•Urea is responsible for almost all of the seasonal variation in the heat stability of milk. The concentration of milk urea is controlled by the level of urea in the blood, which is directly related to diet.
•As mentioned earlier, lipoprotein lipase catalyzes the hydrolysis of triglycerides to free fatty acid. This reaction causes dairy products to have soapy, bitter, rancid, and unclean flavors. A great deal of lipoprotein lipase is present in freshly drawn milk, and under certain conditions, will spoil the milk within a few minutes. The fat globule membrane acts as a physical barrier to lipase, and it is also readily inactivated by heat. Spontaneous lipolysis is influenced by stage of lactation, season, and diet.
•Lactoperoxidase is another enzyme in milk present at high concentrations. It catalyzes the oxidation of unsaturated fatty acids leading to the development of oxidized flavors. However, if thiocyanate and peroxide are supplemented in milk, lactoperoxidase acts as a powerful bacteriocide that can kill coliforms, Salmonellae, Shingellae, and Pseudomonads. This process has been widely used for short-term preservation of milk in developing countries where refrigeration is scarce.
•Xanthine oxidase is also present in milk and can catalyze non-specific oxidation of dairy products. Other that that, its overall significance is low.
•Alkaline phosphatase is almost completely inactivated by pasteurization, and it is therefore used as an index of the efficiency of such heat treatments.
•Vitamins A, D, E, C, B1, B2, B6, B12, pantothenic acid, niacin, biotin, and folic acid are all found in milk.
•The concentrations of fat soluble vitamins in milk (A, D, and E) are dependent on the diet and the breed of the cow.
•All of the vitamins in milk are affected by processing. Vitamins C, B2, and A are deteriorated by light. The fat soluble vitamins are stable to heat, but the water soluble vitamins, B1, B6, B12, and folic acid, are less stable to heat. UHT sterilization leads to a 20-30% loss of vitamin activity and sterilization leads to 50% loss of vitamin activity.
Functions of Vitamins in Milk Vitamin A: Vitamin A prevents eye problems, promotes a healthy immune system, is essential for the growth and development of cells, mucous membranes, skin, bone and tooth health, reproduction, and immunity.
Vitamin D: Vitamin D strengthens bones because it helps the body absorb bone-building calcium.
Vitamin E: Vitamin E is an antioxidant and helps protect cells from damage. It functions in stabilization of cell membranes, support of immune function, protection of polyunsaturated fatty acids, and normal nerve development.
Vitamin C (also called ascorbic acid): Vitamin C is needed to form collagen, a tissue that helps to hold cells together. It is an antioxidant, is restores vitamin E to its active form, it helps to synthesize hormones, it supports immune cell function, and helps in absorption of iron.
Thiamin (also called vitamin B1): Thiamin is part of a coenzyme needed in energy metabolism. It also supports a normal appetite and nervous system function.
Riboflavin (also called vitamin B2): Riboflavin is part of a coenzyme needed in energy metabolism. It also supports normal vision and skin health.
Vitamin B6 (also called pyridoxine): Vitamin B6 is part of a coenzyme needed in amino acid and fatty acid metabolism. It helps to convert tryptophan to niacin and to serotonin, and it helps make red blood cells.
Vitamin B12: Vitamin B12 is part of coenzymes needed to make red blood cells, and it is important for nerve cell function.
Folate (also known as vitamin B9, folic acid, or folacin): Folate is part of a coenzyme needed for synthesis of red blood cells. It is also needed to make DNA.
Niacin (also called vitamin B3): Part of coenzymes needed in energy metabolism. It helps maintain healthy skin and is important for nerve function.
Pantothenic Acid: Part of a coenzyme needed in energy metabolism.
Biotin: A cofactor for several enzymes needed in energy metabolism, fat synthesis, amino acid metabolism, and glycogen synthesis.
Vitamin A: Vitamin A prevents eye problems, promotes a healthy immune system, is essential for the growth and development of cells, mucous membranes, skin, bone and tooth health, reproduction, and immunity. Vitamin D: Vitamin D strengthens bones because it helps the body absorb bone-building calcium.
Vitamin E: Vitamin E is an antioxidant and helps protect cells from damage. It functions in stabilization of cell membranes, support of immune function, protection of polyunsaturated fatty acids, and normal nerve development.
