Normal Microbiological Quality of Foods and Its Significance ◾ 43
in proteins and carbohydrates (lactose), which many microorganisms can utilize for growth.
Because both raw milk and pasteurized milk contain many types of bacteria as predominant
microorganisms, they are refrigerated, yet they have limited shelf life.
In raw milk, microorganisms come from inside the udder, animal body surfaces, feed, air, water,
and equipment used for milking and storage. The predominant types from inside a healthy udder
are Micrococcus, Streptococcus, and Corynebacterium. Normally, raw milk contains <10 3 micro -
organisms/mL. If a cow has mastitis, Streptococcus agalactiae, Sta. aureus, coliforms, and Pseudomonas
can be excreted in relatively high numbers. Contaminants from animals, feed, soil, and water predominantly have lactic acid bacteria; coliforms; Micrococcus, Staphylococcus, Enterococcus, Bacillus,
and Clostridium spores; and Gram-negative rods. Pathogens, such as Salmonella, Mycobacterium
bovis, Lis. monocytogenes, Yersinia enterocolitica, Brucella abortus, Coxiella burnetii, Shiga toxigenic
Escherichia coli (STEC), and Cam. jejuni, can also come from some of these sources and from
milk. Equipment can be a major source of Gram-negative rods, such as Pseudomonas, Alcaligenes,
and Flavobacterium, as well as Gram-positive Micrococcus and Enterococcus.
During refrigerated storage (at dairy farms and processing plants) before pasteurization, only
psychrotrophs can grow in raw milk. They include Pseudomonas, Flavobacterium, Alcaligenes, and
some coliforms and Bacillus spp. They can affect the acceptance quality of raw milk (e.g., by making flavor and texture undesirable). Some of them can produce heat-stable enzymes (proteinases
and lipases), which can also affect the product quality, even after pasteurization of raw milk (see
Chapter 21). Psychrotrophic pathogens (Lis. monocytogenes and Yer. enterocolitica) can multiply in
refrigerated raw milk during storage.
The microbiological quality of raw and pasteurized milk is monitored in many countries by
regulatory agencies. In the United States, the standard plate counts of raw milk for use as market
milk are 1–3 × 10 5 /mL and for use in product manufacturing are 0.5–1 × 10 6 /mL. Grade A pasteurized milk can have standard plate counts of 20,000/mL and ≤10 coliforms/mL.
Microorganisms present in pasteurized milk are those that survive the pasteurization of raw
milk (e.g., the thermodurics) and those that enter after heating and before packaging (i.e., postpasteurization contaminants). Thermodurics surviving pasteurization include Micrococcus, some
Enterococcus (e.g., Ent. faecalis), Streptococcus, some Lactobacillus (e.g., Lab. viridescens), and spores
of Bacillus and Clostridium. Postheat contaminants can be coliforms as well as Pseudomonas,
Alcaligenes, and Flavobacterium. Some heat-sensitive pathogens can also enter pasteurized milk
following heat treatment. Psychrotrophs can grow during refrigerated storage.
Shell eggs and Liquid egg
Shell eggs are contaminated with microorganisms on the outer surface from fecal matter, nesting
materials, feed, air, and equipment. 6,7 Eggshells, depending on the contamination level, can have
10 7 bacteria. Washing helps reduce the bacterial level considerably. Eggshells can harbor different
types of bacteria, namely Pseudomonas, Alcaligenes, Proteus, Citrobacter, Esc. coli, Enterobacter,
Enterococcus, Micrococcus, and Bacillus. They can also have Salmonella from fecal contamination.
Infected ovaries of laying hens can be the source of Salmonella enterica serovar Enteritidis in the
yolk. Liquid egg can be contaminated with bacteria from the shell of washed eggs as well as from
the breaking equipment, water, and air. Pasteurization can reduce the numbers to 10 3 cells/mL.
Bacteria, especially motile Gram-negative bacteria, can enter through the pores of eggshells,
particularly if the shells are wet. Several antimicrobial factors present in egg albumin, such as
lysozyme, conalbumin (binds iron), avidin (binds biotin), or alkaline pH (8.0–9.0), can control
in proteins and carbohydrates (lactose), which many microorganisms can utilize for growth.
