Result Interpretation

This growth curve that we obtained from the experiment is not a common growth curve of the Saccharomyces cerevisiae. 

As we can see from the curve, the cell culture achieved 2 stages of log phase with a stationary phase in between.


Interpretation for the growth curve is as below:

Lag phase
Inoculation was done at 0th hour, and the cells went into lag phase for 2 hours. During this stage, the cells which were originated from a 500 ml shake flask were trying to adjust themselves to the new environment. Rate of growth of cells was low at this stage. Lag phase lasted for approximately 2 hours.

Log phase 1

At around 2nd hour, the cell biomass started to exhibit rapid growth. At this stage, the glucose substrate in the fermenter was fully utilized by the cells for growth and cell division. We observed a gradual increment of optical density from 1.600 to 8.270 within 4 hours.

Stationary phase

During the 6th hour to 12th hour, we observed the cells were experiencing fluctuation of growth. The Optical density of the cell biomass was maintained at 7.360, 8.900 and 8.630 for 8th, 10th and 12th hour respectively. The initial assumption we made was that, the cell biomass had reached its maximum due to depletion of the glucose substrate.

Log phase 2 

At 12th hour, we increased the air flow rate of the fermenter from 1.5 vvm to 3.0 vvm to test for test on our initial assumption. We found out that, increasing the air flow rate had also promoted the cells to enter another log phase. It appeared that our initial assumption on the depletion of glucose that had limited the growth of the cells were inaccurate. The actual limiting factor that had limited the growth of Saccharomyces cerevisiae from 8th to 12th hour was the oxygen concentration in the fermenter.  A rapid increment of cell biomass was observed from 12th to 14th hours, followed by a slower increment of cell biomass from 14th to 16th hour.

Decline phase

Starting from 16th to 18th hour, optical density of the cell biomass started to show decreasing trend. At this stage, we had observed another uncommon phenomenon of cell growth where the microorganism had skipped the stationary phase and directly entered the declining phase. A possible rationale is that, the glucose substrate in the culture had really depleted this time. An increase in air flow rate afterwards didn't increase the optical density of the cell biomass. Another reasoning our group came up with was that, the build up of toxic in the culture has inhibited the growth of the cell biomass. However, future study and experimental testing would have to be done to prove that assumption.