CHEMISTRY OF MILK

Lesson 24. COMPOSITION, CLASSIFICATION AND DISTRIBUTION OF LIPID IN MILK

Module 6. Lipids in milk

Lesson 24

COMPOSITION, CLASSIFICATION AND DISTRIBUTION OF LIPID IN MILK

24.1 Introduction

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

The bulk of milk lipids are triacylglycerols (TGs)which are 97-98 % of the total lipids found in pooled milk. Sterols mostly cholesterol and phospholipids are next in the quantity. The Diacylglycerols (DGs), monoacylglycerols (MGs) and free fatty acids are the products of lipolysis and are more in concentration as compared to the components which are present in traces. The lipid composition of the bovine milk fat is presented in Table 24.1

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.

Fig. 24.1 Structure of triacylglycerol
(Source: www.lipidlibrary.aocs.org)

b) Waxes: Composed of esters of fatty acids with long chain monohydric alcohol. It has industrial and medicinal importance.

24.4.2 Compound lipids

24.4.2.1 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.

Fig. 24.2 General structure of phospholipids
(Source: www.lipidlibrary.aocs.org)

24.4.2.2 Lecithin

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.

Fig. 24.3 Structure of phosphatidycholine (lecithin)
(Source: www.lipidlibrary.aocs.org)

24.4.2.3 Cephalin

It occurs predominantly in nervous tissue. Phosphatidyl ethanolamine; Phosphatidyl serine where phosphatide linkage to ethanolamine or serine; plays a role in blood clotting.

Fig. 24.4 Structure of ethanolamine
(Source: www.lipidlibrary.aocs.org)

24.4.2.4 Plasmalogen

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.

Fig. 24.5 Structure of plasmalogen
(Source: www.lipidlibrary.aocs.org)

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.

24.4.2.5 Inositol(Phosphatidylinositol)

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.

Fig. 24.6 Structure of phosphatidylinositol
(Source: Wikipedia)

24.4.2.6 Sphingomyelin

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. 24.7 Stucture of sphingomyelin
(Source: www.lipidlibrary.aocs.org)

24.4.2.7 Glycolipids

Cerebroside: Cerebrosides is a common name for a group of glycosphingolipds called monglycosylceramides which are an important component of muscle and nerve cell membranes. Theyconsists of a cerimide with a single sugar residue at the l – hydroxyl moiety.The sugar residue can be either glucose or galactose. Accordingly these are named as glucocerbrosides and alactocerebrosides. Galactocerebrosides are typically found in neural tissue while glucocerebrosides are found in other tissues Myelin sheaths of nerves, brain, and othertissues. Yields on hydrolysis of fatty acids, sphingosine, galactose (orglucose), but not fatty acids; includes kerasin and phrenosin or cerebronicacid.

Fig. 24.8 Structure of cerebroside
(Source: www.lipidlibrary.aocs.org)

Ganglioside: Brain, nerve tissue, and other selected tissues, notably spleen; contains aceramide linked to hexose (glucose or galactose), neuraminic acid, sphingosine,and fatty acids.

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.

24.4.3 Steroids

a) Sterols

Cholesterol: found in egg yolk, dairy products, and animal tissues. It is a precursor of bile acids and Vitamin D₃.

Fig. 24.9 Structure of cholesterol
(Source: www.lipidlibrary.aocs.org)

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)

From the table 24.2 it is evident that milk lipids are found in three distinctly different phases of milk namely fatglobule, the membrane surrounding these globules and the milk serum.