Why Do Apples Turn Brown? Enzymatic Browning and How to Stop It

Slice an apple, leave it on the chopping board and within ten minutes the white flesh starts to turn tan and then brown. Bananas, avocados, potatoes and pears do the same. It looks like the fruit is going off, but what you are watching is a defensive chemical reaction that the fruit carries with it, ready to fire the moment it is damaged.

The ingredients are already inside

An apple cell keeps two key ingredients apart, like the two tubes of a glue that only works when mixed:

• phenols (polyphenols), natural compounds that also give apples part of their flavour and health benefits;
• an enzyme called polyphenol oxidase, or PPO.

In an intact cell the phenols sit mostly in the cell’s storage sac, the vacuole, while the enzyme is stored in other structures called plastids. As long as the cell is whole, they rarely meet.

Cutting starts the reaction

When you cut, bite or bruise an apple, the cell walls and membranes break and the contents mix. Oxygen from the air rushes in. Now the enzyme can do its job.

PPO uses oxygen to convert phenols into reactive molecules called quinones. Quinones are unstable. They react with each other and with proteins and amino acids in the apple, linking up into large, complex molecules that absorb light and look brown. These pigments are related to melanin, the pigment in our skin and hair.

Because this reaction is driven by an enzyme, scientists call it enzymatic browning. It is different from the browning of toast or roasted coffee, which comes from the Maillard reaction between sugars and amino acids at high temperatures.

Why would an apple do this?

Browning seems to be part of the plant’s defence system. When an insect bites or a fungus attacks, the quinones formed at the wound are toxic or unpleasant to many pests and microbes, and the brown layer helps to seal the damage, a little like a scab. For a fruit on a tree, that is useful. For a fruit salad, less so.

How to stop apples going brown

To slow browning, you need to block the enzyme, remove oxygen or both.

• Lemon juice. PPO works best at a mildly acidic to neutral pH. Lemon juice makes the surface more acidic, which slows the enzyme. It also contains vitamin C (ascorbic acid), an antioxidant that turns quinones back into colourless phenols before they can form pigments. Pineapple or orange juice work in a similar way.
• Cold. Like most enzymes, PPO slows down at low temperatures. Cut apples last longer in the fridge.
• Water dips. Submerging slices in cold water, lightly salted water or honey water limits oxygen at the surface. Honey contains natural compounds that inhibit the enzyme.
• Keeping air out. Wrapping cut fruit tightly or storing it in a sealed box reduces oxygen.
• Heat. Briefly blanching apples in boiling water destroys the enzyme, which is why apple purée and baked apples do not keep browning in the same way.

Non-browning apples

Scientists have also tackled browning at its source. Using a technique called gene silencing, a Canadian company developed apples in which the genes for PPO are largely switched off. Their flesh stays white for much longer after slicing. They were approved for sale in the United States in 2015. Similar approaches have been used in potatoes to reduce bruising and browning.

Is brown apple bad for you?

No. Browning changes the colour, flavour and texture slightly, and some vitamin C and other antioxidants are used up in the process, but the brown pigments are not harmful. Signs of real spoilage are different: mould, an alcoholic or vinegary smell, or a slimy surface.

In short

• Apples brown because the enzyme polyphenol oxidase (PPO) reacts with phenols and oxygen when cells are damaged.
• The reaction produces quinones, which link into brown, melanin-like pigments.
• It is likely a defence against insects and microbes.
• Lemon juice slows browning by making the surface acidic and supplying vitamin C.
• Cold, water dips and keeping air out also help; heat destroys the enzyme.
• Brown apple is safe to eat.