MARKET MILK

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

Lesson 11
MACRO-COMPONENTS OF MILK – FAT AND LACTOSE

11.1 Fat

Fat is the most commercially significant and most variable constituent of milk. Fat content varies from breed to breed and also among individuals of the same breed, the variation being caused by many factors. Most of the (> 95%) milk fat exists in the form of globules of 0.1-15 µ diameter (cow milk fat – ~ 3-8 µ, buffalo milk fat – ~ 4-10 µ). A thin membrane (8-10 nm thick) covers these liquid fat droplets. The properties of this membrane are vastly different from both milk fat and plasma. The fat globule membrane is rich in phospholipids and also contains lipoproteins and other glycerides. These phospholipids are involved in the oxidation of milk. The membrane decreases the lipid-serum interface to very low values, 1-2.5 mN/m, preventing the globules from immediate flocculation and coalescence, as well as protecting them from enzymatic action. Fat comprises of different glycerides of low melting point. The composition of fat varies with the feed plan, nutrition, stage of lactation, breed and species, the first being the most important.

The size and number of fat globules vary depending on the breed of the animal and method of milking. The globules become smaller and more numerous as lactation advances. Machine milking produces fat globules of more uniform size than hand milking. Homogenization reduces the fat globules to a small size and reduces the tendency of separation during storage. The larger the size of the globules, the quicker they rise as cream to the top of the milk and easier it is to churn such cream into butter. For this reason buffalo milk fat is more easily churned into butter than cow milk fat. The milk of animals in advance lactation is less suitable for being churned into butter. Milk containing small globules is, however, more suitable for cheese making, since less fat is lost in whey. Milk fat is quite bland in taste and imparts smoothness and palatability to fat-containing dairy products.

In milk fat, butyric, caproic, caprylic and capric acids, present in high proportions, are characterized by strong odours and flavour. These volatile acids are not present in such high proportion in other naturally occurring fats. The fatty acid content of milk fat can also be influenced by the amount and type of feeds consumed, stage of lactation and breed of the animal. Milk fat also contains cholesterol, thus differentiating it from vegetable fats, which contain phytosterols. Milk contains 0.1 to 0.23% phospholipids, viz. lecithin, phosphatidyl serine, sphingomyelin, inositol, cerebrosides etc. Some of these phospholipids serve as antioxidants in prolonging the shelf-life of ghee.

The colour of fat depends upon its carotene content and varies with the species, breed and feed of the animal. The yellow colour of cow milk is due to the carotene. Buffalo milk does not contain carotene. Ghee from cow fed on an abundant green fodder is more yellow than when fed on dry food. Similarly some breeds such as Jersey and Guernsey may produce milk deep yellow in colour.

Saturated fatty acids (no double bonds), such as myristic, palmitic and stearic constitute two thirds of milk fatty acids. Oleic acid is the most abundant unsaturated fatty acid in milk with one double bond.

11.2 Lactose

Lactose is the major milk sugar or carbohydrate. Fresh milk also contains other carbohydrates in small amounts, including glucose, galactose, and oligosaccharides. Lactose is present in true solution and, therefore, goes into whey when caseins are separated. Lactose constitutes 4.8 to 5.2% of milk, 52% of milk SNF, and 70% of whey solids. Lactose can be quickly fermented by micro-organisms to lactic acid and is, therefore, essential in the manufacture of cultured dairy products like cheese, dahi and butter-milk. It contributes to the nutritive value of milk and milk products, and is responsible for the texture and miscibility of some milk products. Lactose imparts colour and flavour to dairy products heated to high temperatures. Sucrose is six times sweeter than lactose.

Lactose is a disaccharide made up of two monosaccharides, glucose and galactose (Fig. 11.1). Lactose is hydrolyzed by the enzyme µ-galactosidase (lactase), the results being the two monosaccharides, increased sweetness and depressed freezing point. People suffering from lactose intolerance lack this enzyme and therefore, cannot digest lactose. This sugar crystallizes in an alpha form and results in the defect called sandiness in ice cream.

Fig. 11.1 Molecular structure of lactose

Selected Readings

Fox, P.F. and McSweenwy, P.L.H. 2009. Advanced Dairy Chemistry. Volume 2, Lipids. Springer Publications, USA.
McSweenwy, P.L.H. and Fox, P.F. 2009. Eds. Advanced Dairy Chemistry. Volume 3. , Lactose, Water, Salts and Minor Constituents. Springer Publications, USA.
Robert Jenness, R., Noble P. Wong, N.P. and Elmer H. Marth, E.H. 1999. Fundamentals of Dairy Chemistry. Aspen Publishers, USA.