ORGANIC CHEMISTRY

Module 8.Fatty acids and lipids

Lesson 26
IMPORTANT REACTION OF FATTY ACIDS (SATURATED AND UNSATURATED)

26.1 Introduction

The various reactions of fatty acids are mentioned below

26.2 Saturated Fatty Acids

26.2.1 Saponification value

The process of mixing a strong alkali with glycerides and heating splits the glycerides yielding glycerol and alkaline salts of fatty acids (soaps). This is known as saponification.The saponification value is defined as the number of milligrams of KOH required to saponify 1 gm of fat/oil. The saponification value is inversely proportional to the mean molecular weight of fatty acids. This is because a unit weight of fat containing lower mol. weight fatty acids would contain larger number of molecules than equal weight of fats containing higher mol. weight of fatty acids. Each molecule of glyceride irrespective of mol. weight requires 3 molecules of alkali for saponification. Thus glycerides containing higher mol. weight fatty acids would require less KOH.

Determination of saponification value gives idea about the proportion of fatty acids in respect to molecular weight and chain length. The mineral oil e.g. liquid paraffin is not acted upon by alkali and so there is no uniform solution upon saponification; thus its presence is detected in butterfat. From the knowledge of RM value and Saponification value, as both are higher in for butter fat, the adulteration of animal body fat or vegetable fat/oil can be detected. The Saponification value is also important in speculation about certain physical properties like melting point, solidification point, refractive index etc. of fat.

26.2.1.1 Principle

Heating with alcoholic KOH saponifies the fats/oils. Fats are water insoluble and hence the rate of hydrolysis by aqueous KOH is slow so alcoholic KOH is used for the reaction. The amount of alkali consumed for saponification of fat is estimated by back titration with an acid. Refluxing is the collection of distillate back in the original content from which it has been vaporized.

26.2.2 Free Fatty Acids

Milk fat contains little concentration of free fatty acids in their composition. Upon passage of time due to splitting of glycerides by the action of lipase the concentration of free fatty acids increase resulting into hydrolytic rancidity.

In milk and cream, lactic acid is important, contributing major part of acidity, whereas, in butter and ghee wide range of fatty acids occur which vary widely in molecular weight. It is therefore preferable to present the free fatty acids in terms of acid value.

26.2.2.1 Principle

The free fatty acids present in ghee are estimated by titration with weak alkali using phenolphthalein as indicator.

26.2.3 Reichert-Meissle and Polenske Values

The method does not determine the total quantities of volatile fatty acids, soluble and insoluble in water, present in combination in fat. The amount of these acids actually determined by this process is dependent on strict adherence to the dimensions of the apparatus and the details of the procedure.

The Reichert-Meissl value (R.M. value) is the number of ml of 0.1 N aqueous alkali solution required to neutralize the water-soluble steam volatile fatty acids distilled from 5g of ghee under the precise conditions specified in the method.

The Polenske value is the number of m1 of 0.1 N aqueous alkali solution required to neu-tralize the water-insoluble stream volatile fatty acids distilled from 5g of ghee under the precise conditions specified in the method.

The Kirschner value is the number of ml of 0.1 N aqueous alkali solution required to neutralize the water-soluble steam volatile fatty acids which form water-soluble silver salts distilled from 5g of ghee under the precise conditions specified in the method.

26.2.3.1 Principle

Upon saponification of fats with strong alkali under vigorous heat, the acylglycerols split into glycerol and alkaline salt of fatty acids. The fatty acid salts thus obtained are treated with dilute sulphuric acid to set free the fatty acids during steam distillation. A portion of the fatty acids, being volatile under the given condition, pass into the distillate along with excess of water.

The volatile portion again consists of two types of fatty acids (i) water-soluble and (ii) water-insoluble. The water-insoluble fatty acids are harvested by washing the condenser, still head, receiver and filter paper with alcohol/rectified spirit where as water-soluble fatty acids remain in the filtrate of the distillates. Amount of the water-soluble fatty acids is estimated by titrating the filtrate with standard alkali. That of water-insoluble fatty acids is estimated by titrating the alcohol extract with standard alkali.

