Wednesday, 9 July 2008

Chemical and Physical Properties of Fats and Oils

Fats and oils make up one of three major classes of food materials, the others being carbohydrates and proteins. Fats and oils have been known since ancient times as they were easily isolated from their source. They found utility because of their unique physical properties. Fatty tissues from animal sources liberate free-floating fats on being boiled. Olives and sesame seeds yield oil on being pressed. Such fats and oils add flavor and lubricity to foods prepared with them

The very commonness of fats and oils and the familiarity most people have with them have led to an ironic situation. The average user, whether a housewife, chef, baker, or food manufacturer, often has little real understanding of the characcer of fatty products. There are times when selection of the proper fat for a use situation can be very critical but is done incorrectly. Other times the selection need not be critical but a costly choice is made due to prejudice or to lack of adequate knowledge of the subject.

Much of the terminology used in fat and oil work was developed over the years by processors who had only practical knowledge. Even the early chemists were hampered by incomplete understanding of their field of study. Therefore, a fat was defined as the oleaginous material that was solid at room temperature and the liquid form was called an oil.
The process of separating a fatty mass into a more liquid and a more solid fraction resulted in calling the former the oil and the latter the stearino portion. This led to the anomalous situation that oleo oil from beef fat was fairly solid at room temperature while the stearine separated from cottonseed oil/could be liquid at a somewhat elevated room temperature.

Edible fats and oils are esters of the three carbon trihydric alcohol, glycerin, and various straight chained monocarboxylic acids known as fatty acids. The fatty acids of natural fats have 4 to 24 carbon atoms and, with minor exceptions, have an even number of such atoms. Figure 1 shows the structure of glycerin, fatty acids, and a fat derived from them.

The fatty acids may be saturated, monounsaturated, or polyunsaturated. Saturated acids have all of the hydrogen that the carbon chain
will hold. Table 1 lists them by chemical name, common name, and source. Monounsaturated acids have two hydrogen atoms missing, one from each of two adjoining carbon atoms, giving rise to one carbon-carbon double bond, Polyunsaturated refers to a fatty acid with two or more double bonds. Specifically from a nutritionist's point of view, the polyunsaturates should be the cis,cis-methylene interrupted isomers. These isomers can be distinguished from trans or conjugated isomers by use of the lipoxidase enzyme analysis using a spectrophotometer to determine the oxidation products. These products are conjugated diene hydroperoxides with an absorption wave length of 234 nm.

Natural fatty acids usually exist in specific isomeric forms. Chemi¬cal isomers are compounds that occur in two or more forms although they have the same number of carbon, hydrogen, and oxygen atoms. Unsaturated fatty acids can have positional isomers in relation to where the double bonds appear in the carbon chain. Natural fatty acids most frequently have the first double bond between carbons 9 and 10. the second between C-12 and C-13, the third between C-15 and.C-16.

Table 2 lists the various unsaturated fattv acids, their common names, and sources. The most commonly occurring unsaturated fatty acids are oleic, linoleic, and linolenic.

Another type of isomer is the geometric isomer. Here the carbon chain is bent into a fixed position at each double bond. The carbon chain sections are either bent towards or away from each other. The former is called cis (meaning same side), the latter, trans (meaning across). Figure 2 illustrates these isomeric forms.
The natural acids are found most frequently in the cis form. Trans isomers are usually formed during chemical reactions, e.g., hydrogena-tion or oxidation. Cis acids have a considerably lower melting point than trans acids of the same chain length. Thus, oleic acid (cis-octa-decenoic) melts at 16°C (61°F) and is therefore liquid at room tempera¬ture. Elaidic acid Urans-octadecenoic) has a melting point of 44°C (111°F) and is solid at room temperature. Infrared spectroscopy is used for analysis of trans isomers in fats.

The length of the carbon chain also affects melting point. For satu¬rated fatty acids, melting point increases as the chain becomes longer. Unsaturation lowers melting point of fatty acids of the same chain length with cis acids leaving a considerably lower melting point than trans acids. It is interesting to note that stearic acid, the saturated IS carbon acid, melts at 69°C (156°F). Oleic acid has a melting point close to that of caprylic acid. The former has 18 carbons and one cis double bond. The latter is saturated with 8 carbons. Elaidic acid has 18 car¬bons and one trans double bond. It has a melting point close to that of 1 auric acid with a 12 carbon chain.

The fatty acids are found in natural fats as complex mixtures. There are three positions available on the glycerin molecule for the es-terification of the fatty acids. The physical characteristics of the vari¬ous triglyceride fats depend on the type, quantity, and distribution of the acids on the glycerin molecule. A simple illustration of this was demonstrated by Norris and Mattil (1946). They randomized mixtures of triolein and tripaimitin by interesterification and analyzed the glyceride composition of the final products.

Figure 3 shows the theoretical composition of one random mixture. It should be noted that a distinction has been made between the 1- and 2-positions on the glycerin molecule. The characteristics of a triglycer¬ide depend on which position each fatty acid occurs. This is most important in the sharpness of melting of cocoa butter and in the crystal structure of lard. It is of lesser importance with other typee of facs and oils.

The melting point of fatty mixtures changes on randomization. A mixture of 30% tripaimitin and 70% triolein had a melting point of 58°C (136°F) before interesterification and 48CC (119°F) after the reaction (Norris and Mattil 1946).

There are also a number of minor components incorporated in natu¬ral fats. Simplest of these are mono- and diglycerides. Glycerin is es-terified with only one or two fatty acids, leaving free hydrophilic hydroxyl groups. This makes the monoglyceride an important emulsifier.
Phospholipids are also important as emulsifiers. They are com¬pounds containing fatty acids and phosphoric acid among other chemi¬cal entities. Lecithin is a glycerin ester of two fatty acids and phos¬phoric acid combined with choline, a nitrogenous compound. Cephalin is similar with ethanolamine, a different nitrogen-containing com¬pound, replacing choline. Inositol phosphatides are compounds of inositol, phosphoric acid, fatty acids, ethanolamine, tartaric acid, and sugars.

Soybean oil phospholipids consist of 29% lecithin, 31% cephalin, and 40% inositol phosphatides. They are commonly called "lecithin." Commerical lecithin is obtained primarily from soybean oil. Some are also available from corn oil and egg yolk.

Sterols are complex polycyclic alcohols. They are found as a class of compounds in all oils. Cholesterol is found only in animal fats. Plant sterols are similar in composition but sufficiently different to suppress absorption of dietary cholesterol by the body or to suppress formation of natural cholesterol in the body (Jandacek 1975). Sterols may have some effect as emulsifiers. Joffe (1942) has postulated that cholesterol in egg yolk aids in the formation of the mayonnaise emulsion although lecithin and lecithoproteins are considered to be the major emulsifiers in yolk.

Tocopherols are natural antioxidants found in vegetable oils. They are also known as vitamin E. They are cyclic compounds with a hydroxyl group and heterocyclic oxygen. Tocopherols are easily oxidized to quinones. They, therefore, have natural antioxidant activity.

No comments: