CHEMISTRY OF MILK

Module 6. Lipids in milk

Lesson 27
GENERAL PROPERTIES OF COMPOUND LIPIDS

27.1 Introduction

Although the compound lipids are less in their concentration but they play a very important role in the stabilization of the fat globule being an important constituent of the fat globule membrane.

27.2 Definition

The lipids which on hydrolysis yield phosphates, carbohydrates and nitrogenous compounds like sphingosine, ethanolamine, serine, carbohydrates, choline, etc. in addition to the glycerol and fatty acids are known as compound lipids

27.3 Phospholipids

The phospholipids comprise approximately 1% of the total lipid in bovine milk. Although phospholipids are quantitatively less but they play a very important role in the formation and secretion of the milk fat apart from forming a stable colloidal suspension or emulsions in aqueous solution. These lipids play an important role during storage and processing of milk due to their susceptibility to oxidation of the polyunsaturated fatty acids Their bipolar molecules and the relatively high concentration of unsaturated fatty acids

Phospholipids are also found as lipoprotein complexes in skim milk. The skim milk phase may containing 30-50% of the phospholipids in milk. The phospholipids in milk are synthesized by the mammary cells via pathways that are common to other mammalian cells. The major glycerophospholipids are phosphatidyl ethanolamine, phosphatidyl choline, phosphatidyl serine, phosphatidyl inositol.

The composition of the phospholipids in milk is presented in the Table 27.1. In milk the glycerophospholipids are found predominantly in the diacyl form. However small amounts of plasmalogen which are the vinylether form of glycerophospholipid are also being reported. The studies carried out by some research workers revealed that 4.0% of the phosphatidylethanolamine was in the ether form and 1.3% of phosphatidylcholine was in this form. It was also observed that 1.3 to 2.5% of bovine phospholipids are plasmalogens. Some compounds that are being found as phospholipids in milk lipids which are suspected to be the degradation products or remnants of biosynthesis of lipid material in the mammary glands. Phosphotidic acid is a product formed by the action of phospholipase D on the phosphatidyl choline. The quantity of this product in milk is very low.

Table 27.1 Phospholipids composition of bovine milk

(Source: Fundamentals of Dairy Chemistry, Wong et. al. 1988)

The most abundant and best-known compound lipids in milk are the phos­pholipids or phosphatides, of which the phosphoglycerides makes up the major portion.phospholipids or phosphatides are lipids which contain phosphorus as esterified phosphoric acid.

In other phosphoglycerids, the choline residue is replaced by ethanolamine (-CH2-CH2-NH₃⁺), serine, or inositol (a cyclitol).

The lipid becomes highly polar due to the two charged groups in the molecule where as the rest is very apolar, so that the molecule is amphiphilic and does not dissolve well in either water or fat. The phospholipids, however, form micelles in water and in fat, with the polar ends at the outside and inside, respectively. They are difficult to transform from the hydrated into the other form, and vice versa. Their amphiphilic nature makes the phospholipids very surface active. In milk, they are in contact with water and with proteins as "lipo­proteins."Little is known about the nature of the bonds in such complexes. The phospholipids,particularly the cephalins, have many unsaturated fatty acid residues compared to the neutral glycerides. They include polyenoic fatty acids. This is important for lipid autoxidation.

27.4 Sphingolipids

In addition to phosphoglyceride, milk contains sphingolipids. Their com­mon residue is sphingosine. In all of them, a fatty acid residue is bound to the.-NH group; on average these lipids contain very long-chain fatty acids. In sphingomyelin, aphosphocholine group (as in lecithin) is bound to the terminal oxygen; it is thus a phospholipid.

27.5 Cerebrosides

Glucosyl and lactosyl (cerebrosides) compounds have been isolated from milk by some research workers. They found 1.7 mg / 100ml in the globule membrane and 0.8 mg / 10 ml in skim milk. The membrane bound cerebrosides contained mainly acids of 20 to 25 carbons and those in skim milk contained 18 carbon or less In the cerebrosides or glycosyl ceramides a sugar group is bound. This is mostly glucose, and in milk lipids also lactose. The gangliosides are only a minor fraction of milk fat; they have several ketoses, including N-acetyl neuraminic acid bound to the terminal oxygen. The properties of the sphingolipids are comparable to those of the phosphoglycerides. The entire compound lipids are found mostly in the same places. The distribution between fat globule membrane and milk plasma (presumably in lipoprotein particles) is for all classes similar, but not exactly equal. Compound lipid are absent in the core of milk fat globule.