Why Do Apples Turn Brown? The Enzyme Behind Cut Fruit and How to Stop It
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An apple sliced for a lunchbox at eight o'clock is brown by ten, and a potato peeled and left on the board goes grey while the pan heats. The colour is not rot, and the fruit is still safe to eat; it is the product of a chemical reaction the apple runs against itself the moment its cells are broken, and it is the same reaction that turns a bruise on a banana black and a cup of tea from green leaves to brown.
The enzyme and the oxygen
Plant cells keep two things apart: an enzyme called polyphenol oxidase, stored in small structures inside the cell, and a family of compounds called phenols, held in the cell's central vacuole. As long as the cell is intact they never meet. Cutting, bruising or biting the fruit breaks the membranes, the enzyme and the phenols mix, and with oxygen from the air the enzyme converts the phenols into quinones, reactive molecules that link up with each other and with proteins into long brown polymers called melanins, close relatives of the pigment that colours human skin.
The reaction needs all three: enzyme, phenols and oxygen. Remove any one and the browning stops, which is the principle behind every kitchen trick for preventing it.
Why some fruits brown and others do not
Apples, pears, bananas, avocados, aubergines, potatoes and mushrooms brown quickly because they contain plenty of both the enzyme and the phenols. Citrus fruits, melons and pineapples hardly brown at all, because they have little polyphenol oxidase and, in the case of citrus, are so acidic that what enzyme there is cannot work. Some apple varieties brown far faster than others; Granny Smith and Fuji are relatively slow, Red Delicious and Golden Delicious fast, and a genetically modified apple sold since 2017 does not brown because the gene for the enzyme has been switched off.
In the plant, the reaction is a defence. The quinones are toxic to microbes and the brown polymers seal a wound, so a bruised apple is walling off the damage against infection. From the apple's point of view the browning is the point.
Stopping it
Every method in the kitchen removes one of the three ingredients or slows the enzyme down:
- •Acid: lemon juice or vinegar lowers the pH below the range in which the enzyme works, and the vitamin C in lemon juice also reacts with the quinones before they can polymerise
- •Water: submerging cut potatoes or apple slices keeps oxygen away, and a pinch of salt in the water slows the enzyme further
- •Cold: the fridge slows the reaction but does not stop it; freezing halts it but wrecks the texture on thawing
- •Heat: blanching in boiling water for a minute or two destroys the enzyme permanently, which is why cooked apples do not brown and why frozen vegetables are blanched before freezing
- •Sugar or honey: a syrup coating limits the oxygen reaching the surface, and honey contains a compound that inhibits the enzyme directly
- •Sulphites: the industrial method for dried apricots and pre-cut salads, now restricted because some people react to them
When browning is wanted
The same reaction is deliberately encouraged in several foods. Black tea is made by crushing green tea leaves and letting polyphenol oxidase work on them for a few hours before the leaves are dried; green tea is the same leaf with the enzyme killed by steaming or pan-firing first, which is why one is brown and the other green. Cocoa beans and coffee cherries are fermented and dried in ways that let the enzyme darken them and develop flavour, and raisins and prunes owe part of their colour to it. In each case the reaction is stopped at the right moment by heat.
It is not the same as the browning of toast, seared meat or roast potatoes. That is the Maillard reaction, between sugars and proteins at high temperature, which needs no enzyme and no oxygen and produces the flavours of cooking rather than the dull taste of an old apple slice.
The avocado stone myth and other kitchen claims
A popular tip says that leaving the stone in a bowl of guacamole stops it turning brown. It works only where the stone actually touches the surface, because it blocks oxygen there and nowhere else, and the rest of the bowl browns at the usual rate. Pressing cling film directly onto the surface does the same job over the whole dip, and a squeeze of lime juice adds acid that slows the enzyme. The chemistry predicts which tricks work: anything that keeps air away or lowers the pH helps, and an object merely sitting in the bowl does not.
Another common belief is that a browned apple has lost its goodness. The browning does use up some of the phenols near the cut surface, and a little vitamin C can be consumed as it reacts with the quinones, but the change affects a thin surface layer, and the fruit's fibre, sugars and most of its nutrients are untouched. The main losses are to appearance and a slightly flat taste. To see how fast the reaction runs, cut an apple in half, brush one half with lemon juice, leave the other, and compare them after an hour.
The same enzyme family in animals
Browning is not only a plant habit. Prawns and lobsters develop dark patches on their shells within hours of being caught, a change the seafood trade calls black spot. It is caused by phenoloxidase, an enzyme closely related to the one in apples, acting on compounds in the shell. The spots are harmless, but they make the catch look stale, so processors treat it with anti-browning agents much as cut fruit is treated. Insects use the same chemistry on purpose: a newly moulted insect hardens and darkens its soft outer skin with it, and wraps invading parasites in a dark capsule of melanin.
In your own skin the job is done by tyrosinase, another copper-containing enzyme of the same family, which starts the chain of reactions that turns the amino acid tyrosine into melanin. People with one of the main forms of albinism carry a faulty version of the gene for it. Siamese cats show the chemistry neatly: their version of tyrosinase works only below a certain temperature, so pigment forms on the cooler ears, face, paws and tail but not on the warm body. A Siamese kitten is born almost white because the womb is warm everywhere, and its points darken as it grows.
An apple engineered not to brown
Plant breeders have tackled browning at its source. The Arctic apple, developed by a Canadian company, was genetically engineered to produce much less of the browning enzyme by silencing the genes responsible. When cut, its flesh stays pale for a long time, which appeals to sellers of pre-sliced apples, where browning is a major problem.
It was approved for sale in the United States in 2015 and in Canada around the same time, becoming one of the first genetically engineered fruits aimed directly at consumers' convenience rather than at farmers. Supporters argue it reduces waste, since browned slices are often thrown away, while critics question whether the benefit justifies genetic modification. Similar results are now being pursued with gene editing in other crops.
The takeaway
Cut fruit browns because breaking its cells lets the enzyme polyphenol oxidase meet phenol compounds in the presence of oxygen, producing brown melanin pigments that in the living plant seal wounds against infection. Acid, water, cold, heat and sugar each remove one of the three ingredients, and the same reaction, controlled, is what makes tea black and cocoa brown.