What it shows
Rice and urad dal batter swells and turns sour overnight because tiny microbes in it make gas and acid. The rise is faster in a warm place than in a cool one.
The science
Batter ferments because wild yeast and bacteria, mainly lactic acid bacteria, are present on the grains and the dal. They feed on the sugars from the starch and make carbon dioxide gas, which gets trapped in the thick batter and lifts it, and lactic acid, which gives the sour taste. Warmth speeds up their enzyme activity, so fermentation is faster in a warm place, while very cold slows it. These microbes are helpful and are used in the food, but they are not grown in a laboratory.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Fresh idli or dosa batter, ground by an adult at home (about 1 cup)
- Two identical tall clear jars or steel vessels with lids
- A ruler and a rubber band to mark the level
- A kitchen thermometer (non-mercury)
- A warm spot (near a cooker) and a cooler spot (floor of a cool room)
- A notebook for the diary
Safety
For this project: An adult must grind the batter and handle any hot cooking area. Do not taste the batter if it smells bad or has pink or black patches; throw it away. Wash hands and jars after the test.
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.
Divide fresh batter equally into the two jars, filling each about one third, and mark the batter level with a rubber band.
Cover them loosely with lids so gas can escape.
Put one jar in the warm corner and one in the cool corner, and measure the air temperature near each with the thermometer.
After 4 hours measure the height of the batter above your starting mark in each jar.
Look for bubbles, and smell gently each jar to note any sour smell.
Check again at 8 hours and the next morning, writing all readings in the diary.
Draw a line graph of batter height against time for the two jars and mark the temperatures on it.
Write what the bubbles show and why the warm jar rose faster, then discard any batter you do not use.
What you should see
The warm jar usually rises higher and sooner, with many bubbles and a sour smell, while the cool jar rises slowly. The graph shows the two lines moving apart.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What gas makes the batter rise?
Carbon dioxide, made by microbes as they feed on sugars.
2What makes the batter sour?
Lactic acid made by bacteria.
3Why does warmth speed up fermentation?
Enzymes in the microbes work faster when warm.
4Why were the lids left loose?
So gas can escape and the jars do not burst.
Take it further
Find out how the batter rises with a pinch of salt added, and check whether salt makes it slower or faster.
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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