Milk is a lactal secretion by the mammary glands for the use as food by the young mammals and a highly complex emulsion – essentially fat globules suspended in watery serum.

COMPOSITION OF MILK

Cow’s milk, is largely made up of 4 substances: water, which comprises 87% and gives milk its characteristic fluidity; lactose, a sugar, about 5%; butter or milk fat, which is present to the extent of about 3.7%; and casein, the protein substance of milk of which there is 3% .

QUALITY OF MILK

Milk quality is denoted by character and attribute. It should be free from toxic or antibiotic-like substances, chlorinated and inhibitory sanitizer. It should be free from contaminating flora which could inhibit the growth of the starter either directly by the production of inhibitory substances, or by indirectly by altering the chemical composition. It should be free from bacteriophage but of such composition chemically to support the nutritive growth and development of selected lactic starter culture.

DIFFERENT TYPES OF MILK

HOMOGENIZED Milk – fresh milk that has been pasteurized and subjected to a process that breaks up the fat into a very fine droplets mixes them completely as to make it impossible for the fat to rise as cream.

SKIMMED Milk – fresh pasteurized milk from which the fat has been removed.

FORTIFIED Milk – milk to which one or more nutrients have been added.

CULTURED OR FERMENTED MILK – prepared from pasteurized milk to which certain desirable microorganism have been added to ferment the milk, resulting in the formation of lactic acid from the lactose and some coagulation of casein.

EVAPORATED Milk – obtained from the fresh whole milk after a little more than half of the water has been removed so that it contains not less than 7.9% milk fat and not less than 25.9% of milk solids.

CONDENSED Milk – prepared by adding sugar to milk and reducing its water content by evaporation.

FILLED Milk – prepared by removing nearly all the milk fat from milk and then adding a vegetable oil (usually coconut oil).

DRIED Milk – results from the removal of 95% to 98% of the water from fresh milk.

CREAMMED Milk – fat of milk which has been separated by centrifugation or by gravity.

CONTAMINATION OF MILK

Milk has no native bacterial flora and it is probable that milk as secreted into the udder of a healthy cow is sterile. The milk in the udder is however, rarely if ever bacteriologically sterile, for the microorganisms invade the udder via milk ducts of the teats and the first portion of the milk drawn (fore-milk) always contains more bacteria than the last (strippings). The presence of microorganisms in milk is always consequence of contamination. The bacteria of milk fall into two groups:

1. Those which are present in the tissues of an infected cow and find their ways into the udder
2. Those which enter the milk, usually after it is drawn from sources external to the animal

The internal sources of milk contamination may be bacteria from infected cattle. Of these, perhaps, the most important is Mycobacterium Tuberculosis, the bovine variety of which are short, thick, and solidly stained rods. These bacteria get into the milk directly as a consequence of tuberculosis of the udder, and indirectly by contamination with cow manure when the infectious sputum is swallowed and discharged with the feces. This gives rise to the bones and joints tuberculosis in man. The virus of foot and mouth disease, a virus of cattle, is excreted in the milk and is thus transmitted to man. However, the disease in man is mild although occasionally, the symptoms are severe and some cases end fatally. More important are the streptococcal infections of the udder, one of which is mastitis (inflammation). Mastitis is most often cause by the non-pathogenic, and spherical streptococcus agalactiae, Occasionally, mastitis is caused by typical human streptococcus that gain entrance to the udder directly or indirectly from some person. The re4suslt is that the milk drawn contains pathogenic bacteria, causing septic sore throat and scarlet fever.

Next, the milk is subjected to contamination from the exterior of the animal, especially the exterior of the udder and adjacent paths. Bacteria of manure, soil, and water may enter from this source. Contamination from the air of the barn or milking parlor may contribute organisms of feed, soil, or manure to milk, but the nos. added in this manner is negligible unless an abnormal amount of dust has been raised. Other possible sources of external contamination are dairy utensils, such as the milk pail or milking machines, strainers, etc.; the hands of the milker or another dairy workers; flies, and other utensils and equipments that may add contaminants after the milk leaves the farm or dairy plant. Utensils can be most important of contamination so they may add not only considerable numbers, of bacteria to the milk, but also some of the most undesirable kinds.

PRESERVATION OF MILK

1. Pasteurization

The object of the pasteurization of market milk is to kill all pathogens that may have entered the milk and thereby improve the keeping quality of the milk.

2. Boiling

Boiling of milk or heating in flowing steam destroys all microorganisms except the spores of bacteria, yet changes the appearance, palatability, digestibility and nutritive properties of the milk.

3. Steam Under Pressure

Evaporated milk in canned and then heat-processed by steam under pressure, often with accompanying rolling or agitation. The fore warming of the milk at about 200 o F to 212 o F (43.3 o C to 100 o C) or higher before evaporation, kills all bacteria but the more resistant bacterial spores. The sealed cans of evaporated milk 18 minutes to make the contents sterile.

4. Cooling

For the production of milk of good quality, it is essential that it be cooled promptly after it is drawn from the cow. Grade A raw milk pasteurization shall be cooled to 50 o F (10 o C) or less within 2 hours after drawing and kept that cold until processed. Newly pasteurized milk is to be cooled at 45 o F (7.2 o C) or less and maintained there at.

5. Chilling

The recommended temperature to freshly drawn or newly pasteurized milk should be cooled and really at chilling temperature. This is most recommended to a bottled milk or related products during storage until consumption.

6. Freezing

Milk, concentrated to 1/3 its volume, can be frozen at 0 o F (-17.8 o C) and stored at -10 F (-23.3 o C) or lower, and can be held for several weeks without deterioration. Frozen milk can be concentrated by freeze-drying methods.

7. DRYING

Various milk products are made by the removal of different percentage of water from the whole or skimmed milk. Only in the manufacture of dry products is enough moisture removed to prevent the growth of microorganisms.

8. EVAPORATED AND CONDENSED PRODUCTS:

Evaporated milk is made by removing about 60% of the water from the whole milk, so that about 11.5% lactose would be in solution, plus double the amount of soluble inorganic salts in whole milk. This high concentration of sugar is inhibitory to the growth of some survivors of the heat treatments. Bulk condensed milk is more condensed than evaporated milk and is still a poorer culture medium for organisms not tolerant of high sugar concentrate.

9. USE OF PRESERVATIVES:

The addition of preservatives like salt, ascorbic acid or sorbates, propionic acid or propionates, and the chemical preservatives from smoke may reduce the growth of microorganisms in milk.

10. OTHER METHODS:

Other methods have proven to be effective in the removal or reduction of spoilage microorganisms in milk are the use of radiations (UV rays, X-prays, cathode rays, and gamma rays);sound waves especially ultrasonic vibrations with a frequency of about 8,900 cycles, combined with a temperature of 40 to 50 o C (104 to 122o F) ; and alternating electric currents.