November, Novembre, Noviembre

Hey there,

Against all odds this month we were able to finish our research project and we even prepared a poster. Here’s a briefing of our findings.

First, for the sake of a recap (da cappo) I will remind you a few things. Ethanol has been produced by fermenting sugar cane cellulose since almost forever, at least 30 years if you are thinking about Brazil using ethanol as fuel and many more if you are thinking about rum production. Yet, when ethanol is produced only from cellulose, many parts of the cane are wasted. These wastes are either burned or sent to fill up a space at a landfill. Most of it is known as bagasse which is rich in hemicellulose and lignocellulose. The objective of our project is to make use of these parts of the cane to reduce waste. Now, sugars obtained fron lignin hydrolization have been found to be very hard to ferment by wild yeasts. Nonetheless, hemicellulose can be hydrolized into xylose and glucose. Fermenting glucose is piece of cake (relatively) but xylose fermentation is a tougher case and this case is what we chose as our project.

Unlike glucose, xylose can only be fermented by a limited number of wild yeast strains. During this project, the main objective is to determine the kinetics and ethanol yields of xylose-fermenting yeast strains Pichia stipitis, Pachysolen tannophilus and Candida shehatae to screen for the most efficient xylose-fermenting strain. Batch fermentations for each strain were conducted at a pH range between 4.5 and 5.5 and, temperature and agitation of 32 C and 115 rpm, respectively. Batch culture data and ethanol production were determined by means of periodical optical density measurements, dry biomass weighing and HPLC analysis for xylose and ethanol. 

The maximum specific growth rate, μ_max, overall ethanol yield, Y_P/S and Y_P/X, and overall biomass yield, Y_X/S , were obtained for each strain. Y_P/S is defined as ethanol yield as a function of substrate (xylose) concentration, Y_P/X as  ethanol yield as a function of yeast cell concnetration and as Y_X/S cell concentration as a function of xylose concentration. During experimentation, Pachysolen strain showed random data variability so we were not able to obtain coherent results for it. We did obtain results for Candida and Pichia.
For Pichia stipitis and Candida shehatae μ_max values were 0.036 h^-1 and 0.0586 h^-1, respectively. Based on data analysis, Pichia stipitis offers the highest ethanol yield from xylose fermentation.

The next stage of this project will be to combine Pichia Stipitis and an effective glucose fermenting yeast in a co-fermentation of xylose and glucose. The objective of this stage is to develop a process in which hemicellulosic and cellulosic elements of sugar cane can be employed simultaneously in the production of ethanol.

I hope this was informative and interesting enough for you. Happy holidays. Joyeux Noël =)