Unit
Food Science
Problem Area
Processing Animal Products
Lesson
Souring Milk
Student Learning Objectives
1. Describe bacteria which can be present
in milk.
2. Explain factors that determine how
rapidly milk begins to sour.
3. Describe procedures for maintaining
the quality of milk by reducing the rate of
souring.
Terms
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Mastitis
Pasteurization
Pre-dipping
Sanitized
Streptococcus lactis
What are the bacteria that can be present in milk?
 A. Milk in a healthy cow is free from microorganisms. By
the time the milk reaches consumers, there are bacteria
present. Bacteria are introduced during the milking and
handling process. Some of the common bacteria that
can be present in pasteurized milk and the stages in
which they appear are:
 1. Streptococcus lactis—a type of lactic acid bacteria
usually found in milk. Lactic acid will coagulate the milk
proteins found in liquid milk, causing souring.
 2. Lactobacillus
 3. Yeasts and Molds
 4. Bacillus
 B. Bacteria harmful to man are usually not found
in milk unless they have been introduced from a
diseased cow or an infected milk handler. Some
bovine diseases that can be transmitted to
humans are brucellosis, tuberculosis, and
mastitis.
 Mastitis is a very contagious infection of the
udders of cattle that is estimated to affect 40–
50% of all U.S. dairy cattle in at least one
quarter of their udders.
What factors determine how rapidly milk will begin to
sour?
 A. Bacteria in milk prefer similar environmental
conditions as most other bacteria. Some conditions that
will cause milk to sour more rapidly are:
 1. Warm temperature. Milk will sour more quickly as milk
temperature rises, up to the temperature where milk
bacteria are killed.
 2. Acidity level. Milk with a lower pH creates an
environment that is more favorable for the growth of
bacteria.
 3. Sanitation. Milk that has been contaminated during the
milking or handling process will have higher starting counts
of bacteria, which can in turn, cause rapid spoiling of the
milk.
What procedures are necessary for maintaining the
quality of milk by reducing the rate of souring?
 A. The most common procedure for maintaining the
quality of milk is pasteurization. Pasteurization is the
process of heating raw milk to a certain temperature to
kill bacteria. Even though pasteurization kills many of the
harmful bacteria in milk, all bacteria are not destroyed
and will eventually sour the milk. There are two common
methods of pasteurization.
 1. The low temperature holding (vat) method holds milk at
63°C for 30 minutes.
 2. The high temperature/short time method heats the milk
to 72°C for 15 seconds.
 B. Other methods for controlling bacterial growth in milk are:
 1. Keeping milking employee hands and clothing
sanitized, or free from bacterial contamination.
 2. Keeping cow stalls clean and free from mud and
manure.
 3. Keeping udders clean and udder hairs clipped short.
 4. Washing, drying, and pre-dipping, a process of dipping
udders into an antibacterial solution prior to milking, will
reduce numbers of bacteria for contamination.
 5. Cleaning and sanitizing the milking equipment and
storage tanks regularly.
 6. Cooling milk quickly after milking. Milk should be
immediately cooled to 40 degrees F. for holding before
transporting to a processing plant.
Review/Summary
What are the bacteria that can be present
in milk?
What factors determine how rapidly milk
will begin to sour?
What procedures are necessary for
maintaining the quality of milk by reducing
the rate of souring?
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