CHEMISTRY OF MILK

Module 6. Lipids in milk

Lesson 28
UNSAPONIFIABLE LIPIDS: CHOLESTEROL AND ITS PROPERTIES

28.1 Introduction

The lipid compounds which are not saponifiable in the milk fat are considered under this fraction. The compounds in this fraction have unique properties hence the properties of these compounds needs a special mention.

28.2 Unsaponifiable Matter

When the milk fat is being saponified with an alcoholic base and ether extract obtained is known as unsaponifiable matter unsaponifiable lipids. The actual procedure for the saponification is as follows:

When a weighed amount of fat in an excess of approximately 0.5N alcoholic base for 1 hour would result in the decomposition of triglycerides and other ester materials decompose to fatty acid, salt, glycerol and small amount of other ester components. The resulting soap solution is diluted with water and extracted under specified conditions with ethyl ether. The ether extract after some washing to remove impurities is evaporated and dried to constant weight. This dry residue represents the unsapoinifiable matter which for milk fat generally falls within the range of 0.30 to 0.45 % by weight. As the above procedure involves heating for one hour in an alcoholicbase , there is possibility of having some artifacts during the process along with the saponifiable matter.

Shifting of double bonds of some of the unsaturated fatty acids is likely to occur during this process. The structure and properties of the components of the unsaponifiable matter is taken into consideration. These constituents are usually measured direct under mild conditions than those involved in the isolation of the unsaponifiable matter.

The principal known compounds are cholesterol and related sterols, the fat soluble vitamins A, D, E and K, traces of squalene, unidentified waxes, and homologus series of n-alkylmethyl ketones containing odd number of carbons C₃ to C₁₅

28.3 Sterols

These compounds are found in the unsaponifiable fraction of milk lipids consists mostly of cholesterol with some lanosterol. Research workers who have come with dry saponifiaction have reported that the content of sterols were 0.33 to 0.36% of butter oil. Further investigations in this field have revealed the presence of two new constituents named dihydrolanosterol and β sitosterol. Although researchers were able to identify 17 sterols from bovine milk as trimethylsilylethers. Among these sterols five were identified and reported their content as a percent of the total sterol.

Table 28.1 Main sterols in unsaponifiable matter of milk lipids

These analyses were done by GLC mass spectrometry. In the previous years the reliable data on cholesterol content was not available but with the advancement of various analytical techniques it is now made available by the research work carried out by several researchers in the last tow to three decades. The amount of cholesterol in whole milk fat was 13.49 ± 1.01mg per 100 g of milk which contained 3.47± 0.74 g of fat.

28.3.1 Cholesterol

The principal sterol of milk is cholesterol (C₂₇H₄₅0H),the formula for cholesterol is presented in Figure 28.1.

Fig. 28.1 Structure of cholesterol
(Source: www. lipidlibrary.aocs.org)

The cholesterol content of milk ranges from approximately 0.25 to 0.4% by weight of the fat. Since the total unsponifiable matter ranges from 0.3 to 0.45% (by weight of the fat) giving an understanding that the unsaponifiable matter of milk fat is largely cholesterol.

Many of the methods used for the analysis of cholesterol, in milk are not specific for this sterol alone. There is a possibility that cholesterol values may include plant and animal hormone as well as other sterols all of which may be present in milk. The sterols are very apolar, but they easily associate with phospholipids, a small fraction of cholesterol is esterified, so this is, in fact, saponifiable. There are also conflicting results on the other, minor, sterols in milk.

28.3.1.1 Properties of cholesterol

Cholesterol can be oxidized in various ways and cholesterol oxidized products ( COP)are formed. These oxidized products of cholesterol are found in animal foods including the dairy products. A higher concentration of cholesterol oxidized products can be found only in processed dairy products exposed to harsh storage conditions where the impact of oxygen and light or oxygen and low water activity are associated.

28.3.1.2 Oxycholesterolor 5,6-epoxycholesterol

This a form of oxidized cholesterol implicated in atherosclerosis. It is commonly formed from the reaction of fats and oxygen during high temperature cooking such as frying.

28.3.1.3 Oxysterols

These are oxidized derivatives of cholesterol, which are important in many biological processes, including cholesterol homeostasis, sphingolipid metabolism, platelet aggregation and apoptosis.