Microbes and bread making using yeast
Yeast is used to make bread. Yeast is a microbe which feeds on sugar. Enzymes in yeast ferment sugar forming carbon dioxide and ethanol. The carbon dioxide makes the bread rise. The ethanol evaporates when the bread is baked. This experiment investigates how temperature affects the expansion of bread dough.
It is best if each group does the activity at one temperature and then shares the results with other groups.
Each students or pair of students requires:
Spatula or glass rod
Beaker (100 cm
Measuring cylinder (250 cm
Measuring cylinder (50 cm
Balance (1 d.p.)
Water baths set at 20, 30 and 37°C (Note 1)
Plain flour, 25 g
Yeast suspension, 30 cm
3 ( Note 2)
Sugar, 1 g
Refer to Health & Safety and Technical notes section below for additional information.
Health & Safety and Technical notes
Read our standard health & safety guidance
If thermostatically controlled water baths are not available then large beakers of water maintained at the three temperatures can be used. The water temperature in each beaker needs to be monitored using a thermometer, and access to a supply of hot water, e.g. from a kettle, is needed to top up the beaker as the temperature falls. 1
The yeast suspension is made by stirring 7 g of dried yeast with 450 cm 2
3 of warm water. If fresh yeast is used it should not have been kept too long. Fresh yeast can be kept in the freezer for up to two months.
a Add 25 g of flour to a beaker and then add 1 g of sugar.
Take 30 cm b
3 of the yeast suspension in a 50 cm 3 measuring cylinder. Add the yeast suspension to the flour and sugar. Stir with a spatula or glass rod until a smooth paste, which can be poured, has been obtained.
Pour the paste into a 250 cm c
3 measuring cylinder. Take great care not to let the paste touch the sides – this is very important.
Note the volume of paste in the cylinder. Place the cylinder in one of the water baths. Record the temperature and note the volume of paste every two minutes for about 30 minutes. A results table is useful here. d
Plot a graph to show how the volume of the dough increases with time. Plotting the results from groups, with the water baths at different temperatures, on the same graph will allow comparison of results e
It is important that the paste does not touch the sides of the measuring cylinder when the students pour it from the beaker – this is easier said than done. One way of achieving this is to use a large plastic funnel which has had the stem cut off to leave a hole large enough for the paste to flow through the funnel - or a suitable plastic drinks bottle could be cut to produce a wide mouthed 'funnel'.
After 35–45 minutes the protein (gluten) breaks and carbon dioxide gas escapes.
Possible extensions for this activity could be to investigate the effect of substrate concentration (sugar), enzyme concentration and/or pH value on enzyme reactions.
Health & Safety checked, 2016
This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.
© Nuffield Foundation and the Royal Society of Chemistry
Page last updated October 2015