David Ickringill, of Green Island, asks :-

I was told table spread is only one molecule different to plastic. Is this true and how healthy is it compared to butter?

Robin Smith, an organic chemist at the University of Otago, responded.

Table spread is a general name given to combinations of fats and oils used to anoint breads and the like. The most common components are the familiar butter, obtained by processing cow’s milk, and margarine which is obtained from vegetable sources such as soya bean and olives. Irrespective of the animal or vegetable source the molecules that make up Table Spread have a common general chemical structure: (Fig. 1) that varies in the nature and length of the side chains (R1, R2, R3).The R groups are carbon chains containing from 12 to 18 carbon atoms joined together and often this bonding contains some reactive double bonds and in this case they are termed as unsaturated.

An overall picture of the chemical structure is shown in Figure 2 which represents one of the primary constituents of butter. This is a very regular structure so it packs together tightly and creates a solid form.

In contrast, a common constituent in olive oil (Fig. 3) has a much more irregular structure and is usually liquid. The relative health effects are best discussed with a nutritionist but the essential difference in biological activity and effect is related to these relatively small differences in the structures.

The average mass of these molecules allowing for the different weights of the carbon, hydrogen and oxygen atoms is around 850. This is medium size but is very small compared to the typical plastic which has a mass of several thousand. Plastics and table spreads are very different molecules in size and shape and the statement that they are related is not correct in any way.