Why Does Fruit Turn Brown?
AbstractHave you ever wondered why apple slices turn brown once you cut them, or why a yellow banana gets dark spots over time? In this project you will find out why this happens, and how you can keep your apple slices looking fresh!
To investigate which liquid will keep sliced apples looking the freshest.
Tons of fruits and vegetables are produced, processed, and shipped on a daily basis so that we can buy them fresh. Many of these pieces of produce, however, never make it into stores. This is because many fruits and vegetables such as apricots, mushrooms, lettuce, and pears turn brown or spoil over time. Watch the time lapse video below to see a banana turn brown within 26 days. Would you buy a brown banana? This reaction is also called enzymatic browning and is one of the largest causes of spoilage in fruits and vegetables—even though it does not make the food harmful to eat. The name enzymatic browning comes from the fact that an enzyme within the fruit turns the fruit brown. But what exactly happens during the enzymatic browning process?
The process occurs when chemicals inside the fruit come into contact with oxygen in the air. This produces melanin, the same dark brown pigment that colors human hair, skin, and eyes. The enzyme responsible for the browning is called polyphenol oxidase (or PPO). In the presence of oxygen, the PPO enzyme changes substances known as phenolic compounds (through a process of oxidation) into different compounds called quinones. The quinones then react with other compounds to form melanin. Melanin is what turns the fruit and vegetables brown. This reaction, however, usually does not happen within fresh fruits and vegetables because the PPO and the phenolic compounds are separated in produce plant cells.
The enzymatic browning process is only triggered when PPO, phenolic compounds, and oxygen come in contact with each other. This is exactly what happens when a fruit is cut, falls, or is knocked around too much. When fruit tissue is damaged due to heat, cold, age, or mechanical stress, its cells break open and the phenolic compounds and the enzyme are released and mix with oxygen in the air. As a result, the damaged tissue turns brown almost immediately.
In the food industry, lot of attempts have been made to prevent enzymatic browning to keep produce appearing fresh for a long time. One example is the development of the Arctic apple. This apple was modified so that it no longer produces the PPO enzyme. With no enzyme present, the browning reaction cannot occur. Another solution is to treat the produce in a specific way to inhibit the enzyme, thereby slowing the browning reaction. Enzymes can only work in certain environments; most cannot function if it gets too cold or too warm, and they do not usually like acidic conditions. In this project, you will apply different liquids to apple slices to see which one works best to keep your apples nice and fresh!
Terms and Concepts
- Polyphenol oxidase (PPO)
- What makes fruit turn brown?
- Why do apples turn brown faster when you cut them?
- Which liquids do you think will keep your apples looking the freshest?
- Arctic Apples. (n.d.). Solving the Issue of Apple Browning. Retrieved March 3, 2021.
- Food-Info. (n.d.). Enzymatic browning. Retrieved April 23, 2019.
- Ashish. (Feb 1, 2021). Why Do Apples Turn Brown? How To Keep Apples From Turning Brown?. Science ABC. Retrieved March 3, 2021.
- McLandsborough, L. (2007, July 30). Why do apple slices turn brown after being cut? Scientific American. Retrieved May 10, 2012.
For help creating graphs, try this website:
- National Center for Education Statistics, (n.d.). Create a Graph. Retrieved June 25, 2020.
Materials and Equipment
- Adult helper
- Apples (2 of the same type). Make sure the apples do not have any bruises or dents on the outside and they look similar to each other.
- Cutting board
- Plates (6)
- Masking tape
- Permanent marker
- Lemon juice
- White vinegar
- Apple juice
- Lab notebook
- Optional: camera
Testing Different Liquids on Apple Slices
- Use a piece of masking tape to label each plate: "control," "water," "apple juice," "vinegar," "lemon juice," and "poked apple slices."
- On a clean cutting board, carefully cut both apples in half through the middle. Then cut the apple halves into 18 equally-sized apple slices.
- Immediately after cutting the apple, place three apple slices on each plate. Put the slices on their sides as shown in Figure 1.
- Testing three apple slices separately for each treatment will show that your results are reproducible.
Figure 1. On each plate, place three apple slices.
- Start with your treatment right away, so your apples do not have time to turn brown!
- For each plate, sprinkle about one teaspoon of the corresponding liquid onto each apple slice. Make sure to completely cover the top of each apple slice with the liquid.
- Do not put any liquid on the apples on the control plate. These will show how the apples turn brown without any treatment.
- Take a fork and poke each of the apple slices on the "poked apples slices" plate several times, so that each slice has many holes in it. Do not apply any liquid.
- Prepare a data table as shown in Table 1 in your lab notebook.
|Treatment||Apple Slice||How does the apple slice look?||How brown is the apple slice?
(1 = not brown at all; 5 = very brown)
|Average for treatment group?
(1 = not brown at all; 5 = very brown)
- Set your timer to two hours and start it. Check in on the apple slices every half an hour to have a look at them. After two hours, write down how the apple slices look in your data table.
- Make a note of how each of the apple slices looks. Are they turning brown yet? Where are they brown? Do they look dry or wet? Write down anything you notice. If you have a camera, take a picture of each plate. If you take a picture, make sure that the lighting conditions are always the same, so the coloring is similar in each picture.
- Rate how brown each of the apples slices is. On a scale of 1 to 5, try to rate each apple slice by how brown it seems, with 1 being the least brown and 5 being the brownest. Write the ratings in your data table.
- Optional: Leave the apples slices on the plates overnight. Then repeat the rating for each of the apple slices the next day. Make a second data table for your new data.
Analyzing Your Data
- Look at your data table. Overall, which treatment group has the freshest-looking apple slices? Which treatment group has the brownest apple slices?
- Calculate the average rating for each of the treatment groups. Add the rating number for each of the three apple slices within a treatment group and divide it by three.
- Write this number into the last column of your table.
- Make a bar graph using your rankings for the different treatment groups from your data table.
- You can make a bar graph by hand or use a website like Create a Graph to make a graph on the computer and print it.
- Put the name of each treatment on the x-axis (the horizontal axis going across) and its overall group rank, from 1 to 5, on the y-axis (the vertical axis going up and down).
- If you left the apple slices overnight and have data for the next day as well, make another graph for that data. How does this graph compare to the graph after two hours?
- Why do you think the treatment that had the freshest-looking apple slices overall was able to keep the apple slices the freshest? Why do you think the treatment that had the brownest apple slices did not keep the apple slices fresh?
- Tip: Re-read the Introduction and think about what conditions the enzyme PPO needs to function.
- When you are done analyzing the apple slices, you can compost them or throw them in the trash. Do not eat the apple slices, even if they look edible, because they might have an unpleasant taste after having been treated with the liquids such as vinegar.
Ask an Expert
- What other liquids could you try? Repeat your experiment, but this time, find other liquids to test. Examples could be milk, salt water, or rubbing alcohol. Which of the solutions work well for keeping the apples fresh?
- Instead of rating your apples only once after two hours, rate your apples every 10 minutes for about two hours. How fast do the apples turn brown in each treatment group?
- Test if the temperature has an effect on how fast the apple slices turn brown. Repeat the experiment and prepare three plates with three apple slices each. Then, put one plate into the fridge, one oven-safe plate into the oven at the lowest setting, and keep one plate at room temperature. Does the temperature make a difference?
- Try to find the Arctic apple in your supermarket. Slice one regular apple and one of the Arctic apples. Then observe the apple slices for a couple of hours or even days. Do both of the apples turn brown?
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