December 03, 2020 December 03, 2020/ Have you ever brought apple slices to school for lunch? When you packed them in your lunchbox, they were white on the inside, but by lunchtime, all the slices have turned brown. It doesn’t change the taste, but it certainly doesn’t look as appetizing. So what happened? Today, let’s talk about why sliced fruit turns brown!
Image via unsplash.com/Estúdio Bloom Chemical reactions occur all around us (and within us) in different ways. One of these reactions is oxidation. Oxidation is when, on an atomic level, oxygen is gained, like when iron turns to rust. The specific type of oxidation occurring in your apple slices is called enzymic browning. An enzyme is a substance, usually a protein, in the cell of an organism that speeds up chemical reactions. During enzymic browning, an enzyme called phenolase and another organic compound found in fruit cells called phenols go through an oxidation reaction when they are exposed to oxygen. The phenolase regulates the reaction, turning the phenols into melanin. You might have heard of melanin, since it’s in our bodies, too—melanin is a brown pigment that gives human skin, hair, and eyes color.
Still taken from youtube video “Oxidation in Apples” posted by DNAgeek So now we know how fruits and vegetables turn brown, but why does enzymic browning even occur?
Image via Scientific American Usually the enzymes of fruit are enclosed in tissue. The enzymes are tucked into their cells, working to ripen the fruit. But when those cells break down—either by an outside cause like someone biting or cutting into the fruit or by natural causes like aging—the enzymes are released and come in contact with oxygen, kicking off the chemical reaction and turning the fruit brown. In the food industry, enzymic browning is both a positive and a negative. For some foods and beverages, like dried fruit, chocolate, and tea, enzymic browning give those products their flavor and color. For other foods, like fresh fruit and seafood, enzymic browning can lead to this food being thrown out, creating a lot of food waste and economic loss. So the question of how to slow down enzymic browning is pretty important for farmers, fishermen, and other people who produce food. So let’s explore some ways to slow down the browning of fruit! Materials:
Instructions: 1. Put the lemon juice, sugar water, and plain water in the small containers.
2. With adult supervision, cut four similarly sized slices of apple (and enjoy the rest of the apple as a snack!).
3. Dip one slice in each of the liquids, labeling which slice was dipped in each liquid. Leave one slice as a control, don’t dip it in any of the liquids to see how the apple would normally brown.
4. Let the apples sit out, and check on them periodically. Let the experiment run for about an hour.
The apple slices about an hour after the start of the experiment What did you notice in your experiment? Make your own observations, and then read about our results and an explanation of why we saw what we saw. In our experiment, we found that the the lemon juice prevented browning the most, followed by the sugar water, and the plain water and control browned about the same. So why is that? Like we talked about earlier, enzymic browning is an oxidation reaction that is caused when oxygen is introduced to phenols, which turn into melanin with the help of enzymes. So there are two components of this reaction that we can alter: the amount of oxygen introduced and the speed of the reaction. The amount of oxygen can be reduced by putting a barrier between the air and the apple slice. In our experiment, this takes the forms of the sugar water or the lemon juice. By dipping the fruit in a liquid, it coats it, making a film that allows less oxygen to reach the phenols. Notice that the plain water didn’t slow down the browning, though. Since an apple is already made up of so much water (85%), a quick dip in water doesn’t serve as a strong barrier against oxygen. For water to effectively prevent enzymic browning, it’s better to keep the apple slices in a water bath until ready to use. The speed of the reaction can be reduced by changing the pH level of the apple. Enzymes work best at certain pH levels, so lowering the pH level of the apple slice can slow down those enzymes. When an acid, like lemon juice, is added to the apple slice, it slows down the browning.
Did you try this experiment or put your own twist on it? Share your results with us on Facebook, Instagram, and Twitter! |