CHEMISTRY OF MILK

Module 6. Lipids in milk

Lesson 23
NOMENCLATURE AND STRUCTURE OF GLYCERIDES

23.1 Introduction

Lipids are better energy sources than sugars because they are less oxidized than carbohydrates. These lipids are being synthesized in adipose tissue, mammary gland and liver. Lipids are compounds that are soluble in ether, benzene, hot alcohols, etc. They are not soluble in water. The lipids in bovine milk are present in microscopic globules as an oil-in-water emulsion.The primary purpose of these lipids is to provide a source of energy to the newborn calf. Both the fat content of the milk and the fatty acid composition of the lipids can vary considerably as a result of changes in factors like breed of cow, diet and stage of lactation. The fat content can vary from about 3.0 to 6.0%, but typically is in the range 3.5 to 4.7%. Changes in the composition of the fatty acids (e.g.: 16:0 and 18:1) can be quite marked and can lead to changes in physical properties of the fat. These changes make comparison difficult between different samples of milk fat and ideally comparisons should be made between cows in mid-lactation and fed on similar diets. From a practical viewpoint, milk lipids are very important as they confer distinctive nutritional, textural and organoleptic properties on dairy products, such as cream, butter, whole milk powder and cheese. The common property of all lipids is their hydrophobic nature. Lipids includes fats and oils, waxes, phospholipids, steroids (like cholesterol), and some other related compounds.

23.2 Nomenclature

The fats and oils are made from two kinds of molecules: glycerol (a trihydroxy alcohol) and fatty acids attached to it by ester linkage. The ester of glycerol containing fatty acids are formerly known as triglycerides and now termed as triacylglycerol.

The practice of using common names for various chemical compounds was prevailing earlier but these names often caused confusion in correct identification of these compounds. An example is the use of caprylic to describe 1-octanol and 2-octanol and attempts to qualify the name with “primary” and secondary were not useful in identifying the compounds.The IUPAC names have been given for the chemical compounds to overcome the difficulty in identification of the fatty acids.

23.3 Nomenclature of Fatty Acids

There are different types of nomenclature for the fatty acids. In one method they are classified depending upon their saturation and unsaturation as shown in Table 23.1

Table 23.1 Nomenclature of fatty acid on the basis of saturation

Another type of nomenclature is by using empirical formula and the name of the fatty acids as shown in Table 23.2.

Table 23.2 Systematic nomenclature for the fatty acids and their empirical formula

(Source: http://chel.1f1.cuni.cz/html/Lipids02.pdf)

23.4 General Structure

General structure of Triglyceride (triacylglycerol, TAG or triacylglyceride) is an ester composed of a glycerol bound to three fatty acids. It is the main constituent of vegetable oil and animal fats.

Triglycerides are formed from a single molecule of glycerol, combined with three molecules of fatty acid. The glycerol molecule has three hydroxyl (OH-)groups. Each fatty acid has a carboxyl group (COOH-). In triglycerides, the hydroxyl groups of the glycerol join the carboxyl groups of the fatty acid to form ester bonds.

Fig. 23.1 Structure of triacylglycerol

(Source: www.lipidlibrary.aocs.org)

Where R₁, R₂, and R₃ are alkyl chains of fatty acids. The three fatty acids R₁COOH, R₂COOH and R₃COOH may be different or same.

For understanding the composition of various lipid classes it is necessary to have an idea about the terminology used for explaining the position of the various fatty acids. An abbreviation of fatty acids is used for such purpose. According to this the first figure is the number of carbon atoms and the second number of double bonds. As for example, stearic acid is referred as C18:0 while oleic acid is denoted as C18:1.To locates the fatty acid in acylglycerols stereospecific numbering (Sn) is used. If a glycerol molecule is drawn with the secondary hydroxyl to the left, the hydroxyl above it is Sn-1and that below is Sn-3. Although milk fat contains more than 400 fatty acids most these fatty acids exist in traces while only 20 fatty acids are found in larger concentration. The recent studies carried out on the whole milk fat composition revealed that the short chain acids are selectively associated with the sn-3 position. With increase in chain length of the acid there is greater tendency to be esterfied at position Sn-1.

There are three types of TG The first type of TG are with 48- 54 carbon atoms composed of long chain 1,2 DGs containing 18:0, 18:1 and 18:2. In type 2 TG are containing 36-46 carbon atoms and the Sn-3 position acids composed of 4:0, 6:0 and 8:0, these tri glycerides are enantiomers. Type 3 TG the carbon 26- 34 the 1,2 DG containing medium chain fatty acids and the 3 position acids are short and medium chain acids in sn-3 position that are different from those in Sn-1 are also enantiomers

Chain lengths of the fatty acids in naturally occurring triglycerides can be of varying lengths, but 16, 18 and 20 carbons are the most common. Natural fatty acids found in plants and animals are typically composed only of even numbers of carbon atoms due to the way they are bio-synthesized from acetyl CoA. Bacteria, however, possess the ability to synthesize odd- and branched-chain fatty acids. As a result, ruminant animal fat contains odd-numbered fatty acids, such as 15, due to the action of bacteria in the rumen.

Most natural fats contain a complex mixture of individual triglycerides. Because of this, they melt over a broad range of temperatures. Cocoa butter is unusual in that it is composed of only a few triglycerides, one of which contains palmitic, oleic, and stearic acids in order of concentration.