Designed to provide complete nutrition for growing calves, bovine milk also provides a highly suitable growth medium for a variety of microorganisms. The abundance of carbohydrates, proteins, and fats combined with the neutral pH supports and encourages a microbial ecology that can be both diverse and highly variable. One can find numerous different organisms in raw milk, including psychrotrophs, which can grow at 7°C or less, irrespective of their optimum
Table 1 Human Microbial Pathogens Associated with Milk and Milk Products
Escherichia coli, including O157:H7'
Yersinia enterocolitica (psychrotrophic) Other gram-negative bacteria
Aeromonas hydrophila (psychrotrophic) Brucella spp. Campylobacter jejuni Pseudomonas aeruginosa Gram-positive spore formers
Bacillus cereus (some strains are psychrotrophic) Bacillus anthracis Clostridium perfringens
Clostridium botulinum (type E is psychrotrophic) Gram-positive cocci Staphylococcus aureus Streptococcus agalactiae Streptococcus pyogenes Streptococcus zooepidemicus Miscellaneous gram-positive bacteria Corynebacterium spp. Listeria monocytogenes (psychrotrophic) Mycobacterium bovis Mycobacterium tuberculosis Mycobacterium paratuberculosis Rickettsia
Coxiella burnetii Viruses
Enterovirus, including polioviruses, rotaviruses,
Coxsackie viruses FMD virus Hepatitis virus Fungi
Entamoeba histolytica Giardia lamblia Toxoplasma gondii
Gastroenteritis, hemolytic ure-
mic syndrome Gastroenteritis, typhoid fever Gastroenteritis
Gastroenteritis Brucellosis (Bang's disease) Gastroenteritis Gastroenteritis
Gastroenteritis Anthrax Gastroenteritis Botulism
Emetic intoxication Sore throat
Scarlet fever/sore throat Pharyngitis, nephritic sequelae
Diphtheria Listeriosis Tuberculosis Tuberculosis
Johne's disease (ruminants) Q fever
Foot-and-mouth disease Infectious hepatitis
Amebiasis Giardiasis Toxoplasmosis
Source: Adapted from Boor, 1997, and Johnson et al., 1990.
growth temperature; coliforms and other gram-negative bacteria, which can be associated with unsanitary production and processing practices; thermoduric bacteria, which can survive pasteurization conditions; spore formers, which produce the heat- and dessication-resistant structures known as spores; pathogens that cause mastitis, which can be shed into the milk by infected udders; and various yeasts and molds (Bramley and McKinnon, 1990; Gilmour and Rowe, 1990). As indicated in Table 1, a variety of microbes with human pathogenic potential, including Listeria monocytogenes, Salmonella spp., Staphylococcus aureus and Mycobacterium tuberculosis, can sometimes be found in raw milk (Bramley and McKinnon, 1990; Flowers et al., 1992; Johnson et al., 1990).
Psychrotrophic bacteria belonging to numerous genera have been isolated from milk, including Pseudomonas, Enterobacter, Flavobacterium, Klebsiella, Aeromonas, Acinetobacter, Alcaligenes, and Achromobacter. Certain genera of bacteria isolated from milk are both psychrotrophic and thermoduric, including gram-positive Bacillus, Clostridium, Microbacterium, Micrococcus, and Coryne-bacterium (Cousin, 1982; Suhren, 1989). Coliforms, which are defined as aerobic and facultatively anaerobic, asporogenous, gram-negative rods that ferment lactose with acid and gas production within 48 h at 32 or 35°C, include the genera Escherichia, Enterobacter, Citrobacter, and Klebsiella (Christen et al., 1992; Jay, 2000; Gilmour and Rowe, 1990). Aerobic gram-negative rods commonly found in milk include Pseudomonas fluorescens, P. putida, P. fragi, P. putrefaciens, and less frequently P. aeruginosa (Gilmour and Rowe, 1990).
