PROCEDURE
Objectives:
      After completing this exercise you should be able to
1. Explain how the activities of microorganisms are used to preserve food.
2. Define fermentation.
3. Produce an enjoyable product.

Introduction:
      Microbial fermentations are used to produce a wide variety of foods. When used in industry, fermentation
is any large-scale microbial process occurring with or without air. To a biochemist, fermentation:
      1) is a group of metabolic processes that release energy from sugar or other organic molecules,
      2) does not require oxygen, and
      3) actions of the microbes creates an acidic environment..
In this exercise a lactic acid fermentation will be used to produce a food.
      n dairy fermentations such as yogurt production, microorganisms use the lactose sugar present
for energy, and produce lactic acid as a waste product without using oxygen. In a nondairy
fermentation such as wine production, yeast use the sucrose sugar present for energy and produce
ethyl alcohol and carbon dioxide as wastes, again without using oxygen. If oxygen is available, the
yeast will grow aerobically, releasing carbon dioxide and water instead of alcohol, thus assuring that
no wine will be produced.
      Historically, milk has been selectively fermented, since the resulting acidity prevents spoilage by
most microbes. These "sour" milks have varied from country to country depending on the source of
the milk, the conditions of the culture, and the bacteria used as the "starter" culture. Milk from
donkeys to zebras has been used, with the Russian kumiss (horse milk), containing 2% alcohol,
and Swedish surjolk (reindeer milk) being unusual examples. In general, bacteria will yield lactic
acid products while yeast will yield alcoholic products. Currently, two fermented cow milk products,
buttermilk and yogurt, are widely used.
      Buttermilk is the fluid left after cream is churned into butter. Today, buttermilk is actually
prepared by souring true buttermilk or by adding bacteria to skim milk and then flavoring it with
butterflake. Streptococcus lactis ferments the milk producing lactic acid (sour) while neutral
fermentation products (diacetyls) are produced by Leuconostoc. Yogurt originated in the Balkan
countries, with goat milk being the primary source of milk. Yogurt is milk that has been
concentrated by boiling, and then fermented at elevated temperatures. Streptococcus produces
lactic acid and Lactobacillus produces the flavors and aroma of the yogurt.

Culture
Commercial yogurt or Streptococcus thermophilus and Lactobacillus bulgaricus. Commercial
yogurts with “Active Cultures” on their label contain one or both of these.

Procedure
Be sure all glassware is clean.
1. Add 100 ml milk per person (in your group) to a wet beaker (wash out beaker first with
      water to decrease sticking of the milk).
2. Heat milk on a hot plate or over a bunsen burner until it reaches 80°C for 40 min. Stir
      occasionally. Do not let it boil.
3. Cool to about 65°C and add 3 grams nonfat dry milk per 100 ml of milk. Stir to dissolve.
4. Rapidly cool the milk to about 45°C. Pour the milk equally into two cups.
5. Inoculate each cup with 1-2 teaspoons of commercial yogurt. Cover the cups with aluminum
      foil and label the foil with a marker or wax pencil.
6. Place the yogurt in an incubator set at 43°C for 4 to 18 hours so it may ferment and thicken
      (becomes custard-like).
7. Cool the yogurt to about 5°C by placing it in a refrigerator.
8. Remove the aluminum foil, obtain a clean spoon, and taste the yogurt. Add sugar, fruits,
      jams, etc. to produce any additional flavors you desire. Eat and enjoy!