Market Milk

Module 5. Physico-chemical, microbiological and nutritional properties of milk

Lesson 12
Macro-components of milk – Protein

12.1 Milk Proteins

The caseins, whey proteins and non-protein nitrogen (NPN) make up the nitrogen content of milk (76%, 18% and 6% respectively). Table 12.1 lists the proteins in milk. The minor proteins associated with the fat globule membrane are not included here. The Rowland fraction method facilitates the separation of caseins, whey proteins and NPN. Casein precipitates at pH 4.6 and is thus separated from whey nitrogen. Total proteins may be separated from whey NPN at pH 5 with sodium acetate and acetic acid.

Table 12.1 Concentration of proteins in milk

*Average values derived from different sources

12.1.1 Caseins

Casein constitutes about 80% of the total proteins present in milk. In fresh milk, casein is held in colloidal suspension. Casein contributes in a big way to the viscosity and white colour of milk. Free casein is almost insoluble in water but is rapidly dissolved by dilute alkalies. Casein in milk can be precipitated with dilute acids, salts or rennet. All caseins precipitate at pH 4.6. Casein particles are termed as ‘casein micelles’. Biologically, casein serves as a carrier for calcium and phosphate to nurture the young calf and form a clot in the stomach to aid easier digestion. The micelle also contains citrate, minor ions, lipase and plasmin enzymes, and entrapped milk serum. Caseins can be further classified into three major groups as alpha (?), beta (?) and kappa (?). Alpha casein is further sub-divided into ?s1 and ?s2 fractions. Table 12.2 lists the major casein fractions in milk.

Table 12.2 Major casein fractions

*Average values derived from different sources

12.1.2 Whey proteins

The greenish-yellow clear liquid that separates out of milk after precipitation of caseins at pH 4.6 is called whey and the proteins contained therein are whey proteins. These globular proteins are more water soluble than caseins and are prone to heat denaturation. Denaturation increases their water holding capacity. Native whey proteins have good gelling and whipping properties. The principle fractions of whey proteins are detailed in Table 12.3.

Table 12.3 Major whey protein fractions

*Average values derived from different sources

Beta–lactoglobulins comprises approximately half the total whey proteins. It is coagulated by heat, which is why colostrum curdles when heated. It binds fat-soluble vitamins making them more available to the body and provides an excellent source of essential and branched chain amino acids. These help prevent muscle breakdown and spare glycogen during exercise. Beta–lactoglobulins may also be helpful in controlling certain liver conditions such as cirrhosis.

Alpha lactalbumin is not coagulated by rennet or acids but is precipitated by heat. The extent of coagulation depends on temperature of holding, salt concentration and pH of milk. It is the primary protein found in human breast milk and is a good source of essential amino acids (particularly tryptophan) and branched chain amino acids. This is the only whey protein component capable of binding calcium.

The immunoglobulins provide protective effect to the offspring against enteropathogenic micro-organisms. It is the predominant whey protein component found in colostrum. Bovine milk contains only traces of IgA. Bovine serum albumin is a large sized protein with a good essential amino acid profile and fat binding properties. Lactoferrin is an anti-oxidant that naturally occurs in many body secretions such as tears, blood, breast milk, saliva and mucus. It is a red-coloured iron-chelating protein. It inhibits enteropathogenic organisms due to its ability to bind iron. Iron is an essential nutrient often required for bacterial growth. Lactoperoxidase and lysozyme are enzymes that have antibacterial activities. The latter exhibits immunity-enhancing properties. Although lysozyme from egg white had found more industrial applications in the past, it has now been recognized that the enzyme isolated from human or bovine milk has far greater lytic activity compared to egg lysozyme.

12.1.3 Milk enzymes and non-protein nitrogenous (NPN) substances

Some indigenous enzymes that have been isolated from milk are lipase, aryl esterase, alkaline phosphatase, acid phosphatase, xanthine oxidase, peroxidase, protease, amylase, catalase and lactase. The most significant group is the hydrolases, comprising of lipoprotein lipase, plasmin and alkaline phosphatase. Milk contains some NPN substances that constitute about 5 percent of the total nitrogen in milk. Some examples of this group are amino acids, creatine, urea, uric acid, creatinine and hipuric acid.

Selected Readings

Fox, P.F. and McSweenwy, P.L.H. 2003. Advanced Dairy Chemistry. Volume 1. Proteins. Elsevier Publications, USA.
Robert Jenness, R., Noble P. Wong, N.P. and Elmer H. Marth, E.H. 1999. Fundamentals of Dairy Chemistry. Aspen Publishers, USA.