Lipid Chemistry 1 Dr. Marwa PDF

 Learning objectives Identify definition & biomedical importance of lipids. Describe general classification of lipids. Recognize fatty acids regarding their different classifications. Classify simple lipids into two main subgroups. Describe structure & properties of triacylglycerol. I...

 Learning objectives Identify definition & biomedical importance of lipids. Describe general classification of lipids. Recognize fatty acids regarding their different classifications. Classify simple lipids into two main subgroups. Describe structure & properties of triacylglycerol. Identify rancidity focusing on its definition, causes, types, hazards & ways for prevention. properties of Simple lipids Physical properties of Simple lipids (Neutral Fats = Triglycerides) Solid at ordinary tempreture and are called fats. Liquid at ordinary tempreture and are called oils. Soluble in organic solvents like chloroform, ether and benzene. Relatively insoluble in water ; Fatty acids are carboxylic acids with long hydrocarbon chains. The non-polar hydrocarbon chain is an important counter balance to the polar acid functional group. In short chain fatty acids , the acid functional group dominates and gives the whole molecule a polar character. However, in long chain fatty acids, the non-polar hydrocarbon chain gives the molecule a non- polar character.  Melting point The melting points for the saturated fatty acids follow the Melting point principle: as the molecular weight increases, the melting point increases. Melting Points of Saturated vs. Unsaturated Fatty Acids; Unsaturated fatty acids have lower melting points than Saturated fatty acids. The reason is molecular geometries. Molecular structure of saturated fatty acids allows close intermolecular interactions result in high melting points. Introduction of one or more double bonds in the unsaturated fatty acids results in one or more "bends" in the molecule and the intermolecular interactions are much weaker General Principles: Vegetable oils contain more unsaturated fatty acids. (low melting point – liquid) Animal fats contain more saturated fats. (High melting point – solid) The geometry of the double bond is almost always a cis configuration in natural fatty acids. Lipids- Classification, Simple lipids (Triacylglycerol), Properties of TAG Chemical properties of triacylglycerol (TAG) Hardening: Addition of hydrogen to the double bonds of USFAs in liquid oils to be converted to the corresponding SFAs (solid fats or margarine). Hence the name, hardening which is now widely used as a substitute to natural fats. Lipids- Classification, Simple lipids (Triacylglycerol), Properties of TAG Chemical properties of triacylglycerol (TAG) Hydrolysis of TAG (Acidic- enzymatic- alkaline) Acidic hydrolysis: TAG dil. HCL/ 3 H2O Glycerol + 3 FFA Enzymatic hydrolysis: TAG Lipase enzyme/ 3 H2O Glycerol + 3 FFA Alkaline hydrolysis (Saponification): TAG 3 KOH Glycerol + 3 RCOOK (K soap) Alkaline hydrolysis of TAG produces glycerol and salts of fatty acids (soaps). Soaps cause emulsification of oily material and this help easy washing of the fatty materials. Rancidity Definition: It is a change in the natural properties of the fat leading to the development of unpleasant odor or taste or abnormal color particularly on aging after exposure to atmospheric oxygen, light, moisture, bacterial contamination. Saturated fats resist rancidity more than unsaturated fats that have Types 1-Hydrolytic rancidity: It results from slight hydrolysis of the fat by lipase from bacterial contamination leading to the liberation of free fatty acids and glycerol at high temperature and moisture. O O CH2 O C R1 H2C OH R1 C OH O O Lipase R2 C O C H HO C H + R C OH 2 O O CH2 O C R3 3 H2O H2C OH R3 C OH Triacylglycerol Glycerol Free fatty acids (volatile, bad odor) 2-Oxidative Rancidity: It is oxidation of fat or oil catalyzed by exposure to oxygen, light and/or heat producing peroxide derivatives which on decomposition give substances, e.g., peroxides, aldehydes, ketones and dicarboxylic acids that are toxic and have bad odor. This occurs due to oxidative addition of oxygen at the unsaturated double bond of unsaturated fatty acid of oils. Hazards of Rancid Fats: The products of rancidity are toxic, i.e., causes food poisoning and cancer. Rancidity destroys the fat-soluble vitamins (vitamins A, D, K and E). Rancidity destroys the polyunsaturated essential fatty acids. Rancidity causes economical loss because rancid fat is inedible. Prevention of rancidity is achieved by: Avoidance of the causes (exposure to light, oxygen, moisture, high temperature and bacterial contamination). By keeping fats or oils in well-closed containers in cold, dark and dry place (i.e., good storage conditions). Addition of anti-oxidants to prevent peroxidation in fat (i.e., rancidity). The most common natural antioxidant is vitamin E (tocopherol) Hydrogenation Vegetable oils which contain more unsaturated fatty acids are liquids while saturated fatty acids are solids. Hydrogenation is the addition of pairs of hydrogen atoms to reduce or saturate double bond in hydrocarbons.Usually in the presence of a catalyst ,such as nickel. vegetable oils can be substituted for animal fats such as in butter, by making a product we know as margarine. ASDD ASDD