Thermoduric bacteria belonging to numerous genera have also been isolated from milk, including Microbacterium, Micrococcus, and Alcaligenes. The spore-forming genera most relevant to milk and dairy products are Bacillus and Clostrid-ium. Bacillus spp. have been implicated in spoilage of raw and pasteurized milk; ultrahigh temperature (UHT), concentrated and canned milk products; and ''bitty'' cream and sweet curdling of pasteurized milk. Clostridium spp. have been implicated in the rancid spoilage and ''late blowing'' of numerous cheeses (Gilmour and Rowe, 1990). Species of Streptococcus, Lactobacillus, and Corynebacterium also show some heat resistance, with less than 1% of a given population surviving a heat treatment of 63°C for 30 min (Bramley and McKinnon, 1990). Pathogens that cause mastitis include Streptococcus uberis, S. dysgalactiae, S. agalactiae, Staphylococcus aureus, coagulase-negative staphylococci, P. aeruginosa, Mycoplasma bovis, Corynebacterium bovis, and coliforms (Bramley and Dodd, 1984).
III. BACTERIAL CONTAMINATION A. Raw Milk Contamination
Sources of bacterial contamination of raw milk can be divided into three general categories: environment, udder, and milking equipment. Environmental sources, which include water, soil, vegetation, and bedding material, vary in the numbers and types of organisms that can be introduced into raw milk. In general, contamination with psychrotrophic microflora has been associated with bedding material, untreated water, soil, and vegetation; coliform contamination with soil; and spore formers with bedding material (Cousin, 1982; Suhren, 1989). Poor premilking udder hygiene that fails adequately to clean dirty udders can result in the introduction of vegetation, soil, and bedding material and their associated microorganisms into the milk. Thorough cleaning and drying of the udder immediately before milking lowers total bacterial numbers as well as coliform and Staphylococcus spp. counts and decreases milk sediment (Galton et al., 1984; Pankey, 1989). Bacterial contamination from within the udder is frequently a result of mastitis, an inflammation of the udder that can result in high levels of bacteria being shed into the milk. (see Chapter 1) Currently, E. coli, Staph. aureus, other staphylococci, S. dysgalactiae, and other streptococci are the most prevalent pathogens among dairy herds (Barkema et al., 1998; Sargeant et al., 1998; Waage et al., 1999). Since cows infected with S. uberis can shed up to 107 cfu/mL (Leigh, 1999) and cows infected with E. coli can shed up to 108 cfu/mL (Van Werven et al., 1997), one infected cow can influence total bacterial numbers in an entire bulk tank of milk. Since Staph. aureus is shed in relatively low numbers, typically less than 10,000 cfu/mL (Sears et al., 1990), S. uberis and S. dysgalactiae are often responsible for large increases in the total bacterial count of raw milk (Bramley et al., 1984). Although the microflora of a healthy udder can be shed into the raw milk, these organisms do not typically cause significant increases in the bulk tank total bacterial count.
Common contamination sources associated with milking equipment include milking machines, milk pipelines, bulk tanks, and transport tankers. Ineffective cleaning can leave milk residue throughout these various machines which can provide an excellent environment for microbial growth (see Chapter 14). Bacteria multiply within these residues and contaminate milk passing through the equipment.
Postpasteurization bacterial contamination provides a serious obstacle to maintaining and extending fluid milk product shelf life. Two major sources contribute to postpasteurization contamination: equipment milk residues and aerosols. Ineffective cleaning procedures of the interior of processing equipment create milk residues which can allow bacteria to multiply and contaminate subsequent milk flow (see Chapter 14). Filler nozzles, carton-forming mandrels, and pasteurizers have all been pinpointed as sources of postpasteurization contamination (Gruetz-macher and Bradley, 1999; Ralyea et al., 1998). Bacterial biofilms, which are difficult to remove with clean-in-place (CIP) procedures, can also form within processing equipment and provide a constant source of contamination for both raw and pasteurized milk (Austin and Bergeron, 1995).
Unenclosed milk contact surfaces provide a route for microbial aerosols to contaminate pasteurized milk (Kang and Frank, 1989). During cleaning or operation, airborne yeast, molds, bacteria, and spores can land on a milk contact surface and thus enter the milk flow. An unenclosed filling unit (e.g., a federal-style filler) can allow exposure of the pasteurized milk to airborne bacteria, which can result in levels of postpasteurization contamination higher than those of milk packaged in a self-enclosed system (Douglas et al., 2000).
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