25mL of grape juice, and half a teaspoon of sugar was added to Flask A, then heated to boiling point for one minute. A thermometer was then placed in the flask and plugged with cottonwool. Flask A was then left to cool down to 30°C. Once the flask reached 30°C the cottonwool was quickly taken out and a quarter of a packet of yeast was added, as quickly as possible the cottonwool plug was replaced.
The rest of the grape juice and sugar was then placed in Flask B and then heated at boiling for one minute. The yeast leftover was then added to the boiling mixture, plugged with cottonwool then allowed to cool. Both Flask A and B were then left at 25°C overnight.
The cottonwool was then removed from Flask A and a sterilised dropping pipette was used to place a drop of the suspension onto a slide. A cover slip was then placed on top and the slide was then observed under the microscope. Cells were then observed under the low power lens and yeast was observed under the higher power setting. This process was repeated for flask B and the results were then recorded for both.
Diagram:
Results:
When the flask’s contents were observed after the 12 hours there was a clear difference in colour between flask A and flask B. Flask A was very white, cloudy colour whilst flask B was the same as when boiled. Flask A smelt very sweet (like wine) and flask B still smelt like grape juice. The cells and yeast in flask A were much more tightly packed yet they were very spread out in flask B.
Discussion:
The differences in yeast action can be explained through the difference in procedure. Flask A was cooled to the optimum yeast growth temperature before the yeast was added, this provided better conditions compared to flask B where the yeast was added at 100°C and therefore sterilised or killed. The purpose of boiling the grape juice and sugar is to fully dissolve the sugar so that it provides food for the yeast to live off. It is important to start with sterile flasks because otherwise there is a risk of contamination and possibly brewing pathogens. It is important to replace the cotton wool immediately after the addition of yeast because otherwise airborne contaminants may enter the flask and ruin the experiment. While adding the yeast it was important not to put the cotton wool on the bench because the cottonwool may get contaminated. The yeast was activated by providing the right environment for example the necessary nutrients and temperature.
