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Science project idea

Salt and yeast: how much does salt slow a rising dough?

Yeast makes dough rise by fermenting sugar and giving off carbon dioxide. Adding more salt to the dough slows the yeast, so the dough rises less.

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What it shows

Yeast makes dough rise by fermenting sugar and giving off carbon dioxide. Adding more salt to the dough slows the yeast, so the dough rises less.

The science

Yeast is a living fungus. When it respires without enough oxygen, it ferments sugar to give ethanol and carbon dioxide, and the gas gets trapped in the stretchy dough and makes it rise. Salt draws water out of yeast cells by osmosis and slows their activity, so the more salt, the slower the fermentation and the smaller the rise. Bakers add a little salt to control the rise and improve taste.

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.

  • 4 equal clear tall glasses or cut bottles
  • Plain flour (maida or atta), 200 g
  • Dry baker’s yeast, 4 small pinches
  • Sugar, 4 level teaspoons
  • Salt, in measured amounts of 0 g, 0.5 g, 1 g and 2 g
  • Warm water (about 30 to 35 degrees Celsius), 120 mL
  • A ruler and rubber bands to mark height
  • A kitchen scale

Safety

For this project: An adult must warm the water, because it must not be hot. Do not eat the raw dough. Wash hands and surfaces after handling it.

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 rules

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.

  1. Weigh out four portions of 50 g flour and a pinch of yeast and a teaspoon of sugar in each bowl.

  2. Add salt as 0 g, 0.5 g, 1 g and 2 g to the four bowls and label them.

  3. Add 30 mL of warm water to each bowl and knead each dough for two minutes in the same way.

  4. Roll each into a ball, drop it to the bottom of a glass and press it flat so the surface is level.

  5. Mark the height of each dough with a rubber band and keep the glasses in the same warm place.

  6. Every 15 minutes for 90 minutes, measure the height of each dough in the glass with a ruler.

  7. Record the heights in a table and draw a line graph of height against time for all four doughs on the same axes.

  8. Write which dough rose fastest and explain the result using osmosis and fermentation.

What you should see

The dough with no salt rises the most. Doughs with more salt rise more slowly, and the graph lines spread out over time.

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 the fermentation of sugar.

2Is yeast a plant, an animal or a fungus?

A fungus.

3Why does salt slow the yeast?

It draws water out of the cells by osmosis and slows their activity.

4Why keep all glasses in the same place?

So temperature stays the same and only salt differs.

Free ebook: Viva Questions for Science Projects

Take it further

Keep the salt fixed and repeat with different amounts of sugar to see where the rise is the greatest.

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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