CHEMISTRY OF MILK

Module 6. Lipids in milk

Lesson 26
PROPERTIES OF MILK LIPIDS

26.1 Introduction

In bovine milk fat the triacylglycerols account for about 98% of the total milk lipids. The diacylglycerols, monoacylglycerol and free fatty acids (FFA) are mostly products of lipolysis, and the cholesterol and phospholipids are cellular membrane material which accompanies the fat globule during extrusion from the secreting cell.

26.2 Properties of Lipids

The detailed distribution of the lipid material associated with the fat globule is given here under. The properties of the individual lipids will be discussed here under. During the discussion of the lipid properties the short connotation of fatty acids have been used. Accordingly, for a fatty acid i.e., 18:0, stearic acid; l8:1, oleic acid; etc. The first figure is the number of carbons, the second the number of double bonds.

The composition of lipids in milk is presented in Table 26.1.

Table 26.1 Composition of bovine milk lipids

(Source: Patton and Jenson, Biomedical aspects of lactation, 1976)

26.3 Triglycerides

Triglycerides are the major lipid class accounting for 97 to 98% of the total lipids in many species. Triglycerides can be defined as esters of glycerol and three moles of fatty acids. These triglycerides can be hydrolysed under suitable conditions to yield component fatty acids and glycerol .It is observed that distribution of the fatty acids or groups of residues over the three positions is not random. Particularly the preference of C₄:0 and C₆:0 for the 3 position is not random. Position of the fatty acid residues in the triglyceride molecules affects physical properties particularly crystallization and it determines which fatty acids are predominantly liberated by enzymatic hydrolysis. However the position of these fatty acids has least effect on the chemical reactivity. Triglycerides are not very reactive at room temperature. At higher temperature oxidative attack of the double bonds may occur. The Keto and hydroxyl groups may react.

The triglycerides are very apolar and not surface active. They act as a solvent for many other apolar substances. A small amount of water dissolves in liquid triglycerides (about 0.15% at room temperature in liquid milk fat) but triglycerides do not dissolve in aqueous media at all. They may be a part of some lipoproteins. A kind of mixed micelles largely consisting of protein and compound lipids and thus be present in trace quantities in plasma.

The triglyceride composition in several milk products will be affected as in the case of butter milk which is obtained by churning of cream have a lower melting triglyceride than those of the whole milk fat which is reflected in a higher degree of unsaturated fatty acid residues. The fat can be fractioned by partial crystallization and separation resulting in obtaining fractions with different properties.

26.3.1 Milk fat constants

Physical and chemical constants of fats are helpful in characterizing the fat. The type of the fatty acids present in the fat can also be identified with the help of these constants. These constants would also enable to detect the adulteration qualitatively and some instances quantitatively.

26.3.1.1 Refractive index

This property of milk fat is based on the principle that the degree to which the light waves pass through the liquid fat, which will be the characteristic feature of that milk fat. This physical constant for milk fat is determined by using the Abbey refractometer. The reading is usually obtained at 40℃. This instrument directly gives the refractive index. Butyro refractometer is also useful for determining the refractive index. The reading obtained with this instrument has to be converted in to refractive index or to be used as butyro refractometer reading (B.R. Reading). The reading for the milk fat ranges between 1.4527 and 1.4566.due to the large proportion of saturated glycerides and short chain acids in the bovine milk fat it is low in comparison to that of other fats and oils. There fractive indeed of a fat is influenced by the both the molecular weight and the degree of unsaturation of the component fatty acids.

26.3.1.2 Saponification Number

The saponification number is equivalent to the number of milligrams of potassiumhydroxide required to saponify one gram of fat. This value may range from 210 to 233 for milk fat. This value of a lipid indicates the average molecular weight of fatty acids present in it. The average molecular weight of a triacylglycerol is equals 168,300 / SV. The formula useful for calculating the average molecular weight is:

SV = 56, 100 (45+14 x )

where x average number of C atoms per fatty acid residue

This value for milk fat is much higher for most other fats oils with the exception of coconut and palm kernel oil. Saponification value is more useful in detecting the mineral values such as paraffin in ghee as the are not acted upon by alkali and such a sample does not form a homogenous solution upon saponification.

26.3.1.3 Iodine Number

Iodine number is the number grams of iodine absorbed by 100 g of fat under specified conditions. This value is a measure for the unsaturated linkages present in a fat .Absorption of either iodine bromide (IBr) or iodine chlore (ICl) is used for measuring this value. One molecule of halogen compound absorbed by each unsaturated linkage and the absorption is expressed as equivalent number of iodine absorbed by 100g fat. The iodine value for milk fat ranges between 26 to 42.

26.3.1.4 Reichert-Meissal Number

It is the number of milliliters of 0.1N sodiumhydroxide or aqueous alkali solution required to neutralize the steam volatile water soluble fatty acids distilled from 5g of fat under the precise conditions specified in the method. This value is quite significant for milk fat, since it primarily measures the butyric acid and caproic acid content in the given fat. When compared to other fats this value for milk fat is high. This helps in the identification of milk fat from other fats R.M. for milk fat ranges between 18 to 30.

26.3.1.5 Polenske number

It is the number of milliliters of 0.1N sodium hydroxide or aqueousalkali solution required to neutralize the steam volatile water insoluble fatty acids distilled from 5g of fat under the precise conditions specified in the method. The Caprylic and capric acid although steam volatile, are insoluble in water. Since most of the steam volatile fatty acids in milk fat are water soluble this value helps in identifying the presence of coconut oil content which contains higher proportion of these acids. This value for milk fat ranges between 1.0 to 3.3.

