Unit
Food Science
Problem Area
Processing Animal Products
Lesson
Yogurt Production
Student Learning Objectives
1. Describe the composition of yogurt.
2. Explain how fermentation and
anaerobic respiration are needed to create
the yogurt product.
Terms
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Aerobic respiration
Anaerobic respiration
End product
Fermentation
Food fermentation
Lactic acid
Lactose
Viscosity
Yogurt
What is the composition of yogurt?
 Yogurt, a fermented dairy product whereby milk is
inoculated with bacteria cultures, is an example of a mixed
pure culture fermentation. That is, a controlled mixture of
known cultures of bacteria used in the fermentation
process.
 A. Yogurt is the product of fermentation, a slow
decomposition process of organic substances induced by
microorganisms or enzymes. Food fermentation is the
study of microbial activity, usually anaerobic, on suitable
substrates under controlled or uncontrolled conditions. To
produce yogurt, lactose, a compound sugar found in milk
known as lactin or milk sugar, is fermented by two different
species of bacteria: Lactobacillus and Streptococcus.
 B. Yogurt is commercially produced by adding to milk a
2–4% nonfat dry milk powder that has been inoculated
with a 5% combination of Lactobacillus bulgaricus and
Streptococcus thermophilus (1:1 ratio). The milk mixture
is then incubated at 45 degrees C for 3 to 6 hours. The
product must be chilled immediately.
 C. The fat content of yogurt varies from 0-3.5%; most
yogurt is low fat and contains 1–1.5% fat.
 D. Lactic acid, the end product of anaerobic metabolism
of glucose, provides the tart flavor of yogurt, as well as
the formation of a gel structure. The major flavor
components of yogurt are carbonyl compounds; among
these, acetaldehyde is the most important and gives the
yogurt its green apple or nutty flavor. Yogurt quality is
based on color, appearance, body, texture, and flavor.
 E. Ropy (slime-producing) lactic acid bacteria produce
polysaccharides that are released into the yogurt where
they increase viscosity and improve water retention.
Viscosity is the resistance of a fluid to flow. Yogurt has a
high moisture content of 82–86%.
 F. Milk SNF (solids-not-fat) content of yogurt varies from
9–16%. SNF can be increased by adding milk powder,
and by other means. Increased SNF levels are needed
to increase protein content, which helps to increase
product viscosity to desired levels.
 G. Frozen yogurt is manufactured by mixing varied
amounts of fermented yogurt with ice milk containing
sweeteners, stabilizers, etc.
How are fermentation and anaerobic respiration needed to
create the yogurt product?
 A. Aerobic respiration is the total oxidative degradation
of glucose that must have oxygen to take place.
Fermentation is an example of anaerobic respiration,
which takes place in the absence of oxygen and consists
essentially of the early stages of aerobic respiration.
During this type of respiration, glucose is converted to a
variety of end products, such as lactic acid.
 Since acids produce a sour taste, the result is a sourtasting dairy product. The increased acidity (lowered pH)
of the milk causes milk protein to coagulate and become
more viscous. The bacteria Streptococcus also produces
other compounds that have an effect on final flavor.
 B. Lactic acid bacteria are gram-positive, non-spore
forming bacteria that produce lactic acid as the major
product of fermentation. These bacteria are very
important in pickling, cheese making, fermented dairy
products, and other technologies.
 C. Milk proteins have the unique ability to curdle (form a
gel). Curdling is induced by proteolytic enzymes, lactic
acid, heat, and other means. Each milk gel has a solid
structure consisting of a protein matrix and other
components. This gel matrix has the ability to immobilize
the liquid phase of milk. By modifying this ability, it is
possible to manufacture stable milk products with a high
water content (i.e. yogurt).
 D. Milk to be used in yogurt production is usually heated
to increase the total solids content in order to make a
firm end product. An end product is the final compound
or substance resulting from a chemical reaction. Heating
causes the micelles in the milk to interconnect in chains
to form the gel matrix.
 E. Cooling of the yogurt after incubation (fermentation)
stops bacterial fermentation.
 F. If cows are receiving antibiotic treatments, their milk is
usually discarded for a specific period of time.
Otherwise, residual effects from the antibiotics in the milk
could inhibit the growth of desirable bacteria necessary
for yogurt production.
Review/Summary
What is the composition of yogurt?
How are fermentation and anaerobic
respiration needed to create the yogurt
product?