Because both raw milk and pasteurized milk contain many types of bacteria as predominant
microorganisms, they are refrigerated, yet they have limited shelf life.
In raw milk, microorganisms come from inside the udder, animal body surfaces, feed, air, water,
and equipment used for milking and storage. The predominant types from inside a healthy udder
are Micrococcus, Streptococcus, and Corynebacterium. Normally, raw milk contains <10 3 micro -
organisms/mL. If a cow has mastitis, Streptococcus agalactiae, Sta. aureus, coliforms, and Pseudomonas
can be excreted in relatively high numbers. Contaminants from animals, feed, soil, and water predominantly have lactic acid bacteria; coliforms; Micrococcus, Staphylococcus, Enterococcus, Bacillus,
and Clostridium spores; and Gram-negative rods. Pathogens, such as Salmonella, Mycobacterium
bovis, Lis. monocytogenes, Yersinia enterocolitica, Brucella abortus, Coxiella burnetii, Shiga toxigenic
Escherichia coli (STEC), and Cam. jejuni, can also come from some of these sources and from
milk. Equipment can be a major source of Gram-negative rods, such as Pseudomonas, Alcaligenes,
and Flavobacterium, as well as Gram-positive Micrococcus and Enterococcus.
During refrigerated storage (at dairy farms and processing plants) before pasteurization, only
psychrotrophs can grow in raw milk. They include Pseudomonas, Flavobacterium, Alcaligenes, and
some coliforms and Bacillus spp. They can affect the acceptance quality of raw milk (e.g., by making flavor and texture undesirable). Some of them can produce heat-stable enzymes (proteinases
and lipases), which can also affect the product quality, even after pasteurization of raw milk (see
Chapter 21). Psychrotrophic pathogens (Lis. monocytogenes and Yer. enterocolitica) can multiply in
refrigerated raw milk during storage.
The microbiological quality of raw and pasteurized milk is monitored in many countries by
regulatory agencies. In the United States, the standard plate counts of raw milk for use as market
milk are 1–3 × 10 5 /mL and for use in product manufacturing are 0.5–1 × 10 6 /mL. Grade A pasteurized milk can have standard plate counts of 20,000/mL and ≤10 coliforms/mL.
Microorganisms present in pasteurized milk are those that survive the pasteurization of raw
milk (e.g., the thermodurics) and those that enter after heating and before packaging (i.e., postpasteurization contaminants). Thermodurics surviving pasteurization include Micrococcus, some
Enterococcus (e.g., Ent. faecalis), Streptococcus, some Lactobacillus (e.g., Lab. viridescens), and spores
of Bacillus and Clostridium. Postheat contaminants can be coliforms as well as Pseudomonas,
Alcaligenes, and Flavobacterium. Some heat-sensitive pathogens can also enter pasteurized milk
following heat treatment. Psychrotrophs can grow during refrigerated storage.
Shell eggs and Liquid egg
Shell eggs are contaminated with microorganisms on the outer surface from fecal matter, nesting
materials, feed, air, and equipment. 6,7 Eggshells, depending on the contamination level, can have
10 7 bacteria. Washing helps reduce the bacterial level considerably. Eggshells can harbor different
types of bacteria, namely Pseudomonas, Alcaligenes, Proteus, Citrobacter, Esc. coli, Enterobacter,
Enterococcus, Micrococcus, and Bacillus. They can also have Salmonella from fecal contamination.
Infected ovaries of laying hens can be the source of Salmonella enterica serovar Enteritidis in the
yolk. Liquid egg can be contaminated with bacteria from the shell of washed eggs as well as from
the breaking equipment, water, and air. Pasteurization can reduce the numbers to 10 3 cells/mL.
Bacteria, especially motile Gram-negative bacteria, can enter through the pores of eggshells,
particularly if the shells are wet. Several antimicrobial factors present in egg albumin, such as
lysozyme, conalbumin (binds iron), avidin (binds biotin), or alkaline pH (8.0–9.0), can control