Liz Drew of Waitaki Girl's High School, asks :-
Why does a piece of coal in a pot of peeled potatoes stop them from going brown?
Peter Meredith, of Ilam Potato Sciences, Christchurch, responded.
Peeled potatoes slowly brown because of chemicals called phenols. These are naturally present in the cells of the tubers as a defence against pests and diseases.
The phenols react with oxygen from the air and are oxidised by two different reactions. One uses an enzyme also present in the tubers and the other is by catalysis due to iron in the water. The oxydised phenols join together to form large molecule polymers that are coloured brown-purple-black.
We can only guess how coal stops these reactions, maybe by absorbing the phenols, by reducing the amount of oxygen in the water, by preventing oxydation or by preventing the polymerisation.
Peeled potatoes don't go brown-black anywhere near as often as they used to, partly because growers have fertilizers with a more complete range of trace elements, partly because we no longer peel potatoes hours before cooking them, partly because we have cleaner water supplies and partly because we have learned to acidify the water with vinegar or lemon juice to prevent browning. So we dont use lumps of coal nowadays, it's something largely in grandma's memory.
I dont think anyone has investigated the coal effect, nor are they likely to for who these days will finance the work? Like you, I would still like to know the answer.