Saturated and Unsaturated Fatty Acids

When the fatty-acid molecule contains the maximum of hydrogen possible, the acid is said to be a saturated fatty acid. It is saturated with respect to hydrogen. Myristic, lauric, palmitic, and stearic acids are such saturated acids. They are solids at ordinary temperatures. When, however, the fatty-acid molecule does not contain the maximum amount of hydrogen possible, the acid is said to be an unsaturated fatty acid. It is unsaturated with respect to hydrogen. Such unsaturated acids are oleic, linolic, and linolenic acids. They are liquids at ordinary temperatures. By chemical means these acids may be made to take up, i.e., combine with, hydrogen. This process is known as hydrogenation. It converts a more unsaturated fatty acid into a less unsaturated one, or, if the hydrogenation is carried to completion, into a saturated fatty acid. Thus by hydrogenation oleic acid is converted into stearic acid. Linolic acid when hydrogenated can be made to take up twice as much hydrogen as oleic acid, and linolenic acid three times as much. Linolic acid is, therefore, a more highly unsaturated acid than oleic, while linolenic acid is more highly unsaturated than linolic.

Unsaturated fatty acids can be made to combine with other substances instead of with hydrogen. For example, they may be made to take up iodine or oxygen. Acids of a low degree of unsaturation, such as oleic acid, do not combine with oxygen with any great degree of acidity, but acids of a greater degree of unsaturation, such as linolic or linolenic, combine with it very readily; they do so merely upon exposure to the air.

Measures of Unsaturation

It is obvious, then, that it is important for industrial users of fats to know the degree of unsaturation of a given parcel of fat. This might be ascertained by determining the amount of hydrogen required to convert it into a saturated fat. In practice this is a complicated procedure and so simpler methods are resorted to. The simplest of these is the determination of the amount of iodine that can be made to combine with the fat. The percentage by weight of iodine absorbed by the fat in the natural state is known as the iodine number. It is an index to the degree of unsaturation of the fat. The fats with the highest iodine numbers are the drying oils par excellence, linseed and tung oil, with which must also be classified menhaden fish oil.

REVISION EXERCISES

Ex.1. Answer the following questions:

1. What is the role of food in human nutrition? 2. What are the physical and chemical properties of fats? 3. What does the classification of fats depend on? 4. What is the difference between fats and oils? 5. What is the process of rancidification? 6. What is the difference between saturated and unsaturated fatty acids? 7. What is the iodine number?

Ex.2. Match the words with their definitions:

1. rancid	a. oily in appearance, texture, or manner;
2. greasy	b. any of a class of often fragrant organic compounds represented by the formula RCOOR and that are usually formed by the reaction between an acid and an alcohol with elimination of water;
3. unpalatable	c. any of various substances that are soluble in nonpolar organic solvents (as chloroform and ether), that are usually insoluble in water, that with proteins and carbohydrates constitute the principal structural components of living cells, and that include fats, waxes, phosphatides, cerebrosides, and related and derived compounds;
4. ester	d. the act or process of nourishing or being nourished; specifically the sum of the processes by which an animal or plant takes in and utilizes food substances;
5. lipid	e. having a rank smell or taste usually from chemical change or decomposition;
6. nutrition	f.distasteful, disagreeable, unpleasant.

Ex.3. Say whether the following statements are true or false:

1. The percentage by weight of iodine absorbed by the fat in the natural state is known as the iodine number. 2. The fatty acids without carbon-carbon double bonds are classified as unsaturated, and those containing carbon-carbon double bonds are classified as saturated. 3. Unsaturated fatty acids can be made to combine with other substances instead of with oxygen. 4. Lipids are cellular compounds that are soluble in water. 5. The oleic, linolic, and linolenic acids are liquids at ordinary temperatures. 6. Fats and oils are practically always mixtures of triglycerids in varying proportions.

Ex.4. Insert the necessary word:

Unsaturated Fatty Acids

1. There are about forty naturally … fatty acids. 2. The fatty acids without carbon-carbon double bonds are classified as saturated, and those containing carbon-carbon double bonds are classified as …. 3. Palmitic and stearic acids are the most common saturated fatty acids, and oleic and linoleic acids are the most common unsaturated fatty acids. 4. Oleic acid is monounsaturated because it has only one carbon-carbon double bond. 5. Linoleic, linolenic, and arachidonic acids are polyunsaturated because they have two, three, and four carbon-carbon double bonds, respectively. 6. A way to … the relative degree of unsaturation of a fat or oil is to … its iodine number. 7. The iodine number is the mass of iodine, in grams, that is consumed by (reacts with) 100 grams of a fat or oil. 8. Iodine …with the carbon-carbon double bonds. 9. Thus the greater the number of double bonds, the higher the iodine number. 10. In general, fats have lower iodine numbers than oils because oils have greater … of carbon-carbon bonds that are double bonds. 11. For example, … iodine numbers for butter are 25 to 40, and for corn oil, 115 to 130. В(typical, unsaturated, determine, measure, react, percentage, occurring)