Everybody has seen an apple turn brown after the peel has been broken. I have always been told that this was due to a reaction with the oxygen in the air. If this is true, what is the actual chemical reaction that is taking place. What is the oxygen reacting with?
It’s not a reaction with the air as such. You’ll notice the same reaction ocurs if the apple is dropped, producing ‘brusing’ without the skin being broken.
The potted version is that the apple contains an amino acid called tyrosine. When a cell is damaged it releases an enzyme that converts tyrosine into melanin. Melanin is of course a dark pigment, the same one we have in our skin. It’s also a fairly potent antibiotic, hence the reason why fruit produces it in response to cell damage.
I just realised that I simplified that to the point of incorrectness.
It’s not just tyrosine that gets transformed to melanin, it’s all sorts of phenols, tyrosine is just the most common. Also they don’t all get transformed into true melanin, but they do become the same class of compounds as melanin, with the same colour.
Perhaps most importantly oxygen can play a role and can directly oxidise phenols, but the reaction can also occur without direct exposure to oxygen provided their is sufficient oxygen for the enzymes to work.
great thanks!