properly, especially those due
to culture errors. Yogurt generally has a 10-21 day shelf life when
made and stored properly in the refrigerator below 40°F. Molds,
yeasts and slow growing bacteria can spoil the yogurt during prolonged
storage. Ingredients added to yogurt should be clean and of good
quality. Introducing microorganisms from yogurt add-ins can reduce
shelf life and result in quicker spoilage--"garbage in,
garbage out". Discard any yogurt samples with visible
signs of microbial growth or any odors other than the acidity of
fresh yogurt.
Always use clean and sanitized equipment and containers to ensure
a long shelf life for your yogurt. Clean equipment and containers
in hot detergent water, then rinse well. Allow to air dry.
Kitchen Notes
When making this recipe in our test kitchen we used a saucepan instead
of a double boiler. Despite constant stirring we still had some
minor scorching. We took care not to stir or scrape the scorched
area. During the cooking step milk proteins formed strings that
we scooped out with a slotted spoon. We inoculated our entire batch
of milk with starter and poured the mixture into separate containers.
To some containers we added different amounts of honey or sugar
stirring to dissolve the sweetener, while others we left plain.
Our yogurt reached pH 4.7 in approx. four hours, pH 4.6 in approx.
five hours and pH 4.5 in approx. six hours. The yogurt set was firm
after six hours and the taste was mild. The yogurt was immediately
refrigerated until the next day. On the following day we processed
the yogurt into some of the variations listed above under "Yogurt
Types".
Resources
The following information is provided as a courtesy to the reader.
No endorsements are made or implied for commercial products and
none have been tested in our labs or kitchens. For commercial products
other makes, models, or alternatives are almost certainly available.
Cultures and Probiotics
Commercial Yogurt Sites
Yogurt Makers (Incubators).
Reviewed by Joseph Frank, Ph.D., Department of Food Science and
Technology University of Georgia and Elizabeth Andress, Ph.D. and
Elaine D’Sa, Ph.D. for the National Center for Home Food Preservation.
This material is based upon work supported by the Cooperative State
Research, Education, and Extension Service, U.S. Department of Agriculture,
under Agreement No. 00-51110-9762.
Document Use:
Permission is granted to reproduce these materials in whole or
in part for educational purposes only (not for profit beyond the
cost of reproduction) provided the authors and the University of
Georgia receive acknowledgment and this notice is included:
Reprinted with permission of the University of Georgia. B.A. Nummer.
2002. Fermenting Yogurt at Home. Athens, GA: The University of Georgia,
Cooperative Extension Service.
References to commercials products, services, and information is
made with the understanding that no discrimination is intended and
no endorsement by the University of Georgia, U.S. Department of
Agriculture and supporting organizations is implied. This information
is provided for the educational information and convenience of the
reader.
The University of Georgia and Ft. Valley State University, the
U.S. Department of Agriculture and counties of the state cooperating.
The Cooperative Extension Service, the University of Georgia College
of Agricultural and Environmental Sciences offers educational programs,
assistance and materials to all people without regard to race, color,
national origin, age, sex or disability. An Equal Opportunity Employer/Affirmative
Action Organization Committed to a Diverse Work Force.
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