Fatty acids: classification
Fatty acids are carbon chains with a COOH group at one end, grouped mainly by chain length and by number of double bonds (SFA, MUFA, PUFA).
Ways to classify fatty acids
| Basis | Groups | Example |
|---|---|---|
| Chain length | Short (up to 6 C), medium (8–14 C), long (16 C and above). Cut-offs vary a little between books. | Butyric (ghee), lauric (coconut oil), palmitic |
| Double bonds (saturation) | Saturated (SFA, none), monounsaturated (MUFA, one), polyunsaturated (PUFA, two or more) | Stearic; oleic; linoleic |
| Omega family (first double bond from the CH3 end) | ω-3, ω-6, ω-9 | Alpha-linolenic; linoleic; oleic |
| Shape at the double bond | Cis (bent chain), trans (straight chain) | Natural oils (cis); vanaspati (trans) |
| Number of carbons | Even (most natural fatty acids), odd (rare) | Palmitic (16 C); propionic (3 C) |
| Need in the diet | Essential, non-essential | Linoleic, alpha-linolenic; palmitic, oleic |
Naming and numbering
Carbon 1 is the carboxyl (COOH) carbon. Carbon 2 is the alpha (α) carbon and carbon 3 is the beta (β) carbon. The shorthand 18:2 means 18 carbons and 2 double bonds. The delta (Δ) number gives the position of a double bond counted from the COOH end. The omega (ω or n) number counts from the other end, the methyl (CH3) end. So linoleic acid is 18:2 Δ9,12, and also ω-6.
Common fatty acids and Indian food sources
| Fatty acid | Shorthand | Type | Indian sources |
|---|---|---|---|
| Lauric | 12:0 | SFA, medium chain | Coconut oil (its main fatty acid) |
| Palmitic | 16:0 | SFA | Palm oil (palmolein), ghee, meat fat |
| Stearic | 18:0 | SFA | Ghee, mutton fat, cocoa butter |
| Oleic | 18:1, ω-9 | MUFA | Groundnut oil, gingelly (sesame) oil, olive oil |
| Linoleic | 18:2, ω-6 | PUFA | Sunflower, safflower, soybean, groundnut oils |
| Alpha-linolenic (ALA) | 18:3, ω-3 | PUFA | Flaxseed (alsi), mustard oil, soybean, walnuts |
| Arachidonic | 20:4, ω-6 | PUFA | Meat, egg yolk, liver; also made from linoleic |
| EPA (eicosapentaenoic) | 20:5, ω-3 | PUFA | Fish such as sardine and mackerel, fish oil |
| DHA (docosahexaenoic) | 22:6, ω-3 | PUFA | Fish, fish oil, breast milk |
Cis vs trans fatty acids
Hydrogenation means adding hydrogen to the double bonds of an oil to make it solid.
Use a mix of oils rather than one: groundnut or gingelly oil for MUFA, mustard or soybean oil for some ω-3. Keep ghee, coconut oil and palm oil (mostly saturated) moderate, and avoid vanaspati, the main source of trans fats.
5 marks: “Classify fatty acids with suitable examples.” Classify by chain length, saturation, omega family, cis/trans and even/odd carbons; give one example and one food source for each.
2 marks: “What are MUFA and PUFA? Give one example each.” MUFA have one double bond (oleic acid, groundnut oil). PUFA have two or more (linoleic acid, sunflower oil).
2 marks: “What are trans fatty acids?” Unsaturated fatty acids with hydrogens on opposite sides of the double bond, formed in partly hydrogenated oils like vanaspati; they raise LDL.
To find the omega number, subtract the last Δ position from the chain length: linoleic 18 − 12 = ω-6; alpha-linolenic 18 − 15 = ω-3; oleic 18 − 9 = ω-9.