Milk is synthesized in specialized cells of the mammary gland and is virtually sterile when secreted into the alveoli of the udder (Tolle, 1980). Beyond this stage of milk production, microbial contamination can generally occur from three main sources (Bramley and McKinnon, 1990): from within the udder, from the exterior of the udder, and from the surface of milk handling and storage equipment. The health and hygiene of the cow, the environment in which the cow is housed and milked, and the procedures used in cleaning and sanitizing the milking and storage equipment are all key in influencing the level of microbial contamination of raw milk. Equally important are the temperature and length of time of storage, which allow microbial contaminants to multiply and increase in numbers. All these factors will influence the total bacteria count or Standard Plate Count (SPC) and the types of bacteria present in bulk raw milk.
Microbial Contamination from within the Udder
Raw milk as it leaves the udder of healthy cows normally contains very low numbers of microorganisms and generally will contain less than 1,000 total bacteria per ml (Kurweil, 1973). In healthy cows, the teat cistern, teat canal, and the teat apex may be colonized by a variety of microorganisms although microbial contamination from within the udder of healthy animals is not considered to contribute significantly to the total numbers of microorganisms in the bulk milk or to the potential increase in bacterial numbers during refrigerated storage. Natural flora of the cow generally have little influence on SPCs.
While the healthy udder should contribute very little to the total bacteria count of bulk milk, a cow with mastitis has the potential to shed large numbers of microorganisms into the milk supply. The influence of mastitis on the total bacteria count of bulk milk depends on the strain of infecting microorganism(s), the stage of infection, and the percentage of the herd infected. Infected cows have the potential to shed in excess of 107 bacteria per ml. If the milk from one cow with 107 bacteria per ml comprises 1% of the bulk tank milk, the total bulk tank count, disregarding other sources, would be 105 per ml (Bramley and McKinnon, 1990).
Mastitis organisms found to most often influence the total bulk milk count are Streptococcus spp., most notably S. agalactiae and S. uberis (Bramley and McKinnon, 1990; Bramley et al., 1984; Gonzalez et al., 1986; Jeffrey and Wilson, 1987) although other mastitis pathogens have the potential to influence the bulk tank count as well. Staphylococcus aureus is not thought to be a frequent contributor to total bulk tank counts although counts as high as 60,000/ml have been documented (Gonzalez et al., 1986). Detection of implied pathogens does not necessarily indicate that they originated from cows with mastitis. Potential environmental mastitis pathogens and/or similar organisms can occur in milk as a result of other contributing factors such as dirty cows, poor equipment cleaning, and/or poor cooling. An increase in SCC can sometimes serve as supportive evidence that a mastitis bacterium may have caused an increase in the bulk milk bacteria count. This seems to hold true more for Streptococcus spp. than for S. aureus, which appears to be shed into the milk in lower numbers (Fenlon et al., 1995). Correlations of somatic cell responses and environmental mastitis organisms, including coliform bacteria, streptococci, and certain coagulase-negative Staphylococcus spp., were found to be poor as well. These organisms are by nature associated with the cow’s environment and may influence bulk milk bacteria counts through other means (Bramley, 1982; Zehner et al., 1986). S. agalactiae and S. aureus are not thought to grow significantly on soiled milking equipment or under conditions of marginal or poor cooling. Their presence in bulk tank milks is considered strong evidence that they originated from infected cows (Gonzalez et al., 1986; Bramley and McKinnon, 1990).
Microbial Contamination from the Exterior of the Udder
The exterior of the cow's udder and teats can contribute microorganisms that are naturally associated with the skin of the animal as well as microorganisms that are derived from the environment in which the cow is housed and milked. In general, the direct influence of natural inhabitants as contaminants in the total bulk milk count is considered to be small, and most of these organisms do not grow competitively in milk. Of more importance is the contribution of microorganisms from teats soiled with manure, mud, feeds, or bedding.
Teats and udders of cows inevitably become soiled while they are lying in stalls or when allowed in muddy barnyards. Used bedding has been shown to harbor large numbers of microorganisms. Total counts often exceed 108-1010 per gram (Bramley, 1982; Bramley and McKinnon, 1990; Hogan et al., 1989; Zehner et al., 1986). Organisms associated with bedding materials that contaminate the surface of teats and udders include streptococci, staphylococci, spore-formers, coliforms, and other Gram-negative bacteria. Both thermoduric (bacteria that survive pasteurization) and psychrotrophic (bacteria that grow under refrigeration) strains of bacteria are commonly found on teat surfaces (Bramley and McKinnon, 1990) indicating that contamination from the exterior of the udder can influence Lab Pasteurization Counts (LPCs) and Preliminary Incubation Counts (PICs).
