Brewers' yeast, itself a by-product of the brewing industry, is prepared for commercial distribution in dried or powdered form by means of a spray-drier. Such yeast can only be used in making alcoholic drinks, and is unsuitable for baking bread or cakes.
I will do this by using different concentrations of sucrose. I will be using distilled water, 2%, 4%, 6%, 8% and 10%. I will put the same amount of yeast into each sucrose solution and leave in an incubator for one week. After one week I will take a sample of each, add methylene blue to it to colour the yeast cells and make them easier to see and use a haemocytometer to see how much yeast has grown. I will work this out by counting five out of the twenty-five squares and then multiplying that by twenty-five. I will repeat this five times using a different sample each time to rule out any strange results.
For my experiment I will need one microscope, one haemocytometer slide, distilled water, five different concentrations of sucrose solution, methylene blue, a capillarity pipette, yeast and thirty test tubes.
My hypothesis is that the yeast growth will increase with the increased sucrose concentration. My null hypothesis is that there will be no difference in the yeast growth in each of the sucrose solutions and if there is it is only by chance. My independent variable is the different concentrations of sucrose and my dependent variable is the growth of yeast. I will be keeping certain things the same such as the amount of yeast I use, and then length of time I leave it for and the volume of solution I use.
I will be recording my results in a graph and a table like the ones below:
% of sucrose
Table of Results
I can see a clear pattern from my table and graph. Between 2% and 4% the count almost doubles from 583 to 990. From 4% to 6% it changes very slightly, climbing from 990 to 1110. Then once again it doubles at 8% from 1110 to 1215. At 10% it increases by a substantial amount leaving the total at 1805. I can reject my null hypothesis and accept my hypothesis, which stated that the yeast growth would increase as the concentration of sucrose in the solution increases.
This happens because the yeast uses the sucrose to feed on. With the higher concentrations of sucrose, there is more food so the yeast can eat more and reproduce more. Because there is more sucrose the new generation of yeast cells have enough food to live on so they eat and reproduce as well. This continues until there is no sucrose left and the yeast cells die. This pattern is seen with bacteria.
3
- 1) Lag phase
2) Log phase
2 3) stationary phase
4) Death/decline phase
1
During the lag phase, the yeast or bacteria has been added to the nutrient. They take a while to start reproducing, as adjustments to the new conditions are needed. With my results there was a lag phase but the yeast started reproducing quickly due to the conditions being very favourable with little adjustment needed. The log phase is when nutrient, oxygen and the rest are in plentiful supply and reproduction happens quickly. Waste products are being built up but not to a harmful level. This type of growth is called exponential growth. This is the growth seen on my results, I didn’t leave any of my concentrations long enough to pass this stage as I was not measuring how long yeast cells could last in the solution. The stationary phase is when the population has reached equilibrium where the number of new cells is the same as the number of dead cells. Decline/death phase is when the number of living cells starts decreasing due to lack of nutrients and the build up of toxins
I could have got more conclusive results if I had looked at different solutions going up a % at a time and going higher then 10%. Another experiment I could carry out would be to see how long it would take the yeast to use all the sucrose by counting them day-by-day until they stop increasing in number. I think my data is very reliable because I only changed on thing, which was the concentration of sucrose. I made sure to keep everything else the same such as the temperature, pH, amount of sucrose and yeast used. I also counted a sample of each solution five times and shock it before I took the sample to make sure I got a fair reading of how many yeast cells were in there. However, I did get some strange results. On the 5th and 4th time, the number either stayed the same or increase but did not decrease which is what I would of predicted to happen and fits my average. However, on the 1st, 2nd and 3rd time there was some decrease before it continued to increase. The first decreased between 4% and 6%, the 2nd decreased from 4% to 8% and the 3rd decreased from 6% to 8%. To eliminate this I would need to repeat those three and see if they still decrease half way through.
There are a few limitations to this experiment. One was when we counted the yeast cells you couldn’t tell which were alive and which were dead. To improve my experiment I would only could the live yeast cells.
If I were to do this experiment again I would do the same except count them over a period of time to see how much they grow by. Also only count the live ones because that would give me a better overview. I would also leave them for longer until they reached the decline phase and see how long they took to reach it. In other experiments I could use another sugar instead of sucrose and see what the feed better on and use bacteria instead of yeast.
