What effect does varying the alcohol concentration of yeast have on its fermentation?
Fermentation is the stage during which most CO2 is produced. When no oxygen is available, yeast will switch to an alternate metabolic pathway utilizing sugars for energy and producing, primarily, CO2 and ethanol. Yeast divides rapidly in this phase, reaching its carrying capacity (about 50 million cells/ml) in the wort, or must, and remains suspended in solution in order to expose maximum surface area to nutrients. Assuming no oxygen is added back to the fermenting wort, yeast will continue fermentation until one of two things happen - either alcohol concentration will exceed tolerance, or yeast runs out of food. 1
Figure 1 Process of Glycolysis and Anaerobic cell respiration in Yeast
It is predicted that the fermentation of yeast will decrease with the increase in the concentration of alcohol because the alcohol concentration goes above the yeast cell’s tolerance and hence, the yeast cells die. Also, alcohol denatures the enzymes responsible for the break down of simple sugars i.e the food supply of yeast, as a result of which the yeast cells die.
of alcohol (in ml)
Rate of fermentation
(in number of CO2
Temperature of alcohol
and water in the beaker
Volume of yeast +
glucose solution in the
bottle (in ml)
Time duration of the
experiment (in minutes)
Manipulation of variables
Independent – Concentration/ volume of alcohol (in ml)
The concentration of alcohol is changed to observe its effect on the rate of fermentation of yeast. In this case, the concentration is measured in ml. The different concentrations of alcohol taken into consideration are – 0ml, 1ml, 2ml, 3ml, 4ml, and 5ml.
Dependent – Rate of yeast fermentation (in number of CO2 bubbles released)
With an increase in the concentration of alcohol, the rate of fermentation of yeast will decrease. This is because when the alcohol concentration goes above the yeast cell’s tolerance the yeast cells die. Also, alcohol denatures the enzymes responsible for the break down of simple sugars i.e the food supply of yeast, as a result of which the yeast cells die.
With a decrease in the rate of fermentation, the amount of CO2 released will also decrease and so the number of CO2 bubbles released will decrease with an increase in alcohol concentration. The CO2 bubbles released are an indicator of the fermentation rate. Bubbles were counted manually by releasing it under water for 5 minutes measured using a stopwatch. The rate of bubbles released were calculated and analysed.
Controlled -Temperature of alcohol and water in the beaker (in 0C)
Temperature of the alcohol and water used affects its activity on the yeast cell. The higher the temperature, the higher the posibility of the yeast cell dying. Thus the temperature of alcohol and water was kept constant at 230C measured by an alcohol thermometer with uncertainty ± 0.10C by performing the experiment in an air conditioned laboratory.
Volume of yeast + glucose solution in the bottle (in ml)
The volume of yeast taken in the bottle is kept constant so that the number of yeast particles stays the same in all the cases ( i.e with 0ml, 1ml, 2ml, 3ml, 4ml, and 5ml alcohol concentration). If in any case the volume of yeast used is more, the number of yeast particles reacting with alcohol will increase as a result of which more yeast cells will die and the rate of fermentation will decrease.
The volume of yeast taken in the bottle was 30 ml, measured with the help of a syringe with uncertainty
Time duration of the experiment (in minutes)
The time for which the...