The influence of dirty cows on total bacteria counts depends on the extent of soiling of the teat surface and the wash procedures used immediately before milking. For example, if one gram of teat soil containing 108 bacteria is allowed into the milk of one cow giving approximately 30 lb (~13,400 gm) of milk, the total bacteria count for that cow’s milk, excluding other sources, would be in excess of 7,000 per ml. Milking heavily soiled cows could potentially result in bulk milk counts exceeding 104 per ml. Several studies have investigated premilking udder hygiene techniques in relation to the bacteria count of milk (Bramley and McKinnon, 1990; Galton et al. 1984; McKinnon et al. 1990, Pankey, 1989). Generally, thorough cleaning of the teat with a sanitizing solution (spray, wet towel, or dip) followed by thorough drying with a clean towel is effective in reducing the numbers of microorganisms in milk contributed from soiled teats. Counts of coliform bacteria, although highly associated with manure, barnyard mud, and used bedding, were relatively low in these studies, even for the untreated cows, suggesting that higher coliform counts in bulk milk are more likely to occur due to other factors (i.e., equipment, mastitis).
Influence of Equipment Cleaning and Sanitizing Procedures
The degree of cleanliness of the milking system probably influences the total bulk milk bacteria count as much as, if not more than, any other factor (Olson and Mocquat, 1980). Milk residue left on equipment contact surfaces supports the growth of a variety of microorganisms. Organisms considered to be natural inhabitants of the teat canal, apex, and skin are not thought to grow significantly on soiled milk contact surfaces or during refrigerated storage of milk. This generally holds true for organisms associated with contagious mastitis (i.e., S. agalactiae) although it is possible that certain strains associated with environmental mastitis (i.e., coliforms) may be able to grow to significant numbers. In general, environmental contaminants (i.e., from bedding, manure, feeds) are more likely to grow on soiled equipment surfaces. Water used on the farm might also be a source of microorganisms, especially psychrotrophs, that could seed soiled equipment and/or the milk (Bramley and McKinnon, 1990).
Cleaning and sanitizing procedures can influence the degree and type of microbial growth on milk contact surfaces by leaving behind milk residues that support growth as well as by setting up conditions that might select for specific microbial groups. More resistant and/or thermoduric bacteria may endure in low numbers on equipment surfaces that are considered to be efficiently cleaned with hot water. If milk residue is left behind (i.e., milk stone), growth of these types of organisms, although slow, may persist. Old cracked rubber parts are also associated with higher levels of thermoduric bacteria. Significant buildup of these organisms to a point where they influence the total bulk tank count may take several days to weeks (Thomas et al., 1966), although increases would be detected in LPCs.
Less efficient cleaning, using lower temperatures, and/or the absence of sanitizers tend to select for the faster growing, less resistant organisms, principally Gram-negative rods (coliforms and Pseudomonads) and lactic streptococci. This will result in high PICs and in some case elevated LPCs. Effective use of chlorine or iodine sanitizers has been associated with reduced levels of psychrotrophic bacteria that cause high PICs (Jackson and Clegg, 1965). Psychrotrophic bacteria tend to be present in higher count milk and are often associated with occasional neglect of proper cleaning or sanitizing procedures (Olson and Mocquat, 1980; Thomas et al., 1966) and/or poorly cleaned refrigerated bulk tanks (MacKenzie, 1973; Thomas, 1974).
Milk Storage Temperature and Time
Refrigeration storage, while preventing the growth of non-psychrotrophic bacteria, will select for psychrotrophic microorganisms that enter the milk from soiled cows, dirty equipment, and the environment. Minimizing the level of milk contamination from these sources will help prevent psychrotrophs from growing to significant levels in the bulk tank during the storage period on the farm or at the dairy plant. In general, these organisms are not thermoduric and will not survive pasteurization. The longer raw milk is held before processing (legally up to five days), the greater the chance that psychrotrophs will increase in numbers. Holding milk near the PMO legal limit of 7.2°C (45°F) allows much quicker growth than milk held below 4.4°C (40°F). Although milk produced under ideal conditions may have an initial psychrotroph population of less than 10% of the total bulk tank count, psychrotrophic bacteria can become the dominant microflora after two to three days at 4.4°C (40°F) (Gehringer, 1980), resulting in a significant influence on PICs. Colder temperatures of 1 to 2°C (34 to 36°F) will delay this shift, although not indefinitely.
Under conditions of poor cooling with temperatures greater than 7.2°C (45°F), bacteria other than psychrotrophs are able to grow rapidly and can become predominant in raw milk. Although incidents of poor cooling still occur, this defect is not as common as when milk was held and transported in cans. Streptococci have historically been associated with poor cooling of milk, appearing as pairs or chains of cocci (spherical bacteria) on microscopic examination of milk smears (Atherton and Dodge, 1970). These bacteria will increase the acidity of milk. Certain strains are also responsible for a “malty defect” that is easily detected by its distinct odor. Storage temperatures greater than 15°C (60°F) tend to select for these types of contaminants (Gehringer, 1980). Although poor cooling conditions allow growth of bacteria that normally will not grow in properly refrigerated milk, they will not prevent typical psychrotrophic strains from growing. The types of bacteria that grow and become significant will depend on the initial microflora of the milk (Bramley and McKinnon, 1990).
Microbial contamination of raw milk can occur from a variety of microorganisms from a variety of sources. Because of this, determining the cause of bacterial defects is not always straightforward. Although there is often one source of bacteria that cause high bulk tank counts, high bacteria counts can also result from a combination of factors (i.e., dirty equipment and marginal cooling). In some cases, selective plating procedures or bacterial culturing may be useful in identifying the source of high bacteria counts on the farm.
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