browning fruit experiment

April 11, 2019

Fruits Gone Bad? Discover Enzymatic Browning

A color-changing science project from Science Buddies

By Science Buddies & Svenja Lohner

Turning fruit brown in a matter of seconds? This biochemistry activity is bananas! 

George Retseck

Key Concepts Biology Chemistry Chemical reactions Enzymes Food

Introduction Have you ever wondered why apple slices turn brown once you cut them, or why a yellow banana gets dark spots over time? Both of these phenomena have the same cause: enzymatic browning triggered by an enzyme called polyphenol oxidase (PPO). In this activity you will find out how this enzyme works by turning a banana from yellow to brown in just a matter of seconds. Then you will explore how you can keep your apple slices looking fresh!

Background 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 some fruits and vegetables such as apricots, mushrooms, lettuce and pears degrade over time because of enzymatic browning. You can see this for yourself if you observe a banana for several days or weeks. Would you buy a brown banana? Enzymatic browning is one of the largest causes of quality loss in fruits and vegetables—even though it does not make the food harmful to eat. So what exactly happens during enzymatic browning?

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The process occurs over several steps. 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 the same dark brown pigment that colors hair, skin and the irises of our eyes. It also turns 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 one another. This is exactly what happens when a fruit is cut, falls or is knocked around too much. When fruit tissue is damaged because of 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 this activity you will see this browning process for yourself and investigate how this reaction can be avoided. So grab some fruit and get started!

Banana (yellow with no brown spots)

Adult helper

Cutting board

Lemon juice

Distilled vinegar

Additional one to two bananas (optional)

Fridge (optional)

Tape (optional)

Other fruits and vegetables to test (optional)

Preparation

Fill a pot with tap water.

With the help of an adult, place the pot on the stove and heat the water until boiling. Always use caution and adult help when working around very hot water.

Take one of your bananas and look closely at its peel. What color is it? Do you see any brown spots?

Carefully dip the bottom third of the banana into the boiling water for 30 seconds. What happens to the banana when you submerge it in hot water?

After the 30 seconds remove the banana from the boiling water and observe it for another three minutes. What do you notice? Does the banana look different after a while? How?

When the banana has cooled down, peel the banana. Look at the fruit that was inside the peel. Did you expect the banana to look like that?

With the help of an adult cut two slices from the apple on a cutting board. Place each slice onto its side. How do they look?

Poke one of the apple slices with a fork several times. Then observe both slices for 15 to 20 minutes. How do the apple slices change over time? Do you notice a difference between the two slices? If yes, can you explain why?

Cut five more slices from the apple and place each slice on its side. Immediately after cutting, sprinkle milk on top of the first slice, distilled vinegar on the second slice, lemon juice on the third slice and water on the fourth slice. Keep the last slice as is. Then poke each apple slice several times with a fork. What do you think these liquids will do to the apple?

Observe all five apple slices for another 15 to 20 minutes. How are the apple slices different after 15 to 20 minutes? What did each liquid do to the apple slice? Can you explain your results?

Extra: Instead of heating the banana, try exposing it to cold. Place one banana in the fridge for several days. Take a second banana and cover parts of the banana peel with duct tape. Get creative with the pattern of the tape! Then put that banana in the fridge as well. Check on both bananas every day. Do both bananas change color? How does the second banana look when you remove the tape after a couple of days? What do you think happened?

Extra: Beside bananas and apples other fruits can also undergo enzymatic browning. Test different fruits or vegetables to see if they are prone to enzymatic browning. Or try the same fruit but test different kinds of that fruit. How do different kinds of apples compare?

Observations and Results Were you able to change the color of your banana? Most likely, yes! You probably didn't observe a big difference in the banana right after putting it into the boiled water, but within the next 30 seconds and after taking it out of the water it should have turned pretty dark. You should have noticed that the color change only happened where the banana was submerged in the hot water. This is because the boiling water caused heat stress to the cells in the outer layers of the banana peel and destroyed them. As the cells broke open they released PPO and phenolic compounds, which then reacted with the oxygen of the air to form melanin. Only the peel should have been affected by enzymatic browning as the inner part of the banana was protected by the peel.

