What is enzymic browning?
What is enzymic browning?
Enzymic browning is an oxidation reaction that takes place in some foods, mostly fruit and vegetables, causing the food to turn brown.
Oxidation reactions occur in food and non-food items. Enzymic browning is a reaction which requires the action of enzymes and oxidation in order to occur.
What happens during enzymic browning?
Oxygen in the air can cause sliced fruit to brown, a process called enzymic browning (an oxidation reaction). Phenols and the enzyme phenolase are found in the cells of the apple, and when these are exposed to oxygen in the air, for example through slicing, the oxygen causes a reaction. The phenolase changes the phenols into melanin, which has a brown colour. To stop the oxidative reaction, the phenolase enzymes need to be denatured. This could be done by using heat and acids.
You may have heard of melanin before. Melanin is the pigment that gives human hair, skin and eyes their colour.
Why does enzymic browning happen?
Foods are made up of lots of different molecules including some called enzymes. Enzymes are special proteins which can speed up chemical reactions and act as biological catalysts. They can cause fruit to ripen and over-ripen, which gives the fruit a brown colour.
Fresh fruit and vegetables normally keep enzymes trapped in their tissues. However when the fruit is sliced, or squashed, or when the fruit or vegetable begins to break down with age, the enzymes come into contact with oxygen in the air. This causes the fruit to turn brown.
Enzymic browning causes a lot of food waste but it can also be useful: we would not have tea or chocolate without it!
How can enzymic browning be slowed down?
The browning can be slowed down by preventing the enzyme from working properly. Lemon juice contains an acid which can stop enzymes working properly as enzymes often work best at a certain pH. Water and sugar, in jam for example, stops oxygen in the air getting to the enzymes and prevents the browning.
Effect of oxidation on nutrient content
As well as causing the fruit to change colour, oxidation can also affect the nutrient content of a fruit or vegetable. Vitamin C, found in some fruits and vegetable can be oxidised when it is exposed to air. The longer a fruit is exposed the less vitamins it will have.
- Get some fresh fruit or vegetables – lettuce, apple, or potato
- Have three containers ready, one containing water, one containing sugar and water (say 5g sugar, 50ml water), and one containing a little lemon juice.
- Cut up each fruit or vegetable into pieces about 5cm long.
- Leave one sample of each fruit or vegetable on a plate and quickly place one sample into the water, one into the sugar solution, and one in the lemon juice.
- Leave the experiment running for about 1 hour.
You should see browning in the samples left on the plate. Compare this with the other containers. What conditions prevented browning most? Think of food preparation and cooking where similar conditions are used. Why do chefs often tear rather than cut up lettuce leaves?