Solving ethanol production problems with genetically modified yeast strains

Abstract

The current world demand for bioethanol is increasing as a consequence of low fossil fuel availability and a growing number of ethanol/gasoline flex-fuel cars. In addition, countries in several parts of the world have agreed to reduce carbon dioxide emissions, and the use of ethanol as a fuel (which produces fewer pollutants than petroleum products) has been considered to be a good alternative to petroleum products. The ethanol that is produced in Brazil from the first-generation process is optimized and can be accomplished at low cost. However, because of the large volume of ethanol that is produced and traded each year, any small improvement in the process could represent a savings of billions dollars. Several Brazilian research programs are investing in sugarcane improvement, but little attention has been given to the improvement of yeast strains that participate in the first-generation process at present. The Brazilian ethanol production process uses sugarcane as a carbon source for the yeast Saccharomyces cerevisiae. Yeast is then grown at a high cellular density and high temperatures in large-capacity open tanks with cells recycle. All of these culture conditions compel the yeast to cope with several types of stress. Among the main stressors are high temperatures and high ethanol concentrations inside the fermentation tanks during alcohol production. Moreover, the competition between the desired yeast strains, which are inoculated at the beginning of the process, with contaminants such as wild type yeasts and bacteria, requires acid treatment to successfully recycle the cells. This review is focused on describing the problems and stressors within the Brazilian ethanol production system. It also highlights some genetic modifications that can help to circumvent these difficulties in yeast.

Ethanol production in Brazil: a bridge between science and industry

“…Despite reaching alcoholic fermentation yields as high as 92%, there are several scientific and technological challenges to be overcome for first and second-generation ethanol 22, 100. In comparison to corn fermentations in the USA, Brazilian distilleries run their processes for eight months (or less) in a year because sugarcane cannot be harvested during the rainy season and cannot be stored as corn can.…”

Current progress in high cell density yeast bioprocesses for bioethanol production

“…However, utilization of both flocculating and immobilized cells will always result in mass transfer limitations due to the diffusion‐limited mass transport to cells that are not in immediate contact with the well mixed medium. Flocculation is thus not always desired [21]. On a case to case basis, the benefits of easier cell reuse at high cell density must be weighed against the drawbacks of lower specific productivity because of limited mass transfer rates.…”

Comparative analysis of key technologies for cellulosic ethanol production from Brazilian sugarcane bagasse at a commercial scale

“…The higher fermentation times required for lignocellulose sugars, in comparison with those required for first‐generation sugars or molasses, is a big concern for 2G ethanol production. The stability of the fermenting microorganism in fermenting lignocellulose hydrolysates in repetitive cycles with consistent ethanol production is important because genetically modified microorganisms (GMO) are used currently . Another concern is the disposal of the microbial biomass because GMO cannot be either discarded in an open environment or fed to animals.…”