Spoilage of Specific Food Groups ◾ 261
Milk and Milk Products
Raw Milk
Raw milk contains many types of microorganisms, including pathogens and spoilage coming
from different sources. 5 The average composition of cow’s milk is protein 3.2%, carbohydrates
4.8%, lipids 3.9%, and minerals 0.9%. Besides casein and lactalbumin, it has free amino acids,
which can provide a good N source (and some C source, if necessary). As the main carbohydrate
is lactose, those microorganisms with lactose-hydrolyzing enzymes (phospho-β-galactosidase or
β-galactosidase) have an advantage over those unable to metabolize lactose. Milk fat can be hydrolyzed by microbial lipases, with the release of small-molecular volatile fatty acids (butyric, capric,
and caproic acids).
Microbial spoilage of raw milk can potentially occur from the metabolism of lactose, proteinaceous compounds, fatty acids (unsaturated), and the hydrolysis of triglycerides. If the milk is
refrigerated immediately following milking and stored for days, the spoilage will be predominantly
caused by Gram-negative psychrotrophic rods, such as Pseudomonas, Alcaligenes, Flavobacterium
spp., and some coliforms. Pseudomonas and related species, being lactose negative, metabolize
proteinaceous compounds to change the normal flavor of milk to bitter, fruity, or unclean. They
also produce heat-stable lipases (producing a rancid flavor) and heat-stable proteinases that have
important implications; this is discussed in Chapter 22. The growth of lactose-positive coliforms
produces lactic acid, acetic acid, formic acid, CO 2 , and H 2 (by mixed-acid fermentation) and
causes curdling, foaming, and souring of milk. Some Alcaligenes spp. (Alc. faecalis) and coliforms
can also cause ropiness (sliminess) by producing viscous exopolysaccharides. However, if the raw
milk is not refrigerated soon, growth of mesophiles, such as species of Lactococcus, Lactobacillus,
Enterococcus, Micrococcus, and coliforms, along with Pseudomonas, Proteus, and spore formers, such
as Bacillus and Clostridium, predominates. 6 But lactose-hydrolyzing species, such as Lactococcus
spp. and Lactobacillus spp., generally predominate, producing enough acid to lower the pH considerably and prevent or reduce growth of others. In such situations, curdling of milk and sour flavor
are the predominant spoilages. If other microorganisms also grow, gas formation, proteolysis,
and lipolysis become evident. Yeast and mold growth, under normal conditions, is generally not
expected.
Pasteurized Milk
Raw milk is pasteurized before it is sold for consumption as liquid milk. Thermoduric bacteria (Micrococcus, Enterococcus, some Lactobacillus, Streptococcus, Corynebacterium, and spores of
Bacillus and Clostridium) survive the process. In addition, coliforms, Pseudomonas, Alcaligenes,
Flavobacterium, and similar types can enter as postpasteurization contaminants. Pasteurized milk,
under refrigerated storage, has a limited shelf life, mainly because of growth of these psychrotrophic contaminants. Their spoilage pattern is the same as described for raw milk spoilage. Flavor
defects from their growth are detectable when the population reaches ≥10 6 cells/mL. Growth of
psychrotrophic Bacillus spp., such as Bacillus cereus, has been implicated in the spoilage of pasteurized refrigerated milk, especially when the levels of postpasteurization contaminants are low.
Spores of psychrotrophic Bacillus spp., surviving pasteurization, germinate and multiply to cause
a defect known as bitty. They produce the enzyme lecithinase, which hydrolyzes phospholipids
of the fat globule membrane, causing aggregation of fat globules that adhere to the container
Milk and Milk Products
Raw Milk
Raw milk contains many types of microorganisms, including pathogens and spoilage coming
from different sources. 5 The average composition of cow’s milk is protein 3.2%, carbohydrates
4.8%, lipids 3.9%, and minerals 0.9%. Besides casein and lactalbumin, it has free amino acids,
which can provide a good N source (and some C source, if necessary). As the main carbohydrate
is lactose, those microorganisms with lactose-hydrolyzing enzymes (phospho-β-galactosidase or
β-galactosidase) have an advantage over those unable to metabolize lactose. Milk fat can be hydrolyzed by microbial lipases, with the release of small-molecular volatile fatty acids (butyric, capric,
and caproic acids).
Microbial spoilage of raw milk can potentially occur from the metabolism of lactose, proteinaceous compounds, fatty acids (unsaturated), and the hydrolysis of triglycerides. If the milk is
refrigerated immediately following milking and stored for days, the spoilage will be predominantly
caused by Gram-negative psychrotrophic rods, such as Pseudomonas, Alcaligenes, Flavobacterium
spp., and some coliforms. Pseudomonas and related species, being lactose negative, metabolize
proteinaceous compounds to change the normal flavor of milk to bitter, fruity, or unclean. They
also produce heat-stable lipases (producing a rancid flavor) and heat-stable proteinases that have
important implications; this is discussed in Chapter 22. The growth of lactose-positive coliforms
produces lactic acid, acetic acid, formic acid, CO 2 , and H 2 (by mixed-acid fermentation) and
causes curdling, foaming, and souring of milk. Some Alcaligenes spp. (Alc. faecalis) and coliforms
can also cause ropiness (sliminess) by producing viscous exopolysaccharides. However, if the raw
milk is not refrigerated soon, growth of mesophiles, such as species of Lactococcus, Lactobacillus,
Enterococcus, Micrococcus, and coliforms, along with Pseudomonas, Proteus, and spore formers, such
as Bacillus and Clostridium, predominates. 6 But lactose-hydrolyzing species, such as Lactococcus
spp. and Lactobacillus spp., generally predominate, producing enough acid to lower the pH considerably and prevent or reduce growth of others. In such situations, curdling of milk and sour flavor
are the predominant spoilages. If other microorganisms also grow, gas formation, proteolysis,
and lipolysis become evident. Yeast and mold growth, under normal conditions, is generally not
expected.
Pasteurized Milk
Raw milk is pasteurized before it is sold for consumption as liquid milk. Thermoduric bacteria (Micrococcus, Enterococcus, some Lactobacillus, Streptococcus, Corynebacterium, and spores of
Bacillus and Clostridium) survive the process. In addition, coliforms, Pseudomonas, Alcaligenes,
Flavobacterium, and similar types can enter as postpasteurization contaminants. Pasteurized milk,
under refrigerated storage, has a limited shelf life, mainly because of growth of these psychrotrophic contaminants. Their spoilage pattern is the same as described for raw milk spoilage. Flavor
defects from their growth are detectable when the population reaches ≥10 6 cells/mL. Growth of
psychrotrophic Bacillus spp., such as Bacillus cereus, has been implicated in the spoilage of pasteurized refrigerated milk, especially when the levels of postpasteurization contaminants are low.
Spores of psychrotrophic Bacillus spp., surviving pasteurization, germinate and multiply to cause
a defect known as bitty. They produce the enzyme lecithinase, which hydrolyzes phospholipids
of the fat globule membrane, causing aggregation of fat globules that adhere to the container