An Investigation Into the Effects of Sugar Concentration on Yeast Activity

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An investigation into the effects of sugar concentration on yeast activity


Yeasts are eukaryotic micro organisms belonging to the kingdom fungi. Yeasts live on sugars and produce ethanol and carbon dioxide as by-products. [James Mallory, 1984]When Yeasts are given water and sucrose they convert the sucrose into glucose then convert the glucose into carbon dioxide and ethanol following the following reaction:

C₆H₁₂O₆ ( 2(C₂H₅OH + CO₂

[Brady Burkhart, Terrell Grayson and Eric Kimler, 2009] Because yeasts produce ethanol and carbon dioxide they are commonly used in the fermentation of alcoholic beverages and in baking as a rising agent. [Ron Pickering, 2006]

Research question:

What is the effect of sugar concentration on yeast activity?


The expectation is that the amount of carbon dioxide produced will be directly (and linearly?) inked to the concentration of the sugar


Independent variables:

• Sugar concentration

Constant variables:

• Temperature

• Sugar type

• Yeast type

• Type of bottle used for the experiment

• Volume of water

• Time

• Quantity of yeast

Dependant variables

• Quantity of carbon dioxide produced by the yeast


• 1 Set of scales (accurate to 1 gram)

• 1 Measuring cup

• 5 Bottles

• 5 Thin plastic tubes (approx. 5mm gauge, 1m length)

• 1 Tub of water

• 5 Test tubes with volume markings up to 100ml

• 5 12 g Bags of dried Saccharomyces cerevisiae (Baker’s yeast)

• 472.5 g Sucrose (table sugar)


We started setting up our experiment by measuring different molar solutions (0M, 0.25M, 0.5M, 0.75M and 1M) of sugar and putting them into separate bottles, along with one bag (12g) of yeast and 300ml of water. We then attached a tube going from the top of each bottle to separate test tubes which were entirely full of water and were placed upside down in a tub of water, stopping the tube from emptying and allowing carbon dioxide to be collected at the top of the test tube in bubbles.

The data was collected by removing the plastic tubes leading from the bottles to the test tubes after waiting eight minutes after the experiment was started. The test tubes were then sat vertically upside down and using the measurement markings on the test tubes measured the volume of the carbon dioxide that had been produced by the yeast.


|Sugar Concentration |Amount of carbon dioxide produced. | |0M |0ml | |0.25M |115ml(approx.) | |0.5M |54ml | |0.75M |34ml | |1M |46ml |


The results of this experiment suggest that the ideal sugar concentration for yeast respiration lies around 0.25M. Reasons for this result could include:

1. The alcohol produced by the yeast at higher sugar concentrations could have killed the yeast if the alcohol content in the solutions became enough.

2. Yeast cells may have evolved to favour sugar concentrations of around 0.25M as it may be a common sugar concentration in the natural environment and would therefore be beneficial for the yeast to favour it.

To assess more accurately where the optimal sugar concentration for yeast respiration lies the...
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