NUTRIENTS IN MILK - CONTINUED

Milk fat

The digestibility of milk is comparatively higher than other oils and fats. This can be attributed to existence of fat globules in aqueous phase forming an emulsion. This facilitates its easy absorption through the intestinal tract when compared to other fats which have to be emulsified with bile salts, enzymes from pancreas and fat splitting lipases. Endowed with short and medium chain fatty acids, milk fat can be easily absorbed when compared to long chain fatty acids because of the ability of the lipases to split the ester bonds in the former. Supplementation of milk fat in the diet increases the energy density.

When compared to human milk, the cow milk is low in essential fatty acids such as linoleic and linolenic acids. Short and medium chain fatty acids with 4-12 carbon atoms, which occur at comparatively higher concentration in milk fat, reported to have antibacterial and fungistatic activity. Milk fat plays another important role in preventing tooth decay by forming a protective coat over the surface of enamel.

It is composed of triglycerides of fatty acids.  A fatty acid molecule is composed of hydrocarbon chain and carboxyl group.  Triglycerides are of two types, simple and complex.  In simple, all the three fatty acids are of same nature, whereas complex triglycerides on hydrolysis give glycerol and different fatty acids.

The milk fat exists in the form of small globules of sizes ranging from 2 to 10 microns with different glycerides of low melting points in suspension.  Milk fat varies in amount and composition, according to the breed, species, feed and lactation time, of which, feed being a major factor.  Fat is distributed in globules as triglycerides (98-99 %), fat globule membrane in combination with phoshpholipids and lipoprotein (0.2 to 1.0 %) and also as free fatty acids, cholesterol and phospholipids in the serum.

It is composed of triglycerides of fatty acids.  A fatty acid molecule is composed of hydrocarbon chain and carboxyl group.  Triglycerides are of two types, simple and complex.  In simple, all the three fatty acids are of same nature, whereas complex triglycerides on hydrolysis give glycerol and different fatty acids. The milk fat exists in the form of small globules of sizes ranging from 2 to 10 microns with different glycerides of low melting points in suspension.  Milk fat varies in amount and composition, according to the breed, species, feed and lactation time, of which, feed being a major factor.  Fat is distributed in globules as triglycerides (98-99 %), fat globule membrane in combination with phoshpholipids and lipoprotein (0.2 to 1.0 %) and also as free fatty acids, cholesterol and phospholipids in the serum.

Recently, more light is thrown on the effects of conjugated linoleic acid (CLA), which is a natural constituent of milk and other dairy products. Various experimental models have been devised to elicit the boil active properties of the so called wonder fatty acid, the conjugated linoleic acid. They are said to possess anticarcinogenic property in the fore stomach of mouse, prevention of mammary cancer in rats, anti carcinogenic activity in the colon of rat, anti carcinogenic activity in skin, anti diabetic activity in rats, reduction in body fat and anti atherogenic activity in rabbits and immuno modulation in rats. Apart from conjugated linoleic acid, milk fat has several bioactive and bio-protective molecules such as sphingomyelins, butyric acid, myristic acid, beta carotenes and fat soluble vitamins that show promise in anti-carcinogenic effect and immuno-stimulatory effect. The butyric acid is known for its anticancer properties, butyric acid improves immunity by activating macrophages.

Milk fat constants

For the purpose of characterization, certain well known physical and chemical constants have been derived for the more common fats. The fat constants serve as an indication of the types of component fatty acids present in different fats. They also serve the purpose of detection of adulteration of fat qualitatively and in some cases quantitatively. The following points discusses the fat constants in detail.

Refractive Index

The basic principle in determining the refractive index concerns the fact that the degree of bending light waves passing through a liquid or transparent solid will be characteristic for a particular liquid or solid. With the help of an Abbe refractometer, the refractive index of the milk fat can be easily found out. The reading is generally taken at 40°C. The normal value of the refractive index of milk fat varies between 1.4538 and 1.4578.  When compared to other milk fats and oils, the refractive index value of milk fat is low because of the graeter number of saturated glycerides and short chain acids in milk fat.

Saponification number

It is defined as the number of milligrams of KOH required to saponify one gram of fat. The normal value for milk fat may vary from 210 to 233 and more often the value falls with in the range of 225-230. The constant is an indication of the average weight of the fatty acids present in the milk fat. The constant for the milk fat is well above the values of other fats and oils except coconut and palm kernel oil.

