I work in a restaurant where the chef saves time by parboiling potatoes and leaving them in the water overnight, then roasting them the following day. To stop the potatoes discolouring overnight, he puts a slice of brown bread in the water. Is this an old wives鈥� tale or does this really work? And does it have to be brown bread?
鈥� When you slice a potato, an enzyme called polyphenol oxidase is released and catalyses a chemical reaction between polyphenols in the potato and oxygen from the air. This produces brown compounds in the potato called melanins, but can be prevented in several ways.
The oxidation of the polyphenols can be slowed by keeping oxygen away from potatoes by storing them under water. The water鈥檚 pH can also be made acidic by adding lemon juice or vinegar. As enzymes are pH-sensitive, the polyphenol oxidase loses its catalytic ability, further preventing browning.
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The addition of antioxidants to the water scavenges dissolved oxygen, stopping discolouration. Bread has an antioxidant capacity. That of wholemeal bread is greater than white bread, and fresher bread is better than stale.
鈥淲hen you slice a potato, polyphenol oxidase is released, which produces brown compounds鈥�
Old wives鈥� tales are not to be lightly dismissed 鈥� they are often based on years of trial and error.
Enzymatic browning is not all bad: it develops colour and flavour in teas and, without it, chocolate would be bland.
David Muir, Edinburgh, UK
鈥� My mother used a similar strategy, but without the bread. She would parboil potatoes, place them in a bowl of water, go to church, then roast them for Sunday lunch on her return.
The water is likely to be the key to stopping the browning through preventing oxidation. It is possible that phenolic compounds, responsible for the browning, may bind to the fibre in the brown bread, but we know little about the chemistry of the interaction of phenolics and fibre.
Separating the potatoes into two bowls of water, one also containing brown bread, is the obvious test. I take no responsibility for you getting the sack if the water-only condition fails.
David Cox, Adelaide, South Australia
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