Fermentation and yeast growth rate, what is the link ?

Does fermentation have an impact on the yeast’s growth 

rate? 

This is the problematic of my project. I was interested at the beginning by the possibility of certain bacteria to live in anaerobic condition ( without oxygen), the one which use fermentation to grow. But for some material problems I specifically interested myself to yeast which do fermentation when 02 is present.

Fermentation is a way to produce energy and this energy allows yeast and cells in general to grow. Fermentation does not produce only energy, it uses glucose and casts ethanol and C02. It allows us to measure the fermentation. We can measure the product of the fermentation, because I don't have material to do that, I used the relation between C02 and pH. When C02 increase in the tube, it reacts with H20 and produce H+, so the pH decrease or become more acidic.

In the experiment I did, I tested 4 different concentrations of glucose, 0 %, 0.06 %, 2 % the normal concentration in YPD solution and 5 %. The YPD is a media that we used to grow yeast in lab. 

In this experiment I used baker's yeast ( the yeast we use in some cake ! ). I prepared 4 different tubes with 3 replicas for each one. I put YPD , yeast and glucose in the tubes ( I don't put glucose in all the tubes, just in 0.06% of glucose and 5 % glucose tubes, because I used YPD with glucose already in the media and YPD without glucose in).

Then I took 1mL of each solution and I put it in a cuvette to measure the absorbance of the sample with a spectrophotometer at 600nm . I measure the pH of the sample with pH paper every hours.

Growth rates and pH variation according to the glucose concentration

The first graph represents the absorbance according to the time and the second one represents the pH according to the time. We can see on the two graphs that yeast growths faster when there are 2% of glucose in the media and it is the same for the pH, it is more acid when the media is normal. This is mean that there is more production of C02, so the activity of fermentation is more important.

This project shows that there is a link between yeast growth and fermentation. The results show that pH and growth rate involve in the same way. In fact the percentage of glucose in solution is the same when production of fermentation is the most important and when growth rate is the most important. This is the same for the other concentration of glucose ( 0% and 5%).

 

 Next we can improve the experiment by being more precise. Because of the pH paper my measures are subjective. If the experiment would be done with a pH probe, the measurement would be more precise and more objective. It could be nice to test different concentrations of glucose could in order to see if yeast grow faster with another concentration than 2%.

If you want to know more, you can look at the links :  

- Campbell Biology

- The Crabtree Effect : A Regulatory System in Yeast