I completed a fed-batch fermentation with the addition of spent sulfite liquor (SSL) (a waste stream produced in the paper and pulp process) to S. cerevisiae that terminated with a 60 (v/v) % SSL concentration. Prior to this I completed a batch fermentation with 60 (v/v)% SSL, where I incubated the yeast (in YPX) for 24 hours prior to the addition of the SSL. The yeast was incubated in batch mode (in YPX) for 96 hours before the addition of smaller volumes of SSL (for the fed-batch fermentation).
The ethanol concentration only increased by 3.10 % from the batch mode to the fed-batch mode. I was expecting a much larger increase since the fed-batch mode conditions the yeast to the inhibitory compounds present in SSL. A previous study conducted managed to obtain a 50 % increase in ethanol concentrations using the same substrate. The difference is that the initial batch mode for the fed-batch fermentation was only for 48 hours as opposed to 96 hours (as in my study).
Is there any explanation for the drastic difference in the results obtained and the poor performance in my study?
If a fed-batch mode operated for 48 h gave a 50% increase in ethanol yield, what rationale did you have for extending the fermentation time, if you are sure that the yield increase was solely due to fermentation time? I think you have your answer staring you in the face. Set up the two conditions and let us know, holding every other variable constant. Regards!
The reason for the time extension was limited time in the labs due to Covid. I had to inoculate on a Friday and could not gain access to the labs over the weekend to feed my SSL to the system.
I was possibly thinking that the scientific reasoning behind a small increase is due to the growth phase present when I inserted a portion of SSL. The yeast was already in the stationary phase which may have resulted in poorer performance. Let me know what your thoughts are on this, please.
Thanks.
Ethanol produced in batch process could be consumed by the yeast, and the microorganisms can be stressed for nutrient depletion and metabolites accumulation after long incubación time
Keeping it for longer time will reduce yields... There are literature for this, the yield plummet because ethanol is miscible in water... And it forms an acidic solution which affect the microbes there by leading to a decrease
Hello Danielle Nel;
Regarding your question, congratulations for the detail that you gave to explain your experimental procedure. This is good so that one can give you a more accurate answer or advice.
As other collegues have mentioned, the long time you leave the batch reactor running could be the cause to lower ethanol productivity at the end of the fed-batch reactor. Your guess seems to be right regarding the growth phase at which you started the fed-batch stage; in that case you could have started in the stationary or death phase.
I suggest that you first characterize the cell growth in your batch reactor. It is recommendable to start the fed-batch in an active growth-stage of your batch (full exponential phase), prior to the stationary phase. Inicial cell concentration of the fed-batch stage may be also important for good yields, so that is why commonly it is used the late exponential growth-phase for fed-batch start. After you have this information, you can define the time at which you would start your fed-batch stage. An obligued reference for this is the classical text book by Bailey and Ollis, Biochemical Engineering Fundamentals.
Apart from this, you said that you are comparing your results to a report of the literature. In this regard, you have to take into consideration the biochemical characterization of the SSL used in both studies, as well as the yeast strains used for fermentation. These are both important parameters that would influence the yields of the reactor. As SSL is a biological waste stream, it can vary widely in its chemical composition even from batch to batch.
I hope this helps for your research.
Best regards.
Hello Rogelio Valadez-Blanco
Thank you for your detailed answer and suggestions. The SSL implemented was identical to that of the past student's SSL (my research followed his paper). The yeast strain that I worked with was an improved strain to the one that he worked with, which was interesting for me. I think that I may have obtained better results if I inoculated in the exponential phase, indeed.
I will give the textbook a read! Much appreciated.
Regards
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