Why Do Apples Turn Brown? The Enzyme Behind Cut Fruit and How to Stop It

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.

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.

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

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.

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.