Why do sliced potato and apple turn dark in open air?

Imagine preparing a nice lunch for the day with apple slices and packing it up in your lunchbox, only to find out that by the time you were supposed to eat the apples, they’ve started to look all dark and brown and you end up deciding that you don’t have an appetite. As a matter of fact, this “browning” of apples and potatoes is not only limited to the two. It is found that almost all fruits and vegetables suffer from this effect if left in contact with open air for too long.

Just like any other thing, fruits are made up of a ton of different sorts of chemicals. The darkening of food is a phenomenon that actually occurs because of a series of biochemical reactions that take place after the food has been sliced. The process is called “enzymatic browning.” What exactly is enzymatic browning? It’s a kind of reaction that happens due to the presence of oxygen gas in the air around us. It’s called an oxidation reaction.

Oxidation reactions are very widespread. One very common example is the rusting of iron. Basically, whenever any object is left in the open air, that is, outside of air-tight bags or compartments, it is constantly exposed to the different gases present in the air. Among those, about 20% is oxygen. The molecules of oxygen have a tendency to react in a special way. In the case of eatables, these oxidation reactions tend to be harmful to the food in the long-run.

Enzymatic browning is the reaction that, along with the presence of oxygen, also needs that enzymes take an active part in it. Enzymes are a sort of special proteins that have one main purpose; to support and increase the rate of reactions. They are also called catalysts as they help accelerate the chemical changes caused by a reaction.

In the cells of an apple, two particular types of chemicals are found that cause the browning of the apple. These are phenols (a type of organic compounds) and an enzyme called phenolase. When an apple is sliced or cut open, it is exposed to open air. This causes the beginning of an oxidation reaction. The enzyme phenolase promotes the reaction and causes the phenols to turn into melanin. You may have come across the term “melanin” before. Melanin is a pigment (the substance that is responsible for reflecting off different colors). Melanin, for example, allows our hair, eyes and skin to have a unique coloration.

Hence, if a fruit is not sliced or cut open, it prevents the enzymes from turning the phenols into melanin. Fresh fruit and vegetables can force the enzyme to stay trapped within their tissues. When the food is cut, however, it comes in contact with oxygen, which causes the browning to occur.

Similarly, in the case of potatoes, two substances known as tyrosine and tanninpresent inside the potatoes, are responsible for the dark pigmentation. Oxygen in the air reacts with tyrosine to turn it into a black-colored pigment. Simultaneously, oxygen transforms tannin into polyphenol, within 10-15 minutes of contact, which is dark brown in color.

Although the dark coloring doesn’t primarily affect the taste or properties of the apples or potatoes on a short-term basis, it can still be undesirable. To reduce the darkening, try to store the food in refrigerators or coat them with lemon, pineapple juice or honey. This can effectively block the constant exposure to air and help preserve the food longer.

For your information, in humans, the amount of melanin present in the skin corresponds to how much dark our skin is. There is an abundant amount of melanin in the skin of Negro people, which is why they appear darker than others. Similarly, the lesser the amount of melanin in the skin, the whiter is the complexion. In Asian countries like India and Sri Lanka, the skin of most people contains about an average amount of melanin, which gives off the appearance of having a brownish skin.

The skin of people native to Europe has very little melanin, giving them a much brighter shade of skin. Some people suffer from a rare condition known as albinism, due to which their skin or hair appears unnaturally pale. This is because their body is unable to produce almost any amounts of melanin.