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Why Do Cut Apples Turn Brown

Divya Bichu May 6, 2019
You must have noticed that apples turn brown when sliced or bitten into. Read on to know what causes this to happen and also about the different ways that can prevent them from turning brown.
Eating fresh apples everyday is always good for health, and the nutritional facts associated with it are amazing. You must have come across the proverb 'An apple a day keeps the doctor away'. This proverb urges people to include apples in their daily diet, since it is a very nutritive fruit.
It contains dietary fibers, vitamins A and C, potassium, pectin, water, sugar, and antioxidants in plenty. This makes it the most ideal fruit to be consumed daily.
However, apples must be consumed right away after cutting them into pieces or slices; you can't really store them. This delicious red, crunchy fruit does not taste the same if you cut and store it.
It turns brown when exposed to the air for some time. Even though apples that have turned brown are safe for consumption, one does not enjoy its taste like the original one. Find out why does this happen in the next section.

Why Do Apples Turn Brown After Being Cut

Apple is a pomaceous fruit that has seed chambers inside and the outer part is fleshy and juicy. Apart from vitamins A and C, potassium, fiber, water, and several other nutrients, apples contain an enzyme called 'polyphenol oxidase (PPO) or tryosinase'. These enzymes in particular are responsible for the brown coloration of the apple.
Apples turn brown upon being cut due to the chemical process of oxidation. This oxidation process is driven by the PPO enzyme present in them.
Now, when apples are sliced or bitten into or bruised, the enzymes present in it are exposed to the air, where they react with the oxygen present in it. This oxidizes the phenolic compounds in the apple tissues to o-quinones.
O-quinones then produce brown-colored secondary products that discolor the apple and turn it to brown.

► Thus, this process that converts the naturally present phenolic compounds to quinone compounds gives the apple its brown color upon being cut.

Preventing Apples from Turning Brown

The level of PPO activity and the concentration of substrate varies from apple to apple, and depends upon its maturity and growing conditions. Sometimes, it is necessary to cut and store apples if they are being used for garnishing or presented to guests. Further mentioned a few ways that can reduce or prevent the browning effect.
An approach which the food industry follows to prevent the enzymatic brown coloration of the apple is to choose those varieties that have a low PPO activity and substrate concentration. This one is a very commercial option.
The easiest way to prevent the apples from turning brown is to brush the cut pieces with lemon or pineapple juice. Both these juices being acidic in nature (lower pH) cause the PPO activity to slow down, thus preventing the process of oxidation.
Sprinkling powered sugar or brushing the exposed part with sugar syrup or salt water prevents the browning effect. This method reduces the oxygen diffusion and slows down the oxidation process.
Dip the apple slices in ascorbic acid or lime-lemon soda, as both of these act as an anti-browning solution. Wrapping in plastic or heating them in boiling water (blanching) also prevents the browning process.
The best way to prevent the browning reaction is to place the apple slices in apple juice. The acid in the juice prevents the oxidation process and retains the taste of the apple. Now, that you are aware of how to keep your apples from turning brown, go ahead and savor this wonder fruit any which way you like!