The distillation time should be between 19 and 21min. Overheating results into decarboxylation of fatty acids and migration of double bonds. Hence, prolonged heating or distillation brings decomposition of nonvolatile fatty acids and yield volatile products which makes the determination nonquantitative.

26.3 Unsaturated Fatty Acids

26.3.1 Iodine value

Iodine value is the number of grams of iodine absorbed by 100 grams of fat or oil under the specified conditions. It ranges from 26 to 33 for normal fresh ghee. It is determined as % by weight of halogen calculated as iodine. This value is a measure of degree or unsaturation of fat. The extent of hydrogenation of fat and adulteration of ghee with vegetable oil or animal body fats can also be detected from the iodine value. Iodine value is affected by factors like season, feed, oxidative rancidity, extent of hydrogenation etc.

The volumetric analysis in which the use of standard iodine solution is made to estimate the amount of a substance dissolved in a given solution is known as iodimetry. Iodometry is a volumetric titration in which the iodine liberated during a chemical reaction from the substance containing iodine is determined.

26.3.1.1 Principle

The unsaturated fatty acids absorb halogen and form addition products. The addition of halogen iodide or monochloride to the double bond is quantitative. Iodine is absorbed very slowly by the fat/oil and hence to bring about the reaction carried out, iodine monochloride is used. The method is based on the treatment of known weight of fat/oil with a known volume of standard solution of iodine monochloride and estimation of free ICl.

Iodine has low solubility in water (only 0.335g in 1 lit. water at 25° C). Also an aqueous solution of iodine has an appreciable vapour pressure of iodine and therefore decreases slightly in concentration on account of volatilization. These difficulties are overcome by dissolving iodine in aqueous potassium iodide solution. The increased solubility is due to formation of tri iodate ion (I₂+I^-→I₃^-). The resulting solution has much less vapour pressure. When an iodide solution of iodine is treated with the reducing agent, this displaces the equilibrium to the left and eventually all the tri iodate is decomposed. The solution is therefore behaves as a solution of free iodine. This free iodine is titrated with 0.1 N sodium thiosulphate solution using starch as an indicator.

26.3.2 Peroxide values

The oxidation of unsaturated lipids is initiated by two oxygen addition mechanisms: (a) addition of molecular oxygen on methylene group adjacent to double bond, i.e. autocatalytic radical chain reaction mechanism, or (b) concreted addition “ene” reaction: addition of singlet oxygen directly at double bond in unsaturated lipids. Autoxidation may be considered as a two step process: (i) Primary autoxidation reaction which may lead to the formation of hydroperoxides. (ii) Further reaction of decomposition of hydroperoxides by cleavage of C-C bond to form volatile compounds.

The peroxide value is a measure of the oxi¬dative rancidity in ghee and is expressed as milliliters of 0.002 N sodium thiosulphate per gram of sample, or as milliequivalents of peroxide oxygen per kg of sample. Two methods are recommended by BIS (a) lodometric Method and (b) Oxygen Absorption Method. Here the determinations will be performed by Iodometric Method. The other chemical methods to measure oxidative deterioration in lipid and lipid containing food products include: (i) Determination of peroxide by modified Stamm method, (ii) Loftus-Hill method, (iii) Thio-Barbutyric Acid (TBA) value, (iv) Determination of carbonyl content, etc.

Autoxidation of unsaturated fatty acids involve their reaction with molecular oxygen and as a result several peroxide structure compounds are formed as intermediate products. Amount of the peroxides is estimated iodometrically. In acidic conditions, these peroxides liberate free iodine from potassium iodide. Amount of the librated iodine, which is in proportion to the amount of peroxides present in the sample, is determined by titrating with standard sodium thiosulphate.