Processing Animal Products

Souring Milk

 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.

 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.

 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?