Why do apple slices go brown?
In this experiment, test out the anti-browning abilities of common liquids. And discover the surprising biology of browning!
You will need
- 2 apples (make sure they’re the same type)
- Sharp knife
- Sticky notes
- Water
- Apple juice
- Lemon juice
- Sugary syrup such as golden syrup or maple syrup
- 3 bowls
- 5 small plates
- Timer
Safety
This activity requires a sharp knife to cut the apples into slices. Ask an adult to do this step for you.
When dealing with food, use clean hands and clean equipment.
What to do
Wash your hands with soap.
Prepare for your experiment by gathering five small plates. Label them with sticky notes as follows: “control”, “water”, “apple juice”, “lemon juice”, and “syrup”.
Gather three bowls. In the first bowl, pour ½ cup water. In the second bowl, pour ½ cup apple juice. In the third bowl, pour ½ cup lemon juice. Make sure your syrup is on hand.
Ask an adult to cut your apples into small slices. You will need at least 15 slices to run your experiment.
Place 3 apple slices on the control plate.
Submerge 3 apple slices in the water bowl for at least 30 seconds. Take them out and then place them on the plate labelled “water”.
Submerge 3 apple slices in the apple juice bowl for at least 30 seconds. Take them out and then place them on the plate labelled “apple juice”.
Submerge 3 apple slices in the lemon juice bowl for at least 30 seconds. Take them out and then place them on the plate labelled “lemon juice”.
Place your last 3 apple slices on the plate labelled “syrup” and cover them with syrup. You can use your fingers to make sure the syrup coats the entire surface of the apple slices.
Find somewhere to put your apple slices away from hungry pets and family members.
Set your timer for three hours. You can clean up your bowls and put away your other ingredients.
Once the timer finishes, wash your hands with soap again.
Rinse your syrup apple slices under cool water to remove the syrup. Put them back on the plate labelled “syrup”.
Which slices of apple turned the most brown? The least? Try ordering your plates from most to least browning. Which was the most successful anti-browning liquid?
Once you’ve recorded your findings, you are free to eat (or compost) your apple slices.
What’s happening?
Why do apples slices turn brown? It’s a combination of biology and chemistry.
Like all living things, plants and their fruits are made of cells. Cells contain DNA, proteins and lots of other chemicals. When you cut into an apple, the knife slices open the fruit’s cells and spills their contents all over the fruit. That includes a protein called polyphenol oxidase – or PPO for short.
PPO is a special kind of protein called an enzyme. Enzymes speed up chemical reactions without getting used up. This means that they can keep speeding up the same reaction over and over. PPO speeds up the reaction between oxygen in the air and other chemicals in the fruit. This reaction adds oxygen to these plant chemicals, which then turn brown. Because PPO is an enzyme, scientists call this process “enzymatic browning”.
There are several ways to slow down enzymatic browning.
You can damage the PPO enzyme by exposing it to acid. This explains why acidic lemon juice visibly reduces browning. Apple juice is less acidic than lemon juice but more acidic than water – were your apple juice slices less brown than the ones soaked in water?
You can also stop the reaction by removing one of the other chemicals involved. Covering the apple with syrup will stop oxygen getting to the apple, and without oxygen, the browning reaction can’t happen. This explains why the syrup-covered apple slices didn’t turn as brown as other apples in the experiment.
You may have seen other fruits turn brown, too, like bananas, avocados and pears. In other foods, enzymatic browning can be a good thing, like in tea, coffee and chocolate. In fact, scientists have found PPO in nearly all land plants they’ve tested. Scientists aren’t completely sure what PPO is for, but they think it may be a defence against plant-eating animals and germs
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