26.3.1.6 Kirschner value

It is the number of milliliters of 0.1N aqueous alkali solution required to neutralize steam volatile water soluble fatty acids distilled from 5g of fat which forms water soluble silver salts distilled from 5g fat under specified conditions.

26.3.1.7 Melting point

Triacyl glycerol molecule of a milk Fats are mixture of mixed fatty acids forming. There is no possibility to obtain a specific temperature to be regarded as melting point similar to pure chemical compounds. As such melting point of milk fat is the end of melting range. There are several methods available for the determination of melting points. The following methods are used for the determination of melting point of milk fat:

1. Mettler dropping point method

2. Falling ball method (softening point)

3. Open capillary tube method

4. Slip point method

The research work conducted by De Man et.al.(1983) the Mettle dropping method and softening point methods give reproducible results where as slip point method the results are poor. The average softening point for milk fat milk fats were 30.4°C for soft butter and 38.4°C for hard butter(Parodi,1973).

26.3.2 Crystallization behavior

Milk fat is liquid above 40°C and completely solid below -40°C . Between these extremes milk fat is a mixture of crystals and oil, where oil is the continous phase. Milk fat contains a large number of triacylglyerols, making the process of crystallization complex. The properties of milk fat are the average of the properties of its triacylglycerols. Crystals in fat globules generally cannot grow larger than the globule diameter. Most crystals are much smaller than globule size and they flocculate into a network, giving the globules rigidity. Crystallization starts with the formation of crystal nuclei in the molten fat as few molecules gather in molecular aggregates where the potential energy is reduced to a minimum.

26.4 Lipolysis and Rancidity of Milk Fat

Market milk and some products manufactured from milk may have a flavor which is described as “rancid”. Development of this flvour is due to the accumulation of proper concentration and type of free fatty acids which are released due to the hydrolysis of milk fat due to the action of lipases which are normally present in milk.

26.4.1 Hydrolytic rancidity

The flavor defect commonly referred as “rancidity” or more specifically “hydrolytic rancidity” is caused primarily by the presence in milk of a single enzyme “milk lipoproteinlipase”. Presence of this enzyme in milk is ascribed to leakage from blood through mammary tissues rather than true secretion. Although, most of the milks contain sufficient amounts to cause rancidity, in practice it will not occur because the substrate (triacyl glycerol) and the enzyme are well partitioned.In addition to this there are several factors which affect the enzyme activity. Lipolysis takes place at an oil water interface. The factors which influence enzyme activity are, surface area available , permeability of the emulsion, type of triglyceride employed, the physical state of the substrate( complete solid, complete liquid or liquid-solid) degree of agitation of the reaction medium, pH. temperature, the presence of inhibitors and activators,concentration of the enzyme and substrate, light.

Lipolysis has been classified as spontaneous or induced. Spontaneous lipolysis has been defined as lipolysis caused without apparent mechanical agitation. Induced lipolysis is caused by some form of mechanical agitation of raw milk.

26.4.2 Autoxidation

Lipid oxidation in fluid milk and number of its products has been a concern of the dairy industry for the last several years. To prevent or to retard this defect low-temperature refrigeration of butter and butter oil, inert gas or vacuum packing of dry whole milk. The rate of autoxidation is influenced by the Complex composition of dairy products, the physical state of the product (liquid, solid, emulsion, etc.) and the presence of natural anti or pro oxidants, as well as processing, manufacturing and storage conditions tend to influence. These factors would also influence composition and percentage of products formed due to autoxidation.

26.4.2.1 Mechanism

Chemical reactions involved autoxidation of milk fat are grouped in to three categories. viz. intiation, propogation and termination. The initial step in the autoxidation of unsaturated fatt acid and their ester is the formation of free radical. In the case of monounsaturated and non conjugated polyene fatty acids the reaction is initiated by the removal of hydrogen atom from the methylene group of adjacent to the double bond. The resulting free radical stabilized by resonance adds oxygen to form peroxide containing free radicals these in turn react with another mole of unsaturated compound to produce two isomerichydroperoxides in addition to free radicals capable of continuing the chain reaction.

Oleic acid, having two methylene groups, gives rise to four isomeric hydroperoxides. The preferential points of attack in a polyenenon conjugated systems are the methylene groups located between the double bonds. Autoxidation of linoleic and linolenic acids can lead to the formationof three and six isomeric hydroperoxides respectively by reacting with C₁₁methylene group linoleic acid and C₁₁ and C₁₄ methylene groups of linolenic acids. In a polyunsaturated fatty acid methylenegroups located other than those located between the double bonds can alsoinvolve in these reactions but to a lesser degree. In addition to formation ofpolyperoxides, carbon to carbon polymerization, and the formation of epoxides andcyclic peroxides are also possible in these reactions.

Hydroperoxides formed due to autoxidation beingunstable they readily decompose forming the saturated and unsaturatedaldehydes. The mechanism suggested forthis is that it involves in cleavage of the isomeric hydroperoxide to thealkoxyl radical which undergoes carbon - to - carbon fission to form aldehyde. Formation of other productssuch as unsaturated ketones saturated and unsaturated alcohols saturated andunsaturated hydrocarbons and semialdehydes are also being observed. Saturatedand unsaturated Aldehydes impart characteristic off-flavours in the products.The terms often used to characterize the flavor are “painty”, nutty, melon-like, grassy,tallow,oily, card board, fishy, cucumber etc.,