Vitamin C (also called ascorbic acid): Vitamin C is needed to form collagen, a tissue that helps to hold cells together.
It is an antioxidant, is restores vitamin E to its active form, it helps to synthesize hormones, it supports immune cell function, and helps in absorption of iron. Thiamin (also called vitamin B1): Thiamin is part of a coenzyme needed in energy metabolism. It also supports a normal appetite and nervous system function.
Riboflavin (also called vitamin B2): Riboflavin is part of a coenzyme needed in energy metabolism. It also supports normal vision and skin health.
Vitamin B6 (also called pyridoxine): Vitamin B6 is part of a coenzyme needed in amino acid and fatty acid metabolism. It helps to convert tryptophan to niacin and to serotonin, and it helps make red blood cells.
Vitamin B12: Vitamin B12 is part of coenzymes needed to make red blood cells, and it is important for nerve cell function.
Folate (also known as vitamin B9, folic acid, or folacin): Folate is part of a coenzyme needed for synthesis of red blood cells.
It is also needed to make DNA.
Niacin (also called vitamin B3): Part of coenzymes needed in energy metabolism. It helps maintain healthy skin and is important for nerve function.
Pantothenic Acid: Part of a coenzyme needed in energy metabolism.
Biotin: A cofactor for several enzymes needed in energy metabolism, fat synthesis, amino acid metabolism, and glycogen synthesis.
Physical and chemical properties
Density
Appearance
Freezing point
Density: Density is defined as an object’s mass divided by it’s volume. It depends on the temperature of the object, composition of the material, and whether or not the object contains air.
The density of milk products can be used to convert volume into mass and mass into volume, to estimate the amount of solids present in milk, and to calculate other physical properties.
The density of cows milk usually varies between 1.028 and 1.038 g/cm3.
Appearance: The opacity of milk is due to its content of suspended particles of fat, proteins, and minerals. The color varies from white to yellow depending on the carotene content of the fat. Skim milk is more transparent and has a slightly bluish color.
Freezing Point: The freezing point of milk is lower than the freezing point of water because of the dissolved components in milk. Measuring the freezing point is used as a legal standard to determine if milk has been diluted with water. The freezing point of milk is -0.552oC or 31oF. p. 82,83 Milk Composition
Physical and chemical properties, continued
pH
Titratable acidity
pH:
The pH of milk is higher, or more alkaline, outside of the cow than inside the cow due to loss of carbon dioxide to the air. The pH of milk is never determined immediately after milking because the processing milk goes through removes dissolved gasses. The pH is determined after processing the milk to assure that lactic acid is being produced at the desired rate by added microorganisms during the preparation of cheeses and fermented milk. The casein in milk forms into a curd or a gel at a pH of 4.6.
Titratable acidity:
Titratable acidity is the amount of alkali required to bring the pH to neutrality. This property of milk is used to determine bacterial growth during fermentations, such as cheese and yogurt making, as well as compliance with cleanliness standards. Naturally, there is no lactic acid in fresh bovine milk, however, lactic acid can be produced by bacterial contamination, but this is uncommon. The titratable acidity is due to the casein and phosphates.
Bacteria
Pathogenic
Natural Flora
Affect shelf life and milk quality
Original source of cultured dairy products
•Milk producers are required to exclude bacteria from milk, but some may still gain entry. In the US, the bacterial count in Grade A raw milk may not exceed 300,000/ml. When the cow has mastitis, microorganisms associated with infections are found, including Staphylococcus aureus, Streptococcus uberis, and Streptococcus agalactiae. Milk from cows with mastitis can not be used for human consumption. Pasteurization destroys most of the microorganisms in milk and all of the pathogens. In the US, the upper limit of bacteria in pasteurized milk is 20,000/ml.
•Milk has a pH of 6.6, which is ideal for the growth of many organisms. Milk is sterile at secretion in the udder, but it becomes contaminated by bacteria before it leaves the udder. At this point, the bacteria are few and harmless, unless the cow has mastitis. Further infection of the milk by microorganisms happens during milking, storage, handling, and other activities.
•Pathogenic bacteria: The following pathogenic bacteria are a concern in raw
•These pathogenic bacteria can cause foodborne illnesses by ingesting raw milk that contains these organisms or milk that was not pasteurized properly or was contaminated after processing.
