Newly isolated tolerant Issatchenkia orientalis strains as potential candidates for lignocellulosic bioethanol

Lignocellulosic biomass, abundant and low-cost material, has the great potential to be exploited for ethanol production. However, lignocellulose pre-treatments are needed to provide simple sugars to fermenting microorganism. Some of the pre-treatments result in by-products inhibiting the following fermentation step; additionally, during the process, the fermenting microbes could be subjected to other stresses, such as high ethanol concentrations, pH and temperature fluctuations. As a result, robust yeasts showing tolerance to inhibitors and high temperature, are desirable. This study focused on the isolation and selection of Issatchenkia orientalis strains exhibiting these traits. One hundred and fifty I. orientalis yeasts were isolated from grape marcs incubated at 40 °C in the presence of high levels of weak acids and furans. The strains were tested for their ability to grow at increasing temperatures (30, 40, 42 and 45 °C); their inhibitors tolerance was evaluated using both rich and minimal broths supplemented with increasing concentrations of inhibitors (weak acids and furans), added as single compounds or as cocktails. The most promising yeasts have been studied for their fermentative abilities in minimal medium supplemented with glucose (200 g/L) and an inhibitors cocktail. Few strains, incubated at 42 °C, produced high ethanol yield (about 88% of the theoretical) and could be very interesting as fermenting and inhibitor tolerant yeasts in large scale bioethanol applications.