Milk, Fermentation, and Fermented and Nonfermented Dairy Products 157
The pH of fresh whole milk is around 6.6 but it may reach ca. 6.8 from a cow that has mastitis.
Mastitis is an infection of the udder that is most often caused by Streptococcus agalactiae and S. uberis
but sometimes by Staphylococcus aureus or Streptococcus dysgalactiae. Fresh milk from a mastitic
cow typically contains leucocytes (white blood cells) >10
6 /ml in contrast to nonmastitic milk that
contains leucocytes around 70,000/ml.
Milk contains a very adequate supply of B vitamins with pantothenic acid and riboflavin being the
two most abundant. Vitamins A and D are added for human consumption, and their presence has no
known effect on the activity of microorganisms.
Overall, the chemical composition of whole cow’s milk makes it an ideal growth medium for
heterotrophic microorganisms, including the nutritionally fastidious Gram-positive lactic acid bacteria.
How the milk microbiota utilize these constituents and bring about its spoilage is covered below under
spoilage.
Processing
Milk is processed in a number of ways to produce a variety of products such as cream, cheese, and
butter. Whole fresh milk is processed to produce a number of fluid products. Skim milk (0.5% fat)
or reduced fat milk (up to 2.0% fat) is produced by high-speed centrifugation following heating to
ca. 100
◦ F to remove butter fat as cream, or by use of skim milk to which the desired fat content is
added. The latter is pasteurized either at 150–155
◦ F (65.5–68.3
◦ C) for 30 minutes or at 166–175
◦ F
(74.4–79.4
◦ C) for 15 sec prior to cooling to around 4
◦ C.
78
Evaporated milk is produced by the removal of about 60% water from whole milk which results
in the lactose content being about 11.5%. Sweetened condensed milk is produced by the addition
of sucrose or glucose before evaporation. This leads to a product with a sugar content of about 54%
or >64% in solution.
In the United States, grade A raw milk that is to be pasteurized should not have an APC that exceeds
300,000 cfu/ml for commingled or blended milk, or should not exceed 100,000/ml for milk from an
individual producer. After pasteurization, the APC should not exceed 20,000 cfu/ml, and the coliform
count should not exceed 10/ml.
15 Raw milk should not be held longer than 5 days at 40
◦ F (4.4
◦ C)
prior to pasteurization.
Chocolate milk is processed at a slightly higher temperature than unflavored milk (75
◦ C for 15 sec
rather than 72
◦ C). A study of chocolate milk from four plants revealed that the APC was higher at
14 days post-processing than unflavored milk even though the initial numbers for both types were
essentially the same.
13 On day 14, 76.1% of unflavored and 91.6% of chocolate milk had APCs
>20,000 cfu/ml with 26.1% of the former and 53.7% of the latter products having APCs >10
6 cfu/ml.
These investigators suggested that the chocolate flavor powder contributed to increased growth.
13 The
higher numbers were not due to higher numbers in the chocolate powder per se.
Pasteurization
The objective of milk pasteurization is the destruction of all disease-causing microorganisms. Endospores of pathogens such as Clostridium botulinum and spoilage organisms such as Clostridium
tyrobutyricum, C. sporogenes, or Bacillus cereus are not destroyed. Although pathogens can be destroyed by nonthermal means, milk pasteurization is achieved solely by heating.
The pH of fresh whole milk is around 6.6 but it may reach ca. 6.8 from a cow that has mastitis.
Mastitis is an infection of the udder that is most often caused by Streptococcus agalactiae and S. uberis
but sometimes by Staphylococcus aureus or Streptococcus dysgalactiae. Fresh milk from a mastitic
cow typically contains leucocytes (white blood cells) >10
6 /ml in contrast to nonmastitic milk that
contains leucocytes around 70,000/ml.
Milk contains a very adequate supply of B vitamins with pantothenic acid and riboflavin being the
two most abundant. Vitamins A and D are added for human consumption, and their presence has no
known effect on the activity of microorganisms.
Overall, the chemical composition of whole cow’s milk makes it an ideal growth medium for
heterotrophic microorganisms, including the nutritionally fastidious Gram-positive lactic acid bacteria.
How the milk microbiota utilize these constituents and bring about its spoilage is covered below under
spoilage.
Processing
Milk is processed in a number of ways to produce a variety of products such as cream, cheese, and
butter. Whole fresh milk is processed to produce a number of fluid products. Skim milk (0.5% fat)
or reduced fat milk (up to 2.0% fat) is produced by high-speed centrifugation following heating to
ca. 100
◦ F to remove butter fat as cream, or by use of skim milk to which the desired fat content is
added. The latter is pasteurized either at 150–155
◦ F (65.5–68.3
◦ C) for 30 minutes or at 166–175
◦ F
(74.4–79.4
◦ C) for 15 sec prior to cooling to around 4
◦ C.
78
Evaporated milk is produced by the removal of about 60% water from whole milk which results
in the lactose content being about 11.5%. Sweetened condensed milk is produced by the addition
of sucrose or glucose before evaporation. This leads to a product with a sugar content of about 54%
or >64% in solution.
In the United States, grade A raw milk that is to be pasteurized should not have an APC that exceeds
300,000 cfu/ml for commingled or blended milk, or should not exceed 100,000/ml for milk from an
individual producer. After pasteurization, the APC should not exceed 20,000 cfu/ml, and the coliform
count should not exceed 10/ml.
15 Raw milk should not be held longer than 5 days at 40
◦ F (4.4
◦ C)
prior to pasteurization.
Chocolate milk is processed at a slightly higher temperature than unflavored milk (75
◦ C for 15 sec
rather than 72
◦ C). A study of chocolate milk from four plants revealed that the APC was higher at
14 days post-processing than unflavored milk even though the initial numbers for both types were
essentially the same.
13 On day 14, 76.1% of unflavored and 91.6% of chocolate milk had APCs
>20,000 cfu/ml with 26.1% of the former and 53.7% of the latter products having APCs >10
6 cfu/ml.
These investigators suggested that the chocolate flavor powder contributed to increased growth.
13 The
higher numbers were not due to higher numbers in the chocolate powder per se.
Pasteurization
The objective of milk pasteurization is the destruction of all disease-causing microorganisms. Endospores of pathogens such as Clostridium botulinum and spoilage organisms such as Clostridium
tyrobutyricum, C. sporogenes, or Bacillus cereus are not destroyed. Although pathogens can be destroyed by nonthermal means, milk pasteurization is achieved solely by heating.