Lecture 6 Lipids 2024-2025 PDF

Lecture 6 Lipids Lipids are organic compounds which are esters of fatty acids or associated with these esters in nature. Lipids are classified into three groups: 1- Simple lipids: esters of fatty acids with alcohols 2- Conjugated (compound) l...

Lecture 6 Lipids Lipids are organic compounds which are esters of fatty acids or associated with these esters in nature. Lipids are classified into three groups: 1- Simple lipids: esters of fatty acids with alcohols 2- Conjugated (compound) lipids: esters of fatty acids and alcohol in addition to other non- lipid group 3- Derived lipids: Compounds produced by hydrolysis of simple or conjugated lipids or present associated with lipids in nature. Fatty Acids (R – COOH) Fatty acids are monocarboxylic organic acids, mostly contain even number of carbon atoms.. The carbon skeleton of any fatty acid is numbered either from the carboxylic group or from the terminal methyl group (omega carbon) Fatty acids are classified into 2 main groups: 1- Saturated fatty acids (SFA) with no double bonds in the structure 2- Unsaturated fatty acids (USFA) with one or more double bonds in the structure. Saturated fatty acids (SFA) Saturated fatty acids include the following: 1- Short Chain Fatty acids (up to 10 carbons): Examples Butyric acid……………… C4 Caproic acid……………… C6 2- Long Chain Fatty Acids (more than 10 carbons): 1  Lecture 6 Examples: Palmetic acid…………….. C16 Stearic acid………………..C18 Lignoseric acid…………… C24 Unstaturated Fatty Acids (USFA) Most of the double bonds of USFA are of the Cis type which allow bending and lower their melting temperature i.e. liquid at room temperature. USFA are classified into four families according to the position of the first double bond in relation to the omega (ω) carbon (methyl CH3), ω3, ω6 and ω9. According to the number of double bonds USFA are classified into two types: 1- Monoethenoid acids: They have one double bond. Examples: Oleic acid ……………. (18 :1) Nervonic acid ………… (24:1) 2- Polyethenoid acids (Polyunsaturated Fatty Acids (PUFA): They have more than one double bond (diethenoids, triethenoids, tetraethenoids….etc.). Examples: Linolenic acid………. (18:3) Linoleic acid……………. (18:2) 2  Lecture 6 Arachidonic acid………… (20:4) Nutritional classification of fatty acids Fatty acids are classified into: 1- Essential Fatty Acids: They are not formed in the human body and must be taken in diet. Examples; linolenic and linoleic and arachidonic acids. 2- Non –essential Fatty Acids; They are formed inside the human body. Chemical Properties of Fatty Acids: 1- Ester Formation: Glycerol +3 Fatty acids Triacylglycerol 2- Salt Formation: Fatty acids react with alkalis to form the corresponding salt (soups) 3- Hydrogenation (addition of hydrogen): Each double bond in an unsaturated fatty acid reacts with two hydrogen atoms to give the corresponding saturated fatty acids. Example: Oleic acid (18:1) + 2H Stearic acid (18:0) Linoleic acid (18:2) +2H Oleic acid (18;1) + 2H Stearic acid (18:0) Simple Lipids Simple lipids are esters of fatty acids with alcohols. According to the types of alcohol they are classified into neutral fat and wax. 3  Lecture 6 I- Neutral Fat: They are esters of three fatty acids with glycerol and also called Triacylglycerol or Triglycerides. II- Wax: They are esters of fatty acids with higher mono hydroxyl alcohol Neutral Fat (Triacylglycerol or Triglycerides) TAG Neutral fat are classified into two groups: Oils: They have a low melting point They are liquid at room temperature due to high content of USFA. Examples: corn oil and cotton seed oil. 4  Lecture 6 Solid Fat: They have high melting points. They are solid at room temperature due to high content of SFA: Examples: butter and margarine. Chemical Properties of Neutral Fat: 1- Hardening: It is the addition of hydrogen to convert oils to fats by converting USFA to SFA (hydrogenation) 2- Saponification: It is the hydrolysis of TAG in alkaline medium as NaOH or KOH Hydrolysis leads to release of free fatty acids which react with the alkali giving salts of fatty acids (soaps) 3- Rancidity: It is the development of bad odor and taste (flavor) of fats and oils due to the exposure to moisture, oxygen and high temperature. Two types and mechanisms of rancidity: a- Hydrolytic rancidity: due to moisture and bacteria which release lipase enzyme with hydrolysis of fats and release of volatile fatty acids which gives bad odor. b- Oxidative rancidity: due to formation of peroxides and aldehydes of USFA. Addition of antioxidants as vitamin E protects from this type of rancidity 5