CHEMISTRY OF MILK

Module 6. Lipids in milk

Lesson 25
FATTY ACID COMPOSITION OF MILK LIPIDS AND STRUCTURE OF FAT GLOBULE

25.1 Introduction

Milk and milk products occupy a significant position in the human diet which is primarily attributed to the body, texture and flavour of these foods. The fatty acids composition of the lipid plays a significant role in the characteristics of the products. As such a study of the fatty acid composition and factors which influence them helps us in getting the desired type of fatty acid composition of the milk fat to some extent, alternatively how we can modify the composition to get the desirable attribute to the products prepared from it.

25.2 Fatty Acid Profile of Milk fat

Bovine milk is composed of triacylglycerol (97.0 to 98.5%). These are formed by the esterification of the hydroxyl groups of glycerol with the fatty acids. The nature of these fatty acids varies vastly and has a significant influence on the chemical, physical and organoleptic properties of the fat. Although several fatty acids have been identified in the milk fat only few of them are present in significant quantity and are of nutritional, physical and chemical importance. Further 80% of the total fatty acids are distributed among the five fatty acids namely the oleic, palmitic, butryic, stearic and myristic acids. Nearly 50 of the total 80 fatty acids which are commonly encountered in the milk fat would make only 1% of the total fatty acids. These fatty acids are usually grouped on the basis of saturation (saturated, monounsaturated and polyunsaturated). Similarly they can also be grouped on the basis of geometric isomerism as straight chain, branched chain, on the basis of chain length as short, medium and long chain.

Polyunsaturated fatty acids are further classified as either cis or trans (geometric isomerism) or conjugated or non conjugated positional isomerism. Depending upon the number of carbon atoms present in the fat can also be grouped as even or odd numbered fatty acids. A convenient method of notation of fatty acids two numbers designating the carbon chain length and the unsaturation (Number of double bonds) Thus C_4:0 denotes butyric acid which has 4 carbon atom and is saturated without any double bond. Similarly C_18:1 or C18-2 denotes oleic acid or linoleic acid having a carbon 18 with 1 and 2 double bonds respectively. The chain length of fatty acids in milk fat varies from C₄ to C₂₆. Normally the fatty acids percentage is not expressed on the weight basis but on molar percentage.

Table 25.1 Fatty acid profile of bovine milk fat

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

25.3 Factors Affecting Fatty Acid Composition

The composition of milk fat is not constant and is very variable. The important cause for variation is feed and both lipid and non lipid components affect milk fat composition. Consequently it will vary with the year region farming practice and so on. Therefore it makes little sense to give information on the variations unless we give detailed account of the effect of several factors. Since the fatty acids which are preformed from food fat is transferred to the mammary gland via the blood and lymph in the form of triglyceride and free fatty acids. Most of these fatty acids are having a chain length of 16 or more carbon atoms. Some of the fatty acids are being synthesized by the gland from the acetate and β hydroxyl butyrate produce by rumen bacteria.

The fatty acids synthesized by utilizing the fatty acids producing rumen bacteria will have short or medium chain length C₄ to C₁₄ and part of the C₁₆. Acetate contribute to the increments to all of the C₄ to C₁₄ acids and β hydroxyl butyrate is used primarily for the initial four carbon “primer” units of the most fatty acids synthesized. Low roughage diets diminish acetate and increase propionate production in the rumen. Milk produced by cow on low roughage diets may have only half of the fat content when compared to the milk from cows on high roughage diets and the proportion of the short and medium chain saturated acids which are synthesized from acetate is greatly diminished. To a considerable extent the rumen microorganism hydrogenates the dietary fatty acids. Feeding highly unsaturated oil such as safflower oil increases the C18:2 content of the milk fat. Butyric acid (4:0) shows a maximum value during the first month of lactation, declining thereafter and becoming minimal at the end. Hexanoic acid (6:0) to Myristic (14:0), all had similar variations; the values increased during the first 4 to 8 weeks of lactation, remained relatively constant until the fifth or sixth month, and then decreased again until the end of lactation. There was little variation in 16:0 throughout lactation. Stearic (18:0) and oleic (18:l) acid contents were high in early lactation, then declining throughout the mid lactation and again increasing at the end of lactation.

25.4 Structure Fat Globule

Milk fat is predominantly present spherical droplets which range in diameter from less than 0.2 to 15 µm. The bulk of the fat is in globules 1-8 µm diameter. The size distribution found may depend greatly on the measurement method employed. Globule size can considerably altered by various treatments particularly homogenization. The fat globules of milk differ in composition. The size of the fat globule will also alter the composition. This is because the quantity of membrane lipids (predominantly phospholipids) per unit mass of fat is higher for smaller globules. The fat globules are enveloped by a layer called the fat globule membrane.

Fig. 25.1 Structure of fat globule membrane

(Source: Mc Pherson and Kitchen, J. Dairy Res. 1983)

The membrane consists of a well organized sequence of different components arranged according to their polarity and hydrophobicity. This layer is predominantly proteinaceous in nature.

The composition of the MFGM often varies with the method of isolation or preparation. Protein constitutes nearly 25 to 60 per cent of the total dry weight. 25 different enzymes have been reported and the isolated MFGM is a rich source of enzymes including 5’Nucleotidase, alkaline phosphatase, acid phosphatase, aldolase, xanthine oxidase. Proteins of the MFGM could be in the form of polypeptides of varying molecular weight.

Lipids are the next major constituent of the MFGM and their content could vary from 0.5 to 1.2 mg per gram of protein. The lipid portion comprises of neutral lipids (50 to 80%of total lipid) or phospholipids. Due to their amphiphilic nature, the phospholipids have an important role in maintaining the stability of fatemulsion. Glycerides constitute a major portion of the neutral lipids of the membrane. Other constituents like cerebrosides, gangliosides sialic acid,cytochrome and hexoses have been reported.

The constituents of the MFGM are specifically oriented on the fat surface. The inner most layer consists of the high melting triglycerides in contact with the fat layer. The next layer consists of the phospholipids with their hydrophobic portion oriented towards inwards while the hydrophilic portion oriented outwards. The phospholipids are interspaced with cholesterol, vitamin A etc. The outermost layer is made up of protein enzymes etc absorbed on to the surface. These proteins are in contact with plasma and are even capable of binding water to a some extent. The membrane constituents are very thinly spread over the surface of the fat globule and stabilize the fat in water emulsion. The breaking or disruption of the membrane also makes the lipids more susceptible to enzymatic action due to greater accessibility.