Characteristics of Milk

Destabilization of the casein micelles

Change in pH

Proteolysis

Cottage cheese

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Casein destabilization

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•The most important reactions of the milk proteins are those which involve destabilization of the casein micelles.

•As we previously mentioned, reactions can occur within casein micelles as the pH is reduced. As the pH if milk is reduced, the native micellar structure disintegrates. At the isoelectric point of casein, (the pH at which casein carries no net electrical charge – a pH of 4.6), the casein precipitates out of solution, along with any heat-denatured whey. This reaction may be reversed as the pH is increased, but native form of the micelle is not recovered.

•Acidification of milk may be used to form many fermented products such as yogurt and to fractionate the milk proteins to produce acid casein. Acid casein can be purified and used in foods for its emulsifying and water binding properties.

•At a neutral pH, milk is very chemically stable. However, when milk is heated, added to alcohol, or concentrated, stability may be reduced.

•Destabilization of casein micelles also occurs when milk is subjected to selective proteolysis.

In selective proteolysis, a specific enzyme, chymosin, cleaves off the part that stabilizes the kappa casein. This proteolysis results in an increase in sensitivity of the casein micelles to calcium, so the two coagulate and a firm gel is formed. These reactions form the basis of cheese production. However, this process is actually made up of two reactions: proteolysis and calcium-induced aggregation. If the reaction is carried out at refrigeration temperatures, only proteolysis occurs and the milk remains stable until heating takes place and the calcium- induced clotting can occur.

Lactose

Most abundant of milk solids

Lactose is a disaccharide composed of galactose and glucose

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•Lactose is the most abundant of the milk solids.

•Lactose is a useful source of energy and it is thought by some to promote the absorption of calcium from the diet. However, some people lack the enzyme lactase and have a difficult time digesting lactose. When people who are lactose intolerant eat a substantial amount of lactose containing foods, they can suffer from GI distress. Some milk products are available that have lactase introduced aseptically into the previously sterile product before packaging.

•The relative sweetness of lactose is small (only 20%) when compared to sucrose (100%). (http://www.lactose.com/basic/physiological_properties.html)

Minerals

 Calcium: Calcium functions in the mineralization of bones and teeth, muscle contraction and relaxation, nerve functioning, and blood clotting.

 Iron: Iron carries oxygen as a part of hemoglobin in blood or myoglobin in muscles, and it is required for cellular energy metabolism.

 Magnesium: Magnesium is a factor involved in bone mineralization, the building of protein, enzyme action, normal muscular function, transmission of nerve impulses, proper immune function and maintenance of teeth.

 Phosphorus: Phosphorus is involved with mineralization of bones and teeth, it is important in genetic material, in cell membranes as phospholipids, in energy transfer, and in buffering systems.

 Potassium: Potassium facilitates reactions, including the making of protein. It is involved in the maintenance of fluid and electrolyte balance, the support of cell integrity, the transmission of nerve impulses, and the contraction of muscles, including the heart.

 Sodium: Sodium, sodium chloride, and potassium (the electrolytes) maintain normal fluid balance in the body. Sodium is critical to nerve impulse transmission.

 Zinc: Zinc is associated with hormones, it is needed for many enzymes, it is involved in making genetic material and proteins, immune cell activation, transport of vitamin A, taste perception, wound healing, making of sperm, and normal fetal development.

Calcium: Calcium functions in the mineralization of bones and teeth, muscle contraction and relaxation, nerve functioning, and blood clotting.

Iron: Iron carries oxygen as a part of hemoglobin in blood or myoglobin in muscles, and it is required for cellular energy metabolism.

Magnesium: Magnesium is a factor involved in bone mineralization, the building of protein, enzyme action, normal muscular function, transmission of nerve impulses, proper immune function and maintenance of teeth.

Phosphorus: Phosphorus is involved with mineralization of bones and teeth, it is important in genetic material, in cell membranes as phospholipids, in energy transfer, and in buffering systems.

Potassium: Potassium facilitates reactions, including the making of protein. It is involved in the maintenance of fluid and electrolyte balance, the support of cell integrity, the transmission of nerve impulses, and the contraction of muscles, including the heart.

Sodium: Sodium, sodium chloride, and potassium (the electrolytes) maintain normal fluid balance in the body. Sodium is critical to nerve impulse transmission.

Zinc: Zinc is associated with hormones, it is needed for many enzymes, it is involved in making genetic material and proteins, immune cell activation, transport of vitamin A, taste perception, wound healing, making of sperm, and normal fetal development.

Minor components and micronutrients

 Non-Protein Nitrogen

Nucleotides

Urea

Free amino acids

 Enzymes

Lipoprotein lipase

Lactoperoxidase

Xanthine oxidase

Alkaline phosphatase

Heme group of lactoperoxidase

http://www.sacs.ucsf.edu/home/Ortiz/images/peroxfigure300.gif

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