Cooking For Engineers

[timgower]

Sorry if this is a repost, but I'm new here. My question: If polyunsaturated fat is liquid at room temperature (such as vegetable oil), why is fish fat solid?

[Guest]

[watt]

guest, I think you mean all animal FATS will be a mixture, in fact, so will vegetable oils.

To answer the poster, fats/oils are made up from a backbone of glycerol (3 carbon atoms, 3 OH groups, rest hydrogen). Onto each original OH group is chemically attached a fatty acid (FA), which comprises a carboxylic acid group (originally) and a long chain of carbon atoms, surrounded by hydrogen atoms, except when the FA is unsaturated when two adjacent carbon atoms are linked with a double bond, with the reduction of 2 hydrogens. So in one fat/oil molecule (usually refered to as a tri glyceride) there are three FA moieties, these can vary in the number of carbon atoms, from about 6 up to 24 is usual. The longer the FA chain, the higher the melting point (and the double bonds have different melting points to single, saturated FA). It is common to have the chain lengths different in each molecule, but we usually quote a total, and therefore average figure.
Each fat/oil has a well characterised FA profile, fats have longer chain lengths and oils shorter, though the degree of saturation (double bonds) will complicate matters a little. So it is the total of all the FA which determines whether the fat is solid and the oil liquid, not the degree of saturation, though, in general, everything else being equal, saturated fats will have a higher melting point (see margarine).
HTH
watt