What it shows
Yeast feeds on sugar and releases carbon dioxide gas, which inflates a balloon over the bottle. Comparing plain sugar, jaggery, honey and an artificial sweetener shows which foods the yeast can actually turn into gas, the same reaction that makes bread rise.
The science
Yeast is a living single-celled fungus. Without air it breaks sugar down into alcohol and carbon dioxide gas, a reaction called fermentation. Yeast cells carry enzymes that fit ordinary sugars such as sucrose, glucose and fructose, so sugar, jaggery and honey all feed them. Artificial sweeteners taste sweet to us but their molecules do not fit the yeast enzymes, so no gas is made. Warmth speeds up the enzymes, which is why the water must be warm but not hot.
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.
- Dry yeast, 4 teaspoons (from a bakery or grocery)
- Four small bottles and four balloons
- Warm water, 2 cups (adult pours)
- A teaspoon each of sugar, grated jaggery, honey and a sugar-free sweetener
- A string and ruler to measure the balloons
- A timer
Safety
For this project: An adult must pour the warm water; it should be only as warm as bath water or the yeast dies. Do not seal the bottles with caps, use balloons only.
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.
Pour half a cup of warm water into each bottle and add one teaspoon of yeast to each.
Add one sweetener to each bottle, one teaspoon of sugar, jaggery, honey or artificial sweetener, and label them.
Swirl each bottle gently to mix and immediately stretch a balloon over its neck.
Stand the bottles together in a warm spot and start the timer.
Every ten minutes measure the balloon widths with the string and ruler and write them down.
Continue for an hour or until the balloons stop growing.
Plot a line graph of balloon size against time for the four bottles.
Smell the bottles at the end (carefully, from a distance) and note the bread-like smell of fermentation.
What you should see
Balloons on the sugar, jaggery and honey bottles inflate within twenty minutes, with sugar and jaggery usually the largest, while the artificial sweetener balloon stays flat, showing the yeast could not use it.
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 inflates the balloon?
Carbon dioxide produced by the yeast during fermentation.
2Why did the artificial sweetener make no gas?
Its molecules are not a food the yeast enzymes can break down.
3Why must the water be warm but not hot?
Warmth speeds up the yeast, but hot water kills it.
4Where is this reaction used every day?
In making bread, idli and dosa batter rise.
Take it further
Keep the sugar fixed and run three bottles in cold, warm and hot water to find the temperature yeast likes best.
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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