CHEMISTRY OF MILK

Module 6. Lipids in milk

Lesson 29
VITAMINS IN MILK

29.1 Definition

Vitamins may be defined as carbon compounds of diverse structures, which are not used for energy or fixed into the tissue frame work but which are required in minute amounts for the normal functioning of the living organisms. Vitamins are also recognized as nutrients for animals obtained from different foods etc. These can not be synthesized in the body. Vitamins are also synthesized by several other living organisms such as microorganisms like yeast and molds. Sun light also helps in the synthesis of vitamins like Vitamin D. Since milk being considered to be nearly complete food it is consumed by human beings of all ages.

29.2 Vitamins in Milk

Milk contains various fat soluble vitamins like vitamin A, D, E, K, along with water soluble vitamins like vitamin C and B complex vitamins and provitamins. However, concentration of some of these vitamins is very less. The concentration of the vitamins in the milk is shown in Table.29.1

Table 29.1 Concentration of vitamins in milk

*Aretinol equivalent (RE) is equal to 3.33 IU retinol or 10 IU β-carotene

(Source: Text book of Dairy Chemistry by Mathur/et.al., 2005)

The abbreviation IU stands for the international units which is the measurement for some vitamins. It is the same idea as milligrams, though theyare not equivalent. These units exist in order to account for the fact that certain preparation of the same vitamin is not bioequivalent. Regardless of the preparation 1 IU will provide the same biologica activity.Eg. 1 IU of vitamin E equals the biological equivalent of about 0.667 mg d-alpha-tocaopherol or 1 mg of dialpha to copherol acetate. Though the mass of each preparation is different, the biological activity is the same.

29.3 Classes of Vitamins

On the basis of their solubility vitamins are distinguished into two:

1. Class A: Fat soluble vitamins A, D, E and K,

2. Class B: Water soluble vitamins, the B-complex and vitamin C (Ascorbic acid).

The B-complex vitamins includes thiamine,riboflavin, niacin, pantothenic acid, pyridoxine biotin, choline, inositol, folic acid, p-aminobenzoic acid. B_I2. All the above vitamins are present in milk, however, vitamin A and B₂ group vitamins are present in appreciable amounts

29.4 Vitamin A (Retinol)

It is a primary alcohol derivative of a polymer consisting of four isoprene units. It is formed by the cleavage of provitamin A carotenoids, polymers of eight isoprenes, of these βcarotene has the highest vitamin A potency because it consists of identical halves and thus yields two retinol molecules, while other carotenoids yield only one. Plants synthesize carotenoids but not retinol, animals cannot synthesize carotenoids but can split them to retinol (vitamin A). Vitamin A levels are frequently expressed in International Units (I.U.), although this unit is officially no longer accepted. One I.U. equals 0.344μg of crystalline vitamin A acetate, or 0.300 μg vitamin A alcohol; or 0.600 μg β-carotene. Current usage is to express vitaminA potency as retinol equivalents (RE)

1 RE = l µg Retinol

= 6 µg β carotene

=12 µg other provitamin A carotenoids

= 3.33 IU from retinol

= 10.00 IU from β carotene

Fig. 29.1 Structure of renitol
(Source http//www//chemistry.about.com/bio,Anne Marie)

Vitamin A is present in cow’s milk as retinol, retinol ester and carotene. The content depends strongly on the amount of carotenoids in the feed. The ratio of retinol to carotene in milk varies with breed and species of animals. Vitamin A is relatively stable to heat in the absence of oxygen, because of the highly unsaturated character of the molecule, It is quite susceptible to oxidation,especially in the presence of sunlight or artificial light.

VitaminA is unstable in the presence of mineral acids but stable in alkali Vitamin A and the carotenoids have good stability during various food processing operations. Losses may occur at high temperatures in the presence of oxygen. These compounds are also susceptible to oxidation by lipid peroxides, and conditions favouring lipid oxidation will also result in vitamin A breakdown. Pasteurization of milk does not result in vitamin A loss but exposure to light will result in loss. It is essential,therefore, that sterilized milk be packed in light-impervious containers. Possible losses during storage of foods are more affected by duration of storage than by storage temperature. There are several forms of provitamin A,belonging to the carotenoid pig­ments. The most important one is β-carotene and some of the pigments that can be derived from it are of practical importance. These are β-apo-8' carotenal and β-apo-8'-carotenoic acidethyl ester. For example, Apo carotenal (R = CHO) and apo-carotenoic acidester (R=COOC₂H₅). Other provitamins are α-and γ-carotene s and cryproxanthin.

