Fats – saturated and unsaturated fatty acids
Fats refers to the class of nutrients known as lipids. All fats are esters (links) of fatty acids and glycerol. The lipid family includes triglycerides (fats and oils), phospholipids (lecithin) and sterols (cholesterol). Triglycerides (glycerol and three fatty acids) are the most abundant in foods and in the body. Chemically lipids are composed of five elements: carbon, hydrogen, oxygen, nitrogen and phosphorus.
Lipids are characterized by the presence of fatty acids (and derivatives) and solubility in fat solvents (alcohol, acetone, chloroform ether). Fatty acids are derived lipids, since they are constituents of phospholipids, sphingolipids, glycolipids, waxes and steroids ( cyclic alcohol, contain no fatty acids). Lipids are essential structural components of the cell membrane, are stored in fat depots and serve as major energy sources for man and animals.
Fatty acids may be 4-24 (even numbers) carbons long: the 18- carbon ones are the most common in food. Fatty acids in nature are straight-chain organic acids that may be saturated or unsaturated.
Unsaturated fatty acids have one or more points of unsaturation – they may be monounsaturated OR polyunsaturated. The polyunsaturated fatty acids are known as omega-3 and omega-6 fatty acids: A polyunsaturated fatty acid with its closest double bond three carbons away from the methyl end is an omega-3 fatty acid. A polyunsaturated fatty acid with its closest double bond six carbons away from the methyl end is an omega-6 fatty acid. Monounsaturated fatty acids belong to the omega-9 group, with their closest and only double bond, nine carbons away from the methyl end. The 18-carbon polyunsaturated fatty acids are linolenic acid (omega-3) and linoleic acid (omega-6) – both are essential fatty acid that the body can’t make.
All fatty acids have the same basic structure – a chain of carbon and hydrogen atoms with an acid group (COOH) at one end and a methyl group (CH3) at the other end.
Fatty acids differ in the length of their carbon chains and in the number and position of double bonds in the molecule. The long chain (12-24 carbons) fatty acids of meats, seafood, and vegetable oils are most common in the diet. Medium chain (6-10 carbons) and short chain (fewer than 6 carbons) fatty acids are primarily in dairy products.
Saturated fatty acids are devoid of double bonds, have higher melting points and are solid (fats) at room temperature. e.g. lauric acid (coconut), myristic acid (nutmeg), palmitic acid (palm), stearic acid (animal fat) and butyric acid (butterfat).
Unsaturated fatty acids are those with one or more double bonds in the carbon chain, have lower melting points and are liquid (oils) at room temperature:
monounsaturated fatty acids (one double bond)
- palmitoleic acid (butter)
- oleic acid fats (olives, peanuts, canola) – or omega 9 fats.
polyunsaturated fatty acids (two or more double bonds)
- linoleic acid (soybean, sunflower, safflower and corn oils) – or omega-6 fats
- linolenic acid (flaxseed, walnut, canola and soybean) – or omega-3 fats
- arachidonic acid (meat, eggs, fish)
- eicosapentaenoic acid (seafood)
- docosahexaenoic acid (seafood).
Linolenic acid (omega-3) and linoleic acid (omega-6 fatty acids) must be obtained from the diet, therefore they are essential fatty acids. The body cannot produce unsaturated fatty acids of the linoleic series, unless a dietary precursor is furnished. If linoleic acid is supplied in the diet, the tissues can synthesize arachidonic acid from it. Arachidonic acid and linoleic acids are essential for maintenance of a normal skin structure in the young. Breast milk contains 6-9% of calories as linoleic acid. The desirable intake of linoleic acid for a growing infant is 3-4%.
Palm oil and coconut oil are saturated, but firmer than most vegetable oils, because of their saturation, but softer than most animal fats, because of their shorter carbon chains (8-14 carbons long). Generally, the shorter the carbon chain, the softer the fat is at room temperature.
The degree of unsaturation also influences stability. All fats become spoiled when exposed to oxygen. The oxidation of fats produces a variety of compounds that smell and taste rancid. Polyunsaturated fats spoil most readily, because their double bonds are unstable: monounsaturated fats are less susceptible. Saturated fats are most resistant to oxidation and least likely to become rancid.
Manufacturers protect fat-containing products against rancidity in three ways – none of which are perfect:
- Products are sealed in air – tight, non-metallic containers, protected from light and refrigerated;
- adding antioxidants (BHA, BHT and vitamin E) to protect the oil
- hydrogenation – unsaturated fatty acids are artificially modified by the saturation of the double bonds with additional hydrogen atoms (hydrogenation), which converts them into single bonds. Saturation of the unsaturated fatty acids by hydrogenation will convert a liquid oil, into a hard fat (margarine, white vegetable shortenings). Hydrogenation offers two advantages: firstly, it protects the foods against oxidation (prolonging shelf life); secondly, it alters and improves the texture of foods, making margarine more spreadable.
The sources of food fat are: butter, milk, lard, cheese, eggs, meat, fish, nuts and vegetable oils (sesame, olive, soy, corn, peanut and cottonseed oil).