Bernard Wilkinson, of Oamaru, asks :-

A brown build-up has occurred over a period of use on the sole plate of my iron. What is this? Is it the same as the brown stains on my pots and pans?

Richard Hartshorn and Juliet Gerrard, a chemist and a biochemist at the University of Canterbury, responded.

If your iron is rather old, it will simply be a shaped lump of iron and the brown stain would most likely be rust. However, the vast majority of modern irons have a Teflon (or similar) coating on the metal plate of the iron, and, indeed, different metals may be used for that sole plate as well. Your iron could well not be made of iron at all!

The coating is the same sort of “non-stick” coating that is used on frying pans and other cookware, and it both protects the metal against corrosion and makes a “slippery” surface that dirt, dust and other things find it difficult to stick to. Difficult, but not impossible, as this is what leads to the brown stain. An initial small piece of some material may get caught on the hot surface of the iron and then be cooked or decomposed by the heat. More dust or dirt sticks on or near the decomposed (and possibly sticky) material and the brown stain gets larger.

So yes, the brown stain is similar, but not the same as what you get on your pots and pans. Cooking involves a huge variety of chemical reactions in which beneficial flavours and aromas are produced, and food components are made more digestible. Unfortunately there are sometimes also side reactions in which nutritional quality is lost and toxic compounds may be produced. In such cases, the food is eventually rendered unpalatable.

When a pot boils over, the temperatures to which the food involved is subjected are far higher than required for beneficial cooking effects, and a series of uncontrolled reactions takes place, in which the food eventually burns. This process is too complex to have been properly understood, although some research has been carried out on what occurs.

Put simply, a series of dehydration, condensation and oxidation reactions occur, eventually producing brown and charred residues. These organic residues are carbon based, and tend to be tar-like, eventually turning to charcoal (and hence smoking if reheated later). Once again a brown mess is produced, but much more quickly than the kind of build-up that you have seen on your iron.

If you want to clean your iron or the non-stick pans, the most important thing is not to scrub or scratch so hard that you damage the coating. Any defects in the coating will lead to much faster and worse build-ups of the brown stain. Gentle scrubbing with soft cloths, and perhaps using warm vinegar rather than water, seems to be recommended, but others may have better suggestions.