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Experiments with yeast - 3

To monitor the progress of fermentation of different sugars by yeast

Background information

If yeast is added to a liquid containing sugars and other nutrients, kept at an appropriate temperature [and deprived of oxygen], it will turn the sugars into ethanol (alcohol) and carbon dioxide.

A hydrometer is a special piece of apparatus which measures the specific gravity of a liquid - the same concept as density (units of which are g per cm3). Specific gravity has no units, so pure water has a specific gravity of 1.000, sometimes expressed as 1000 units of gravity.


In brewing, a hydrometer can be used to monitor the disappearance of sugars (which raise the specific gravity over 1.000) and their replacement by alcohol (which has a specific gravity less than 1.000).
Sometimes, extra sugars are added during fermentation, so in these cases the specific gravity will increase dramatically, and the sugars may be gradually used up as fermentation proceeds. This is likely to take several days or weeks. Added sugar may also simply contribute to the sweetness of the beverage, if the yeast can be prevented from continuing fermentation.

It is in fact the (total) change in specific gravity, rather than the actual measurements themselves, that indicate the actual alcohol production.

Suggested action

Each group to work as a team.

Individually, think about the problem and try to come up with a hypothesis about the effect of different sugars on fermentaton by yeast.

The measuring and other apparatus should ideally be sterilised before use, using sodium metabisulphite solution or some other method.
CAUTION: Do not inhale fumes from this liquid. This solution should then be rinsed off with clean water before the hydrometer is inserted into the yeast growth medium.

The same procedure must be followed each time a reading is taken using the hydrometer, including sterilising the hydrometer and spinning it in the liquid in order to dislodge any bubbles.
It is especially important for every member of the team to check the significance of the divisions on the hydrometer stem, and the direction of the scale!

Try 1 spatula of yeast per 100 ml of sugar solutions in a 100ml measuring cylinder, covered with a disc of polythene, held in place with a rubber band. These measuring cylinders will fit into the incubator, but care must be taken to avoid knocking them over (best kept on a tray).

Background information: Wines are often started at a specific gravity of 1070, which corresponds to about 180 g sugar per litre (18 g per 100ml).
The sugar may need to be dissolved in warm water before making up to the final 100 ml mark.

Decisions needed:

1) Which sugars to use?

2) What concentration(s) of solution to use?

3) What factors are to be kept constant (and how) ?

4) How to organise team work to gather results.

The same criteria apply to this as to other assessments, so it is important to research and justify each stage.

Suggested ( flexible?) results format

Time Elapsed
time from
by (name)

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