If you put a banana in the fridge, the whole banana should have turned brown. As the banana is a tropical fruit, it is evolved for warm temperatures, which is why the banana cells get damaged in the cold. If you taped parts of the banana, however, you should have noticed that underneath the tape the banana kept its yellow color. This is because the tape sealed the banana from the oxygen, which is necessary for the enzymatic browning reaction to happen.

When you cut an apple its tissue is damaged, and its cells are broken due to mechanical stress. This again triggers enzymatic browning, which you should have observed on the apple slices. When poking the apple slices with a fork, you damaged even more cells and released more enzyme and phenolic compounds, which is why this apple slice should have turned noticeably darker. The PPO content inside a fruit or vegetable determines the degree of its enzymatic browning. This is why some fruits or vegetables, even different types of apples that contain more of these compounds, become darker than others.

When you sprinkled, milk, lemon juice, vinegar and water over your apple slices you should have noticed that acidic solutions such as vinegar and lemon juice prevented enzymatic browning. This is because PPO doesn't work well in acidic environments, which means that the enzyme stops working or slows down considerably. So next time you eat an apple and don't want it to get brown you know what to do!

Cleanup If you would like to and if you used clean implements, you can eat the experimental fruit. You can compost uneaten fruit.

More to Explore Why Do Apple Slices Turn Brown after Being Cut? , from Scientific American Why Does Bruised Fruit Turn Brown? , from Scientific American Enzymatic Browning , from Food-Info, Wageningen University Prevention of Enzymatic Browning in Fruit and Vegetables , from European Scientific Journal STEM Activities for Kids , from Science Buddies

This activity brought to you in partnership with Science Buddies

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Fruit and vegetables: enzymic browning

What is enzymic browning?

Enzymic browning is an oxidation reaction that takes place in some foods, mostly fruit and vegetables, causing the food to turn brown. 

Oxidation reactions occur in food and non-food items. Enzymic browning is a reaction which requires the action of enzymes and oxidation in order to occur. 

What happens during enzymic browning?  

Oxygen in the air can cause sliced fruit to brown, a process called enzymic browning (an oxidation reaction). Phenols and the enzyme phenolase are found in the cells of the apple, and when these are exposed to oxygen in the air, for example through slicing, the oxygen causes a reaction.  The phenolase changes the phenols into melanin, which has a brown colour. To stop the oxidative reaction, the phenolase enzymes need to be denatured.  This could be done by using heat and acids.

You may have heard of melanin before.  Melanin is the pigment that gives human hair, skin and eyes their colour.

Why does enzymic browning happen?

Foods are made up of lots of different molecules including some called enzymes. Enzymes are special proteins which can speed up chemical reactions and act as biological catalysts. They can cause fruit to ripen and over-ripen, which gives the fruit a brown colour.

Fresh fruit and vegetables normally keep enzymes trapped in their tissues.  However when the fruit is sliced, or squashed, or when the fruit or vegetable begins to break down with age, the enzymes come into contact with oxygen in the air. This causes the fruit to turn brown.

Enzymic browning causes a lot of food waste but it can also be useful: we would not have tea or chocolate without it!

How can enzymic browning be slowed down?

The browning can be slowed down by preventing the enzyme from working properly.  Lemon juice contains an acid which can stop enzymes working properly as enzymes often work best at a certain pH.  Water and sugar, in jam for example, stops oxygen in the air getting to the enzymes and prevents the browning.

Effect of oxidation on nutrient content

As well as causing the fruit to change colour, oxidation can also affect the nutrient content of a fruit or vegetable.  Vitamin C, found in some fruits and vegetable can be oxidised when it is exposed to air.   The longer a fruit is exposed the less vitamins it will have.  

Get some fresh fruit or vegetables – lettuce, apple, or potato

Have three containers ready, one containing water, one containing sugar and water (say 5g sugar, 50ml water), and one containing a little lemon juice.

Cut up each fruit or vegetable into pieces about 5cm long.

Leave one sample of each fruit or vegetable on a plate and quickly place one sample into the water, one into the sugar solution, and one in the lemon juice.

Leave the experiment running for about 1 hour.

You should see browning in the samples left on the plate.  Compare this with the other containers.  What conditions prevented browning most? Think of food preparation and cooking where similar conditions are used.  Why do chefs often tear rather than cut up lettuce leaves?

More information

See our video exploring the effect of different variables on enzymic browning.

