Learning Thru Research: Yeast Recovery after a Heat Shock

May 3, 2017

Yeast Recovery after a Heat Shock

On the 13th of april, our uncle, a baker, realized that he didn’t have a lot of yeast left. Although, he still had many breads to bake as they had already been ordered by his customers… So he couldn’t wait until his next delivery. He decided to cultivate his bakery yeast and make them divide in order to have more of them. But his yeast had been left next to the oven, and exposed to very high temperatures. Therefore he asked us, scientific students, if this heat shock would have an impact on the growth rate of his yeast.

We went to our lab, and wondered: how long do yeast take to recover from a heat shock ? A heat shock can denature proteins, and therefore affect the ability of yeast to function normally. But in which proportions can they survive and progressively recover from that heat shock ? How much and how long does it affect their metabolism ? We knew that when yeast are in good conditions, they divide faster; but damaged yeast don’t divide, so we focused on the impact of a heat shock on their growth rate. The optimal temperature for yeast is between 25 and 37°C, and at this temperature yeast usually divide every 1h30/2h. To study the growth rate, we also took in consideration the fact that a heat shock is a short exposition to a high temperature, 45°C or above for yeast.

We prepared the medium with 150ml of YPD, 150ml of water, 6g of glucose. we prepared an erlenmeyer with 100ml of medium with 5g of yeast, incubated this erlenmeyer during 2 hours. We put 1ml of the erlenmeyer in 9 tube ependorff. 3 sample during 5minutes at 50°C, 3 at 55°C and 3 with no change of heat. Put 200ul of each ependorff in 9 erlenmeyer, incubate. Every 20-30minutes take 200μL of each erlenmeyer and make a spectrophometry. Take 8μL of the first erlenmeyer ad 0,8μL of phloxine B and count the dead cells at the microscope.

Our uncle came to join us to know our results !

- Look, Uncle, We used Sagemath, to make a graph to see our results. But we had a big problem,we observed that the green curve decreases, but, normally it should increase ! The red curve corresponds to the sample of yeast exposed to 50°C. The blue curve show the sample of yeast exposed to 55°C, and the green curve corresponds to the sample of yeast without heat shock. Normally, the sample unexposed to heat shock, should grow... But, we have a curve which decreases.

- Ho.. and what is the reason of this result? ask the baker.

- We don’t know… Maybe we made a mistake of manipulation in the dilution…


Absorbance measured in function of time

No heat shock 50°C heat shock 55°C heat shock

Counting Chamber

- Here we tried to count all the dead cells, we used phloxine B that colored the dead cells. We can see that our results are weird, particularly for 50°C

- Why ?

- Because we can see only one cell, we must have made a mistake with the dilution. But we can see that for no heat shock there are more white cells, living cells than for 55°C. For 55°C there are more cells but more colored cells so it means that a lot died.

We can conclude that our experience has not really worked because we probably made mistakes and we ran out of time. We had a problem with our protocol: we found it the day we did the experiment so we lost a lot of time. But it was interesting; if we had had more time and a better comprehension of our protocol we could have observed better results. We can say that we had a good idea but we had a lot of problems. Although, we can say that after a heat shock at 50°C, 55°C the yeast can recover.

Maya Saidi, Anais Martin, Camille Bonne

If you want to know more :

Yeast ? But what is it more precisely ? : Click here

Heat shock on Yeast: McAlister L., Finkelstein D. B. (1980) Heat shock proteins and thermal resistance in yeast. Biochemical and Biophysical Research Communications.