What it shows
High concentrations of salt or sugar draw water out of microbes by osmosis, so they cannot grow and food keeps longer. You compare how well cut carrot or raw mango keeps with salt, sugar, both or nothing over two weeks.
The science
A strong salt or sugar solution has far less free water than the cells of microbes. Water moves out of the microbe through its membrane by osmosis, and the cell shrinks and stops growing. This is why jam, pickle and salted fish keep for months without a fridge, as long as the concentration stays high.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 3 carrots or 2 raw mangoes, cut into cubes by an adult
- Common salt, 6 spoons
- Sugar, 6 spoons
- Four clean, dry glass jars with lids
- A kitchen scale or a spoon measure
- Labels, marker and a notebook
Safety
For this project: An adult must cut the vegetables. Do not eat any food from these jars, and never taste jars that smell sour or show mould or fuzz. Do not open mouldy jars close to the face. Wash all jars with hot water beforehand.
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.
Step 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.
Wash and dry the jars and cut equal weights of vegetables into similar cubes.
Fill jar A with plain cubes, jar B with cubes and three spoons of salt, jar C with cubes and three spoons of sugar and jar D with both.
Shake each jar gently and screw the lid on loosely.
Keep the jars in a shaded place in the kitchen at room temperature.
Observe every two days for liquid, smell, colour and any fuzzy mould, noting the date.
Photograph each jar at the same times to build a visual record.
After two weeks rank the jars by how fresh the vegetable looks and smells.
Compare the liquid collected at the bottom of each jar as proof of osmosis.
What you should see
Jar A spoils first, with mould or a sour smell in days. The salt and sugar jars last much longer and collect liquid drawn out of the vegetable, showing osmosis at work.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01How does salt preserve food?
It pulls water out of microbes by osmosis so they cannot grow.
02Why does liquid collect in the salted jar?
Water is drawn out of the vegetable cells into the salt.
03Why is a jar with nothing added included?
It is the control that shows spoilage without preservative.
04Name three foods preserved this way.
Pickles, jam and dry fish.
Take it further
Add oil or a spoon of lemon juice to jars and find out whether acid and oil improve the keeping time.
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.
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