Saturated, Unsaturated and Trans Fats

The distinction between saturated and unsaturated fats comes down to molecular shape and how closely the fat molecules can pack together. Animal fats are mostly built from saturated fatty-acid chains, while vegetable oils typically contain unsaturated fatty-acid chains as well. A saturated fatty-acid chain is a long, straight, all-single-bonded run of tetrahedral carbon atoms; long chains like this can pack closely side by side against neighbouring molecules, and the van der Waals attraction between such closely packed chains is strong enough to hold the fat as a solid at room temperature -- this is shown schematically in Fig. 16.1(a). An unsaturated fatty-acid chain, by contrast, contains one or more C=C double bonds, and each such double bond puts a 'kink' or bend into the chain (Fig. 16.1(b)); these kinks prevent unsaturated chains from packing as closely together as saturated ones can, which weakens the van der Waals attraction between molecules and lowers the fat's melting point. Because natural fats are always mixtures of many different triglycerides rather than one pure compound, they do not show one sharp melting point but instead melt gradually over a range of temperatures -- and, as a rule, the more unsaturated a fat's fatty-acid mixture is, the lower its melting point and the less crystalline (more liquid-like) it is, exactly as Table 16.1 illustrates across coconut fat, olive oil and sunflower oil. A C=C double bond can additionally exist as either of two geometrical isomers: in the cis form, the two hydrogen atoms on the double-bonded carbons sit on the same side of the double bond; in the trans form, they sit on opposite sides. The cis form is by far the more common form found in natural unsaturated fats, whereas the trans form occurs only in animal fats and in processed (industrially hydrogenated) unsaturated fats. This geometry has real health consequences. Fats are transported around the body as lipoproteins, which also carry cholesterol; when low-density lipoprotein (LDL) is present in excess, cholesterol tends to be deposited in blood vessels, raising the risk of cardiovascular disease. There is evidence that eating l …