OCR topic exploration pack: teacher instructions: Preparation techniques: preventing enzymixc browning (worksheet 8)

Enzymic browning experiments

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Effect of Acids and Bases on the Browning of Apples

Everything You Need for This Simple Experiment

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Apples and other fruit will turn brown when they are cut and the enzyme contained in the fruit (tyrosinase) and other substances (iron-containing phenols) are exposed to oxygen in the air.

The purpose of this  chemistry laboratory exercise is to observe the effects of  acids and bases  on the rate of browning of apples when they are cut and the enzymes inside them are exposed to oxygen.

A possible hypothesis for this experiment would be:

Acidity (pH) of a surface treatment does not ​affect the rate of the enzymatic browning reaction of cut apples.

Gather Materials

The following materials are needed for this exercise:

• Five slices of apple (or pear, banana, potato, or peach)
• Five plastic cups (or other clear containers)
• Vinegar (or dilute acetic acid )
• Lemon juice
• Solution of baking soda ( sodium bicarbonate ) and water (you want to dissolve the baking soda. Make the solution by adding water to your baking soda until it dissolves.)
• Solution of milk of magnesia and water (ratio isn't particularly important - you could make a mixture of one part water one part milk of magnesia. You just want the milk of magnesia to flow more readily.)
• Graduated cylinder (or measuring cups)

Procedure - Day One

• Lemon Juice
• Baking Soda Solution
• Milk of Magnesia Solution
• Add a slice of apple to each cup.
• Pour 50 ml or 1/4 cup of a substance over the apple in its labeled cup. You may want to swirl the liquid around the cup to make sure the apple slice is completely coated.
• Make note of the appearance of the apple slices immediately following treatment.
• Set aside the apple slices for a day.

Procedure and Data - Day Two

• Observe the apple slices and record your observations. It may be helpful to make a table listing the apple slice treatment in one column and the appearance of the apples in the other column. Record whatever you observe, such as the extent of browning (e.g., white, lightly brown, very brown, pink), the texture of the apple (dry? slimy?), and any other characteristics (smooth, wrinkled, odor, etc.)
• If you can, you may want to take a photograph of your apple slices to support your observations and for future reference.
• You may dispose of your apples and cups once you have recorded the data.

What does your data mean? Do all of your apple slices look the same? Are some different from others?

If the slices look the same, this would indicate that the acidity of the treatment had no effect on the enzymatic browning reaction in the apples. On the other hand, if the apple slices look different from each other, this would indicate something in the coatings affected the reaction.

First, determine whether or not the chemicals in the coatings were capable of affecting the browning reaction .

Even if the reaction was affected, this does not necessarily mean the acidity of the coatings influenced the reaction. For example, if the lemon juice-treated apple was white and the vinegar-treated apple was brown (both treatments are acids), this would be a clue that something more than acidity affected browning.

However, if the acid-treated apples (vinegar, lemon juice) were more/less brown than the neutral apple (water) and/or the base-treated apples (baking soda, milk of magnesia), then your results may indicate acidity affected the browning reaction.

Conclusions

You want your hypothesis to be a null hypothesis or no-difference hypothesis because it is easier to test whether or not a treatment has an effect than it is to try to assess what that effect is.

Was the hypothesis supported or not? If the rate of browning was not the same for the apples and the rate of browning was different for the acid-treated apples compared with the base-treated apples, then this would indicate that the pH ( acidity, basicity ) of the treatment did affect the rate of the enzymatic browning reaction. In this case, the hypothesis is not supported.

If an effect was observed (results), draw a conclusion about the type of chemical (acid? base?) capable of inactivating the enzymatic reaction.

Additional Questions

Here are some additional questions you may wish to answer upon completing this exercise:

• Based on your results, what substances in each apple treatment affected the enzyme activity responsible for the browning of the apples? Which substances did not appear to affect the enzyme activity ?
• Vinegar and lemon juice contain acids. Baking soda and milk of magnesia are bases. Water is neutral, neither an acid nor a base. From these results, can you conclude whether acids, pH neutral substances, and/or bases were able to reduce the activity of this enzyme (tyrosinase)? Can you think of a reason why some chemicals affected the enzyme while others didn't?
• Enzymes speed the rate of chemical reactions. However, the reaction may still be able to proceed without the enzyme, just more slowly. Design an experiment to determine whether or not the apples in which the enzymes have been inactivated will still turn brown within 24 hours.
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