Milk

A. Milk Composition

In the United States, milk has a strict legal definition: ''the lacteal secretion, practically free of colostrum, obtained by complete milking of one or more healthy cows'' (Office of the Federal Register, 1995). Parallel definitions are provided for milk from goats and sheep (United States Public Health Service, 1993). Because of the central role of milk in the food supply and its ease of microbial contamination, production and processing of milk used for consumption is subject to tight regulation in most developed countries. In the United States, most milk is regulated according to the Grade A Pasteurized Milk Ordinance (United States Public Health Service, 1993), a document that sets the standards for all aspects of milk production and processing. From a microbiological standpoint, the Pasteurized Milk Ordinance is important primarily in its setting the standards for acceptable numbers of viable microorganisms in milk before and after pasteurization. The ordinance sets limits for microbial counts in raw milk for pasteurization at 1 X 105/mL for milk from an individual producer and 3 X 105 /mL for commingled milk from multiple producers. The ordinance also establishes the permissible levels of antibiotic residues in milk, which affects the selection and implementation of antibiotic therapies to control infectious diseases in dairy animals.

In addition to the direct contamination of milk with pathogens, many microorganisms that are themselves not pathogenic can be responsible for altering the composition of milk after its synthesis. One example of a deleterious effect on milk is provided by mycotoxins. These compounds are secondary metabolites of fungi that can produce various toxic effects which can range from acute poisoning to carcinogenesis. The most widely known mycotoxins are the aflatoxins, which are produced by Aspergillus flavus, A. parasiticus, and A. nomius. Numerous structurally distinct aflatoxins have been identified (Fig. 3). The most notorious of these is aflatoxin Bj, one of the most potent carcinogens known. Milk and dairy products may be contaminated by mycotoxins either directly (by contamination of milk or other dairy products with fungi followed by their growth) or indirectly (by contamination of animal feed with subsequent passage of the mycotoxin to milk) (van Egmond, 1989). In either event, contamination is largely dependent upon environmental conditions that determine the ability of the fungi to grow and produce toxins.

Two of the more potent aflatoxins, B1 and B2, can be converted in the rumen to their respective 4-hydroxy derivatives, the somewhat less carcinogenic M1 and M2 (see Fig. 3). The extent of this conversion varies greatly among cows. For example, Patterson et al. (1980) reported that the M1 concentration in the milk of six cows fed approximately 10 |g aflatoxin B1/kg feed varied from 0.01 to 0.33 |g/L milk; on average ~2.2% of the ingested B1 was converted to M1. Applebaum et al. (1982) administered B1 ruminally to 10 cows at higher doses (425-770 mg B1/kg feed) and detected higher amounts of B1 in milk (1.1-10.6 |g M1/L). Feeding of, or ruminal dosing with, high concentrations of B1 have significantly reduced feed intake and milk yield (Mertens, 1979). The effect is more powerful with impure B1 than pure B1; suggesting the synergistic effects of other mycotoxins present in the impure preparation. Several other researchers have noted substantial differences in M1 concentration among cows at similar or

Figure 3 Bioconversion of aflatoxins B1 and B2 to M1 and M2, respectively.

different stages of milk production and milk yield and between milkings of the same cow (Kiermeier et al., 1977; Lafont et al., 1980).

B. Milk Biosynthesis

In evaluating the microbial role in providing the animal with milk precursors, it is useful briefly to describe the biosynthesis of milk. A more detailed treatment of the process is provided by Bondi (1983).

Although the mammary gland comprises only 5-7% of the dairy cow's body weight, it represents perhaps the animal's highest concentration of metabolic activity. Careful breeding and advances in nutrition over the years have resulted in the annual production of milk nutrients from a single cow sufficient to provide the nutrients required by 50 calves.

Milk is produced in secretory cells clustered in groups known as alveoli. These cells feed milk through an arborescent duct system that collects milk into the udder. Production of milk is strongly controlled by endocrine hormones. Following parturition, the cells secrete antibody-rich colostrum for several days until milk secretion begins. Continued production of milk is stimulated by suckling or by milking through the stimulation of several hormones, particularly prolactin.

Nutrients for milk synthesis are provided to the udder through the blood via a pair of major arteries. The ability of the mammary gland to capture milk precursors effectively from the arterial blood supply—expressed as a ''per cent extraction'' calculated from the difference of precursor concentrations in arterial and venous blood—is truly impressive (Table 2) when one considers the rapid flow of arterial blood through the udder, which in dairy cows can approach 20 L/min. Production of 1 L of milk requires approximately 500 L of arterial blood flow through the udder.

Milk is predominantly (80-87%) water. The major components of milk solids are lactose, protein, and fats. The composition of milk varies with feeding regimens, individual animals, and breed. Marked differences are also noted among different ruminant species as well, with sheep's milk having substantially greater content of protein and fat than the milk of cows or goats (Table 3). Much of the energy required for biosynthesis of milk in the udder is produced by oxidation of glucose (30-50%) or acetate (20-30%). In the ruminant animal, glucose is not derived directly from dietary carbohydrate, but is instead produced by gluconeogenic pathways, primarily using propionate, a major product of the ruminal fermentation.

Lactose, a disaccharide of D-glucose and D-galactose linked by an a-1,4-glycosidic bond, is synthesized by a series of reactions using D-glucose as the starting substrate. Approximately 60% of the glucose consumed in the mammary gland is used for lactose synthesis. Lactose concentration in milk is relatively invariant with diet and stage of lactation, although its concentration declines substantially in mastitic cows (see Sec. VI.A).

Table 2 Arterial Concentrations of Milk Precursors and the Efficiency of Their Extraction in the Udder of Goats

Arterial Extraction concentration efficiency Precursor (mg/L) (%)

Blood:

O2 119 45

Glucose 445 33

Acetate 89 63

Lactate 67 30 Plasma:

3-Hydroxybutyrate 58 57

Triglycerides 219 40

Source: Bondi, 1983.

Table 3 Mean Composition of Milk from Domestic Ruminants

% by weight in milk

Table 3 Mean Composition of Milk from Domestic Ruminants

% by weight in milk

Component

cow

goat

sheep

Fat

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