Lesson 24. COMPOSITION, CLASSIFICATION AND DISTRIBUTION OF LIPID IN MILK
The lipids in milk are unique for the species although they are termed as milk lipids or milk fat. This is because of the fact that the fatty acid composition and its biosynthesis is different in each species. The importance of milk fat cannot be over emphasized.The role of milk fat in nutrition is its capability to yield approximately ~37 kJ per g (9 Kcal/g) apart from carrying the fat soluble vitamins viz., A, D, E and K. The presence of significant amounts of essential fatty acids viz., linoleic and arachidonic acids also play an important role in the nutrition of the new born. The rich pleasing flavour contributed by the milk fat to the milk products render them acceptable by all the consumers and further no other fat can be used to duplicate it. The body and texture of the milk products is largely being influenced by this milk component.
24.2 Composition of Milk Fat
Knowledge about the physical state of milk lipids is essential before proceeding to the detailed study of its composition. The bulk of the milk fat exists in the form of small globules with a average size of approximately 2 to 5µm . This is water in oil type of emulsion; the surface of these globules is covered by a adsorbed layer of material commonly known as fat globule membrane. Small quantities of milk fat also occur in the milk serum in combination with proteins.
24.3 Composition of Lipids in Bovine Milk
Table 24.2 Lipid composition of bovine milk fat
(Source: Patton and Jensen, Biomedical aspects of lactation, 1976)
24.4 Classification of Lipids
24.4.1 Simple lipids
The lipids which yields only fatty acids and glycerol upon hydrolysis
a) Neutral fats: Found in adipose tissue, butterfat, lard, fish oils, olive oil, cornoil, etc. These are esters of three molecules of fatty acids with one molecule of glycerol. This includes triacylglyerol, diacylglyerol and monoacylglyerol. The structure of a triacylglyerol is shown in figure 24.1.
24.4.2 Compound lipids
22.214.171.124 Phospholipids (phosphatides)
Found chiefly in animal tissues. Substituted fats,consisting of phosphatidic acid; composed of glycerol, fatty acids, and phosphoric acid bound in ester linkage to a nitrogenous base.
Found in brain, egg yolk, and organ meats. Phosphatidyl choline or serine; phosphatide linked to choline; a lipotropic agent; important in fat metabolism and transport; used as emulsifying agent in the food industry.
It occurs predominantly in nervous tissue. Phosphatidyl ethanolamine; Phosphatidyl serine where phosphatide linkage to ethanolamine or serine; plays a role in blood clotting.
Plasmalogen is a predominant component of the membrane phospholipids of many animal and microbial species. The vinyl ether bonds in position sn -1 In animal tissues, the highest proportion of the plasmalogen form is usually in the phosphatidylethanolamine class with rather less in phosphatidylcholine, and commonly little or none is in other phospholipids such as phosphatidylinositol. In phosphatidylcholine of most tissues, a higher proportion is often of the O -alkyl rather than the O -alkenyl form. Found in brain, heart, and muscle.
a) Lipositol: Found in brain, heart, kidneys,and plant tissues together with phytic acid. Phosphatidyl inositol; phosphatide linked to inositol; rapid synthesis and degradation in brain; evidence for rolein cell transport processes.
Inositol (Phosphatidylinositol) is a negatively charged phosphor lipid and a minor component in the cytosolic side of eukaryotic cell membrane. Inositol can be phosporylated to form phosphatidylinoistol phosphate, phosphatidylinoistolbisphosphate, phosphatidylinoistoltrisphosphate,all these are collectively called phosphoinositides. phosphoinositides play important roles in lipid signaling, cell signaling and membrane trafficking.
Found in nervous tissue, brain, and red blood cells. Sphingosine-containingphosphatide; upon hydrolysis it yields fatty acids, choline, sphingosine,phosphoric acid, and no glycerol; source of phosphoric acid in body tissue.
Fig. 24.8 Structure of cerebroside
Sulfolipid: white matter of brain, liver, and testicle; also plant chloroplast.Sulfur-containing glycolipid; sulfate present in ester linkage to galactose.
Proteolipids: Brain and nerve tissue. Complexes of protein and lipids having solubility properties of lipids.
24.4.4 Derived lipids
a) Fattyacids: They occur in plant and animal foods; also exhibit in complex forms with other substances. Obtained from hydrolysis of fats; usually contains an even number of carbon atoms and are straight chain derivatives. Classification of fatty acids is based on the length of the carbon chain (short, medium, orlong); the number of double bonds (unsaturated, mono, or polyunsaturated); or essentiality in the diet (essential or non essential). A current designation is based on the position of the double bond, counting from the methyl (-CH3)group, called the omega end. The most important omega fatty acids are: Omega-6 fatty acid and Omega-3 fatty acid, eicosapentaenoic, and docosahexaenoic acids.
24.5 Distribution of Lipids in Milk
In milk different classes of lipids are distributed in various phases and the Table 24.2 will give us an idea about their range of occurrence and their location in milk. It must be remembered here that the lipids mentioned in this represent a group of compounds and they should not be considered as a single constituent.
Table 24.2 Distribution of lipids in milk
(Source: Principles of Dairy Chemistry Jeeness ans Patton, 1969)