More bioethanol with yeasts?


A great amount of ethanol fuel is currently produced by starch fermentation. This starch comes from grains such as wheat and maize. Fermentation is produced by a yeast culture. The digestion of starch by yeasts is done in two stages: the starch is initially hydrolyzed in sugars by a chemical or enzymatic process then sugars are converted into alcohol and carbon dioxide by yeasts. This operation is effective and extracts the solar energy stored in the starch.

This transformation uses noble agricultural products and competes with their food use. It would be more interesting to use cellulose contained in the stems and the sheets of the cereal plants, in particular maize. This cellulosic material is much more abundant than the grain and does not have a great economic value. However the yeasts are not usable for this operation because they do not have the capacity to hydrolyze cellulose in sugars. Many researches are currently made to industrialize this stage of hydrolysis of cellulose. The realization of this stage will give access to the biofuel of second generation, produced in greater quantity and at lower cost.

A part of bioethanol fuel is produced by a discontinuous process. The digestion of the hydrolyzed starch by yeasts is done in large industrial bioreactors. When the totality of the starch is digested, the bioreactor is emptied. The yeasts are recovered to treat the next batch. Produced alcohol is concentrated by distillation.

It seems to me that this discontinuous process could be used for experimentation with the aim to obtain bioethanol with cellulose. Because of this discontinuous operation, the yeasts are submitted to a food stress when they finish transforming the sugars in the bioreactor. This state of stress is favourable to impose a controlled evolution on yeasts.

The technique of controlled evolution consists, by combining stress and food, to obtain a genetic transformation of a biological culture. This process has already obtained the evolution of bacterial cultures for the digestion of organochlorinated compounds which, normally, are not degraded by the bacteria. The bacterial cultures, put in a state of food stress and in the presence of organochlorinated compounds, developed a capacity to digest these compounds.

The controlled evolution applied to yeasts could consist in adding in an industrial bioreactor producing bioethanol a small quantity of cellulosic material. The food stress that the yeasts endure when they digested the most part of sugars could make them develop a capacity to hydrolyze cellulose.

The hydrolysis of cellulose is made via several enzymes named cellulases. A study published in may 2008 (*) has shown the presence of genes coding several cellulases in the genetic code of Sacchromyces Cerevisiae, the yeast used for the production of bioethanol. The goal of the controlled evolution is to push the expression of these genes coding cellulases.

The great volume of industrial bioreactors should be a favourable factor to the evolution of yeasts. The probability of appearance of a genetic mutation of yeasts is proportional to the number of yeasts submitted to the experimentation, and this number is much greater in industrial bioreactor than in a laboratory bioreactor.

The cost of this experimentation could be moderate because it could be inserted in the normal process of production of bioethanol:
- at the start of fermentation, a small quantity of cellulosic material is added in the bioreactor with the hydrolyzed starch and the yeast culture,
- at the end of fermentation, when the bioreactor is emptied, a filtration permits to recover yeasts and the no digested cellulosic material, whereas a distillation concentrates the produced ethanol,
- the recovered yeasts are used to treat the following batch of starch, the presence of cellulosic material in the culture maintains the evolutive pressure on yeasts.

By this cheap experimentation on the industrial production of bioethanol, we could perhaps obtain a slow evolution of yeasts toward a biofuel based on cellulose.

(*) : Genome sequencing and analysis of the biomass-degrading fungus Trichoderma reesei, Nature Biotechnology, Volume 26, Number 5, May 2008.