Bioethanol production from mixed sugars by Scheffersomyces stipitis free and immobilized cells, and co-cultures with Saccharomyces cerevisiae

Bioethanol can be produced from several biomasses including lignocellulosic materials. Besides 6-carbon sugars that represent the prevalent carbohydrates, some of these feedstocks contain significant amounts of 5-carbon sugars. One common limit of the major part of the xylose-fermenting yeasts is the diauxic shift between the uptake of glucose and xylose during the fermentation of mixed syrups. Thus, optimized fermentation strategies are required. In this paper the ability of Scheffersomyces stipitis strain NRRLY-11544 to ferment mixed syrups with a total sugar concentration in the range 40-80 g/L was investigated by using mono cultures, co-cultures with Saccharomyces cerevisiae strain Bakers Yeast Type II and single cultures immobilized in silica-hydrogel films. The experimental design for the fermentations with immobilized cells included the process analysis in function of two parameters: the fraction of the gel in the broth and the concentration of the cells loaded in the gel. Furthermore, for each total sugars level, the fermentative course of S. stipitis was analyzed at several glucose-to xylose ratios. The results indicated that the use of S. stipitis and S. cerevisiae in free co-cultures ensured faster processes than single cultures of S. stipitis either free or immobilized. However, the rapid production of ethanol by S. cerevisiae inhibited S. stipitis and caused a stuck of the process. Immobilization of S. stipitis in silica-hydrogel increased the relative consumption rate of xylose-to-glucose by 2-6 times depending on the composition of the fermentation medium. Furthermore the films performances appeared stable over three weeks of continuous operations. However, on the whole, the final process yields obtained with the immobilized cells were not meaningfully different from that of the free cells. This was probably due to concurrent fermentations operated by the cells released in the broth. Optimization of the carrier characteristics could improve the performances of the process with immobilized cells.