What it shows
A cut apple turns brown when enzymes in it react with oxygen in the air. Lemon juice, salt water, sugar water and cold slow or stop the change, which shows how cooks keep fruit looking fresh.
The science
Cut cells release an enzyme that joins with oxygen and turns colourless compounds in the apple into brown ones. Lemon juice has acid, which slows the enzyme and also stops the oxygen reaction, while a cold fridge also slows it. Salt water stops the enzyme moderately, and water alone only covers the slice from the air.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 1 large apple (or a banana or potato)
- Lemon juice, salt water, sugar water, plain water
- Five small plates or saucers
- A butter knife (adult cuts)
- A marker and sticky labels
- Phone camera or a notebook for timed notes
Safety
For this project: An adult must cut the fruit. Do not eat slices that have been left out for the experiment, and wash hands after handling the lemon juice.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Ask an adult to cut one apple into ten thin, equal slices.
Place two slices on each of the five plates and label them: nothing, lemon juice, salt water, sugar water and plain water.
Dip or brush each slice in its liquid, except the plate that has nothing.
Put the plates side by side on a table in the same light.
Photograph or sketch the slices every fifteen minutes for two hours.
Score the browning from 0 for none to 5 for deep brown at each time.
Put one extra slice in a covered box in the fridge and compare it at the end.
What you should see
The slice with nothing turns brown first, plain water and sugar water help slightly, and lemon juice keeps the apple pale for the longest time. The fridge slice browns slowly.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the apple turn brown?
An enzyme in the cut cells reacts with oxygen to make brown pigments.
2How does lemon juice help?
Its acid slows the enzyme and also stops the reaction with oxygen.
3Why is the plate with nothing needed?
It is the control that shows what happens without any treatment.
4Does cold help?
Yes, enzymes work more slowly at low temperature.
Take it further
Test other liquids such as vinegar, orange juice and honey water and rank all of them.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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