What is the stoichiometry of bioethanol fermentation like? If CO2 concentration suddenly decreased, what happened to the ethanol concentration. A shift in equilibrium is probably what happened if bioethanol concentration was as usual. Several factors could be responsible for equilibrium shifts during fermentation. As suspected by you, inhibitory compounds may have gathered. The limit of ethanol tolerance of the producing organism may have been exceeded. You could look at the substrate consumption rate of the organism at that point and may have to measure deacarboxylase activity too.

Hello, the initial CO2 production is high because of availability of easy consumable substances like sugar after that gradually by decreasing the easy cosumable substances the process the yeasts stars metabolize othe substances more slowly as result the CO2 production gradually decrease.

There doesn’t look like inhibition - I am assuming that the green triangles represent biomass? Have you tried a log-linear plot? Determining the doubling time would add to the evidence. Do you have any GC/MS data on the composition of the hydrolysate?

I agree with Dr. Karen A. Darbinyan - the initial CO2 production is because of the availability of the sugar and equivalent compounds which is consumed and the result is CO2 production. Once the metabolizable sugars are consumed, the yeast finds it very difficult to get sugar for its growth as well as for alcohol production. So, you may have to feed sugar/s slowly for the production of the required compound.

Thank you very much for your answers. Dr Prabakaran Krishnamurthi and Dr Karen A. Darbinyan , the initial sugar concentration was 110 g/L of glucose, mannose and xylose combined. By the time the CO2 rate dropped, only 20 g/L of glucose and minimal amounts of xylose and mannose were consumed. Is it common for yeast to decrease its sugar uptake rate when 90 g/L of easily metabolisable sugars are still available?

Thank you very much for your answer Dr Maurice Ekpenyong . The ethanol concentration (green triangles in the figure) continued to increase, and the ethanol production rate was rather constant in the first 24 hours. Unfortunately my labwork period is over, so I will not be able to measure the decarboxylase activity, but I kindly appreciate your input.

Thank you for your insight Dr Jim C Philp . The green triangles represent the ethanol concentration; the blue circles represent the biomass. The specific biomass yield was surprisingly low, only 0.01 g/g of substrate consumed. Regarding the hydrolysate composition, I only have some HPLC data available, which showed around 1 g/L of HMF, furfural, formic acid and levulinic acid, and around 6 g/L of acetic acid in the start of the fermentation. The acetic acid concentration remained unchanged, and the other inhibitors were slowly reduced through the course of the fermentation.

Thank you for your answer Dr Amit Kumar Mukherjee