Iodine number

The iodine number is the number of grams of iodine absorbed by 100 g of fat under specified conditions. This constant is an indication of the unsaturated linkages present in the fat. The value of iodine number of milk fat falls within the range of 26 to 35, wich is generally lower when compared to most of the other fats.

Reichert-Meissl number (R-M Number)

It may be defined as the number of milliliters of 0.1 N alkali solution required to neutralize the volatile, soluble fatty acids distilled from 5 g of fat under specified conditions. It is primarily a measure of butyric acid. The value for milk fat ranges between 17 and 35 and this value is well above that for all other fats and oils.

Polenske number

This value may be defined as number of milliliters of 0.1N alkali solution required to neutralize the volatile and insoluble fatty acids distilled from 5 g of fat under specified conditions. While   R-M number is primarily a measure of butyric and caproic acid content, caprylic and capric acids, which are some what steam volatile but largely insoluble in water, are indicated mainly by Polenske number. The normal value of Polenske number falls between 1.2 and 2.4.

Melting point

This term which is generally used in connection with fats and oils may be considered as a misnomer. Fats and oils do not exhibit sharp melting points like pure chemical compounds. Fats being a mixture of glycerides, exhibit  a melting range that may vary considerably depending upon the nature of fat and method of melting. As regard to any fat constant, the melting range for the fat should be determined under specified conditions. The melting point of the milk fat ranges between 30 and 41°C.

Physico-chemical constants of some fats

*At 25°C

Minerals present in milk

The portion left after ashing of milk at 150ºC is known as the ‘ash of milk’ and is composed of various inorganic constituents. Ash forms about 0.75 % of milk and plays a very important role in milk and is basic in character. The minerals in milk consist principally of the chlorides, citrates and bicarbonates of calcium, magnesium, potassium and sodium.

Major minerals in milk (mg/100ml)

The percentage of ash in an average sample of milk has been connected always with the amount of other constituents. Thus the proportion of lactose, protein and ash has been given as 13:9:2 (Vieth Ratio). The minor constituents present in traces are Copper, Iron, Cobalt, Manganese, Silica and Zinc. Salt constituents of milk exist in two states namely colloidal and soluble forms. Colloidal minerals are found attached with the casein micelles while soluble minerals are found dissolved in serum of milk.  In general, about 33% of Ca, 33% of P, 75 % of Mg and 90% of citrates of milk are present in dissolved state.  The colloidal salts are in equilibrium with dissolved salt; heating causes shift of minerals from soluble to colloidal form while souring of milk carries the reverse effect.  If the ratio of soluble Ca and Mg to citrates and phosphates (salt balance ratio) is disturbed the stability of milk also changes accordingly.

Phospholipids

These contain phosphorus in their molecules in addition to the fatty acids and glycerol; they also contain a nitrogenous base.  Principally milk phospholipids are the Lecithin, Cephalin and Sphingomyelin.  Though fat-soluble, they are hydrophilic and imbibe large quantity of water and swell.  They are used as antioxidants for fat rich dairy products.

Vitamins

Various vitamins present in milk are as follows. Fat Soluble Vitamins - include vitamins A, D, E and K. Water-soluble vitamins are the, B1 (thiamine), B2 (riboflavin), B6 (pyridoxine), Biotin, Niacin (nicotinic acid), Pantothenic Acid, Para-amino benzoic acid, Inositol, Choline, Folic acid, B12, and Ascorbic acid. Fat rich milk products contain large quantities of fat soluble vitamins, whereas whole milk, skim milk, buttermilk and whey are a good source of water soluble vitamins.

Vitamin A

Vitamin A is otherwise known a retinol from the chemistry point of view. The supply of this vitamin to human beings is through animal products such as cod liver oil / liver or indirectly through vegetable products such as carrots and spinach. The vegetable sources don’t have Vitamin A as such but in the form of its precursor or pro-vitamin called alpha and beta carotene. These pro-vitamins are transformed in to retinol in the liver and intestine of humans. Milk and cheese are remarkable as well as rich source of Vitamin A for the consumers, more so for the new born. Carotenoids present in the green fodder are converted into Vitamin A, which has antioxidant property. It is present as beta carotene in the milk of cows which is responsible for the yellow colour of cow milk; the buffalo milk is white in colour due to the existence of Vitamin A as such rather than as its pro-vitamin.