Secret tanning sessions? The mysteries of the darkening of apples and bananas.

Why does chopped fruit go brown?

We’ve all been here before. Life is busy, you prepare yourself a snack and forget about it for a while. Normally this wouldn’t be a big deal, but in the case of some fruits you have them so nicely sliced up, you remember you have them waiting for you in the kitchen, go back to grab your tasty snack, and then you see your delicious treat has turned an unappealing shade of brown.

Now, though it may seem the fruit has decided to wander off and tan under the sun for a bit while you were gone (unlikely unless you’ve been somehow transported into the world of Veggie Tales), there is actually a much more scientific explanation for this phenomenon: enzymatic rust.

Now, if we think of apples then you know the colour of the outer part remains quite stable unless the apple is damaged, but the internal tissue changes rapidly once cut. Though apples come in many different colours such as the green of Granny Smith, the yellow of golden delicious, or the red of idar, to name a few, the internal colour of the flesh is always a bright, off-white colour.

Why does this discolouration occur and how can it be prevented?

In apples, there are substances known as polyphenols. These compounds are very important in healthy nutrition as they protect our body from oxidative stress and radicals, thus functioning as antioxidants. It has been shown that individuals who consume a range of antioxidants in their diet are less likely to develop cancer and also tend to have less severe cardiovascular issues. Antioxidants work by “sacrificing” themselves by binding when they come in contact with the reactive oxygen compounds in our bodies thus rendering these substances harmless.

When you slice into an apple, a similar reaction occurs as oxygen from the air reacts with the antioxidants in the apple flesh. This reaction is enabled by the polyphenol oxidase enzymes in the apple.

As a result of this reaction, brown polymers (also known as melanin) form. Though these structures are not the same as the melanin that causes pigmentation in human skin, it likewise leads to a darkening of the apple’s flesh. Several other types of fruit also undergo this process, which is generally seen as undesirable as it decreases the aesthetic value of the fruit. That being said, a similar process is responsible in the creation of black tea where here it is very important! Here it converts the colourless polyphenols from green tea into the red and brown phenols that are typical of black tea.

If you want to serve apple slices that will maintain their color, this rusting reaction needs to be stopped. This can be done in many ways, but all follow two basic routes: preventing access to oxygen or stopping the enzymes. Oxygen access to the apples, for example, can be prevented by immersing the slices in water. A frequently-used method involves adding another substance to the slices that will react more quickly with the oxygen than the polyphenols. Here, we are talking about vitamin C (ascorbic acid), for example in lemon or orange juice. This is why such juices are used in fruit salads to keep the apple pieces from changing color. Not only does vitamin C stop the oxidation of polyphenols in apples, but it can also reverse it, thus rejuvenating already browning apple pieces.

If you have a bit more technology at your disposal, you can also keep the slices in a nitrogen environment from which all the oxygen has been removed. Under normal pressure, nitrogen is a harmless gas, and one that actually makes up 78% of the air around us (while oxygen is only 21% and other gases a mere 1%). Apples cut in containers of pure nitrogen will not brown as there is no oxygen to react with their polyphenols.

Besides these methods, there are also many ways to stop the actions of the polyphenol oxidase enzymes that make the reaction possible. Enzymes are proteins and, like all other proteins, are susceptible to elevated temperatures. Above 70 degrees celsius the enzymes change so that they can no longer catalyse the reaction. Colour can thus be preserved through a thermal treatment, for example blanching (a brief immersion in boiling water).

Most enzymes also will only work in a neutral environment (though exceptions exist, of course, such as our digestive enzymes), so apples put in any form of acid (lemon juice, wine, vinegar, etc.) will remain unchanged (although if you use red wine they may pick up colour from that!). You can also use acetic acid, but this will give the apples an unpleasant odour.

Chemical inhibitors can also stop the action of the enzymes. One such example is sulphur oxide, used for wood barrels and wine. But despite all these options, the most efficient and least demanding way is put the apple pieces into your stomach! Here the acids will prevent the apple from browning (although hopefully no one will be seeing it ever again once it goes down there).

In some cases, the fruit can brown even when not cut. This can happen rapidly when the cell membranes are damaged, thus allowing the enzymes to mix with the polyphenols. This also explains the confusing banana paradox: though food in a refrigerator should deteriorate more slowly, bananas actually brown faster inside the fridge!

How is this possible?

Food is basically broken down in four ways: microbiological, biochemical, chemical, and physical. Microbiological deterioration involves bacteria and molds developing on food, while biochemical involves enzymes degrading the product (as we have just seen in our apple example). Chemical deterioration is similar to this previous method but involves food substances reacting with each other or oxygen without the aid of enzymes or bacteria. In some rare cases, we can also talk about physical deterioration, which does not change the chemical composition of the food, just its physical structure. For this, we can use butter as an example: though it melts when warmed, it has not chemically changed. the loss of carbonation in beer or other beverages left open is another such example.

The speed of these processes almost always depends on the temperature in which they are taking place, and thus the handy refrigerator slows down the degradation immensely. That being said, bananas are the exception, with their biochemical degradation occurring faster in cool temperatures.

How exactly does this happen?

The process by which bananas darken is actually similar to that responsible for browning in apples; the enzymes that drive the reactions are separated from the substances they catalyse by biological membranes. Though in apples it takes cutting or cooking to break these membranes, they are actually much weaker in banas and will often dissolve on their own.

This process occurs in two stages: in the first stage, the breakdown of the membranes results in the release of the enzymes. In the second stage, these enzymes catalyze the reactions that produce the brown color that you’ve surely seen on bananas that no one wants to touch (thankfully the gods of cooking invented banana bread specifically for these less-than-appealing fruits).

Biological membranes are thin barriers composed of only two layers of highly specific lipids that are similar to lecithin. Though they may be thin, they are mighty! These membranes are what keep our cells together. In order for them to work as well as possible, they are adapted to the temperature at which the organism they compose will live. Human membranes, for example, contain lipids that function at 37 degrees celsius, our natural body temperature. This allows them to not be too fluid nor too fragile. Plants, on the other hand, adapt their membranes to the conditions as they grow. Bananas grown in tropical places are composed to function at high temperatures, and thus the coolness of a fridge causes them to become brittle and break. Though the low temperatures in a fridge do limit the activity of enzymes, it is not enough to overcome this effect in bananas. Though the enzymes may work faster at room temperature, the membranes remain stronger and thus prevent browning for a longer period of time.

But just what temperature is perfect for a banana? Fortunately, science has answered this great mystery of life for us: experiments have shown 13.3 degrees to be ideal. That being said, lowering in temperatures is much more harmful than raising, so bananas at 20 degrees will last much longer than those at 6 degrees. As a result of the processes behind these changes, we also know that bananas cook best when cooled (to break the membranes) and then allowed to sit in warmth (to let the enzymes work). All this to say, don’t keep your bananas in the fridge if you want them to keep their lovely yellow colour.

The above processes also affect the inside of the banana and the peel in different ways. We’ll leave it up to you guys to experiment and see if you can find out how. Feel free to write your ideas and answers in the comments 🙂