Fig. 29.2 Structure of β-carotene
(Source: http://www//chemistry.about.com/bio,Anne Marie)

It has been found that vitamin A added to milk, is more easily destroyed by light than the native vitamin A. This is not because natural and synthetic vitamin A is different, but because these two types of vitamin A are dispersed differently in the milk. The form in which vitamin A is added to food products may influence its stability. Vitamin A in beadlets form is more stable than that added as a solution in oil. The beadlets are stabilized by a protective coating. If this coating is damaged by wetting with water, the stability of the vitamin is greatly reduced.

29.4.1 Importance

Vitamin A is important in the diet as it is necessary for growth, health and reproduction. It keeps the epithelial tissues healthy and thus aids in preventing infections. Lack of this vitamin will cause night blindness. Vitamin A aids in maintaining normal glandular functions such as mainte­nance of adrenal cortex and steroid hormones synthesis. It acts to initiate vision from light energy. Vitamin A is present as carotene in plant and must be converted to vitamin A for its function. Vitamin A is a growth factor. It has an important role in the vision of mammalian organisms.

Table 29.2 Vitamins A and carotene content of some foods

(Source: Text Book of Dairy Chemistry, Mathur et. al., 2005)

29.5 Vitamin D

Vitamin D includes a number of compounds with antirachitic activity. Ergocalciferol (D₂)occurs in plant material and is formed by UV irradiation of ergosterol. Cholecalciferol (D₃) is formed by UV irradiation of7-dehydrocholesterol in animals particularly in the skin. Both forms of this vitamin occur in milk, D₂ coming from the feed and D₃ from their radiated skin of the cow. The recent research shows, that the active form of vitamin D is identified as 1,25-dihydroxycholicalciferol which triggers the synthesis of calcium binding proteins for the calcification of bones. Thus this vitamin prevents disease called rickets in children and osteomalacia in pregnant women and osteoporesis in old. The potency is sometimes expressed as international units where, 1 IU is 0.25ng of cholecalciferol. Average value of this vitamin in bovine milk is 13.0 and 31.2, I.U. for winter and summer milk respectively.

The potency of thevitamin D in milk can be increased by

1. Exposure of cow to sunlight
2. Feeding higher levels of irradiated plantmaterial
3. Irradiating milk
4. Fortification in milk with vitamin D.

Itis stable in milk, not affected by pasteurization, boiling or sterilization.Storage of fluid milk for 30 months at -17.8°C, irradiated evaporated milk for2 to 3 years or of frozen butter for over 2 years resulted in little or notloss of the vitamin.

29.5.1 Importance

Vitamin D is necessary to aid in calciumand phosphorus retention so as to make strong bones and prevent rickets. It increases tubular preabsorption, and citrate blood levels. It maintains and activates alkaline phosphatase in bones and serum calcium and phosphorus level.Vitamin D is sometimes called the sunshine vitamin because the ultraviolet radiation of the sun can convert an active precursor of the skin(7-dehydrocholesterol) to vitamin D. Children need vitamin D for their growth and to a lesser extent mothers require this vitamin during pregnancy and lactation.

29.6 Vitamin E

Vitamin E consists of a group of to copherols,the most potent of these and the principal one in milk is α-tocopherol C₂₉H₅₀O₂ this is strong reductant and serves as an antioxidant protecting lipids from oxidation. The vitamin E content is rather low. It depends to some extent on feed. Summer milk has higher concentration than winter milk. It is stable to heat but may be partly destroyed by intensive illumination in the presence of O₂ the vitamin E content of human milk is about ten times that of cow milk.

Fig. 29.3 Structure of vitamin E
(Source: http://www//chemistry.about.com/bio,Anne Marie)

29.6.1 Importance

It is astrong antioxidant, prevents oxidation of unsaturated fatty acids and vitamin A in intestinal tract and body tissues. It acts in oxidation­ reduction reactions, plays role in human nutrition and is associated with reproductive factor, necessary for nutrition of muscles and better utilization of vitamin A. It maintains integrity of vascular system and central nervous system. Vitamin E is a detoxifying agent and maintains kidney, tubules, lungs, genital structure,liver and red blood cell membranes.

29.7 Vitamin K

Vitamin K is present only in traces in milk if at all. Human needs for this vitamin are supplied from consumption of plant materials containing it and by microbial synthesis in the digestive track .