Composition – proteins, lipids, lactose, minerals, vitamins, enzymes, etc.
Factors affecting composition – species breed, feed, stage of lactation
Physical and chemical properties – density, freezing point, color, pH, acidity, flavor
Bacteria
Pathogenic Bacteria
Somatic Cells and Microorganisms – lactobacilli, history, significance in cultured products
Milk Fraction Terms
Plasma: milk – fat (skim milk)
Serum: plasma – casein micelles (whey)
solids-not-fat (SNF): proteins, lactose, minerals, acids, enzymes, vitamins. It is the total solids content minus the fat content.
Total Milk Solids: fat + SNF
•“Plasma” and “skim milk” are interchangeable terms, but “skim milk” is more commonly used.
Composition of Milk
Ash 7.0 5.5
Fat 37.0 28.9
Protein: casein 27.6 21.6
Lactose 48.0 37.5
Total solids 127.9 100.0
Non-protein 1.9 1.5
nitrogen
Protein: whey 6.4 5.0
Proportion solids (%)
Constituent Concentration (g/L)
Technology of Dairy Products, 1998
•The biological function of milk is to supply nutrition and immunological protection to the young mammal. In some species, milk is the only food consumed for weeks or months. Therefore, it must furnish all nutritive requirements for maintenance and growth, such as energy, amino acids, minerals, and vitamins.
•This chart shows the composition of milk, detailing the major components as proportions of solids.
Basic Properties of Milk
Emulsion
Colloidal Solution
Emulsion:
•An emulsion is a suspension of droplets of one liquid into another liquid. Milk is an emulsion of fat in water. Butter is an emulsion of water in fat. The solute is known as the dispersed phase and the solvent is known as the continuous phase. Other examples of emulsions include margarine, mayonaise, cream, and salad dressing.
Colloidal solution:
•A colloidal solution is when matter exists in a state of division in between a true solution, which is sugar in water, and a suspension, which is chalk in water. The characteristics of a colloid are small particle size, electrical charge, and affinity of the particles for water molecules.
•In milk, the whey proteins are in colloidal solution. The casein proteins are in colloidal suspension.
The composition of milk may change due to differences in relative rates of synthesis and secretion of milk components by the mammary gland. Variations are due to differences among species, between individuals within a strain, and between conditions affecting an individual. Conditions affecting the cows may include the weather or seasons and the stage of lactation.
Breed:
The US mostly uses milk from cows of the larger breeds, such as Holsteins and Brown Swiss’ because of the lower fat content and greater milk production. Breeds such as the Guernseys and the Jerseys have higher fat contents in their milks. Both the Guernseys and the Jerseys have a fat content of 5.2%, where as the Holsteins and the Brown Swiss’ have fat contents of 3.5%.
Diet:
The composition of the cows’ diet and the form in which they are fed affect the composition of milk and especially milk fat. High fat and/or low roughage diets can reduce the fat content of milk. Diet has small effects on protein content and none on lactose content.
The seasonal effect is due to the changes in the diet throughout the year.
Stage of lactation
•When mammals give birth, their first secreted milk is called colostrum, and it differs greatly in composition from regular milk. Colostrum contains more mineral salts and protein and less lactose than normal milk. Also, fat content, calcium, sodium, magnesium, phosphorus, and chloride are higher in colostrum than in normal milk. Whey content is about 11% in colostrum as opposed to 0.65% in normal milk.
•Colostrum contains an extremely high imunoglobulin (Ig) content. Igs accumulate in the mammary gland before parturition and transfer immunity to the baby cow. These immunoglobulins protect the baby cow until it can establish its own Immunity.
•Figure 5.1 in your assigned reading
Milk Fat
Milk fat globule
Membrane
Rupture of FGM
Density
Low magnification milk fat droplets (2-10 _m in diameter)
•Milk fats consist of a mixture of compounds including triacylglycerols, diacylglycerols, monoacylglycerols, phospholipids, cerebrosides, gangliosides, sterols and sterol esters and derivatives, carotenoids, tocopherol, vitamins A, D, E, C, B1, and B2 and free fatty acids. They are not soluble in water, or aqueous liquids.
•Milk fat is secreted from the mammary epithelial cells as fat globules.
•The average size of a milk fat globule is about 3um in diameter, but they can range from 1 to 18 um.
•The milk fat globule is encapsulated by a lipid bilayer membrane made up of proteins, phospholipids, lipoproteins, cerebrosides, nucleic acids, enzymes, trace elements, and bound water. The FGM (fat globule membrane) provides stabilization for the fat globule in the aqueous environment of the milk serum.
•When the FGM membrane is ruptured, the fat globules join together into a solid mass of fat. (This is what happens during the production of butter).
•Milk fat globules are the largest particles in milk but they are also the lightest. They have a lower density than milk serum, so the globules rise to the surface, which is how cream is formed. The density difference is used to separate milk fat from whole milk in high speed, continuous flow, centrifugal separators.
