What it shows
Yeast in dough eats sugar and gives out carbon dioxide gas, which stretches the sticky gluten and raises the dough. By marking the height in glasses you measure how sugar and salt change the speed of rising.
The science
Yeast cells ferment sugar into carbon dioxide and alcohol. The gas bubbles are trapped in the stretchy gluten network and raise the dough. A little sugar feeds yeast faster, but very much sugar or salt pulls water out of the yeast cells by osmosis and slows them. Warmth speeds up the enzymes that do the work.
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.
- Plain flour (maida) or atta, 4 cups
- Instant dry yeast, 1 small packet
- Sugar and salt
- Four identical tall glasses or cups
- Rubber bands to mark height
- Warm water at about 35 C, tested on the wrist
- A ruler and a stopwatch
Safety
For this project: Wash hands before and after handling dough. Do not eat raw dough because flour can carry germs. Use warm, not hot, water, which an adult checks.
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.
Mix four batches of dough, each with 100 g flour, 1 g yeast and 60 ml warm water, and label them plain, sugar, salt and both.
Add one spoon of sugar to the second batch, half a spoon of salt to the third and the same amounts to the fourth.
Knead each batch for the same five minutes and roll it into a ball.
Press each ball into the bottom of a glass and put a rubber band at the top level of the dough.
Keep all glasses together in a warm room away from drafts.
Every fifteen minutes measure the rise from the rubber band to the top of the dough and record it.
Continue for two hours or until the dough reaches the top of a glass.
Draw a graph of rise against time for each dough on one sheet.
What you should see
The dough with a little sugar rises fastest early on, the plain dough rises steadily, and the salted batches rise most slowly. Differences in the first hour are clear on your graph.
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 dough rise?
Carbon dioxide from yeast fermenting sugars.
2Why does salt slow the rise?
Salt draws water out of yeast cells and slows their activity.
3What does gluten do?
It forms a stretchy network that traps the gas bubbles.
4Why is the water kept warm and not hot?
Warmth speeds yeast but heat above about 50 C kills it.
Take it further
Compare dough at 10 C in the fridge, room temperature and a warm place to find the best rising temperature.
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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