What happens chemically when an apple browns?

The browning of an apple, known as enzymatic browning, is a chemical reaction that occurs when the flesh of the fruit is exposed to oxygen. This reaction is catalysed by an enzyme called polyphenol oxidase (PPO), which is naturally present in apples.

When the apple is cut or bruised, the cells are damaged, releasing PPO and other compounds that react with oxygen to produce melanin, a dark pigment responsible for the brown colouration.

The overall chemical reaction can be summarised as follows:

Substrate + Oxygen → Melanin + Water

The rate at which an apple browns depends on several factors, including the variety of apple, the temperature, and the amount of oxygen present. Some apple varieties, such as Granny Smith apples, have higher levels of PPO and tend to brown more quickly than others. Higher temperatures also accelerate the browning process, as they increase the activity of PPO.

To prevent or slow down the browning of apples, various methods can be employed, such as:

Storing apples in a cool environment: Refrigeration slows down the enzyme activity, helping to preserve the freshness and colour of the fruit.

Using anti-browning agents: Some substances, such as lemon juice or ascorbic acid (vitamin C), can inhibit the action of PPO and prevent browning. These can be applied to the cut surfaces of apples to maintain their colour.

Minimising exposure to oxygen: Reducing contact with oxygen can help slow down the browning process. This can be achieved by tightly wrapping cut apples in plastic wrap or storing them in airtight containers.

By understanding the chemistry behind apple browning and using appropriate storage and handling techniques, it is possible to maintain the freshness and visual appeal of apples for a longer period.