What it shows
Fresh pineapple contains bromelain, an enzyme that breaks down proteins, so it stops gelatin jelly from setting. Tinned or boiled pineapple, whose enzyme has been destroyed by heat, lets the jelly set normally.
The science
Gelatin is a protein. When it cools it forms a net that traps water and sets as a gel. Bromelain, a protease enzyme in fresh pineapple, cuts protein chains into shorter pieces, so the net cannot form and the jelly stays runny. Heating above about 60 to 70 degrees Celsius changes the shape of the enzyme (denatures it), so tinned pineapple, which is cooked during canning, and boiled pineapple no longer digest the gelatin. The same reasoning explains why papaya, which has papain, behaves like pineapple.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Gelatin powder (enough for 4 small portions; check the pack for the ratio)
- Fresh pineapple, tinned pineapple and a few pieces of fresh pineapple boiled for 5 minutes
- Four small clear cups
- Hot water (an adult must prepare it)
- A refrigerator
- A spoon, a marker and labels
- A notebook
Safety
For this project: An adult must handle the hot water. Fresh pineapple can sting the lips and tongue, so do not taste it in quantity. Gelatin is made from animal protein, so check whether the family avoids it. Do not eat the experiment jellies.
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
8 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 prepare the gelatin as on the pack and divide it equally in four labelled cups while still warm and liquid.
Let the liquid cool slightly until it is warm and not hot, so the fresh enzyme is not destroyed.
Add chopped fresh pineapple to cup 1, tinned pineapple to cup 2, boiled fresh pineapple to cup 3 and nothing to cup 4 as the control.
Stir each lightly, cover the cups and place all four in the refrigerator on the same shelf.
After 4 hours, tilt each cup gently and note whether the jelly has set or still flows.
Check again after 24 hours and record any changes, such as a partly set top layer.
Fill the results table with columns for the cup, treatment, set or not set and your notes.
Write the explanation of each result using the ideas of protein, enzyme and denaturation.
What you should see
The cup with fresh pineapple stays liquid or only partly sets, while the tinned, boiled and control cups set firmly. Your table links each outcome to what the heat did to the enzyme.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does bromelain do?
It breaks down proteins into smaller pieces.
2Why does tinned pineapple not stop the jelly setting?
The heat of canning destroyed (denatured) the enzyme.
3What is the role of the control cup?
To show how the jelly sets with nothing added.
4Name another fruit with a protein-digesting enzyme.
Papaya, which has papain.
Take it further
Use agar instead of gelatin, which is not a protein, and see whether fresh pineapple affects the setting.
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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