"Chemostat Evolution" Used to Develop an Ethanol-fermenting Yeast in Lignocellulosic Hydrolyzates at High Solids Loading

Scientists from the Microbiology Department, University of Georgia (United States) report the use of "chemostat evolution" for the development of a Saccharomyces cerevisiae strain which can ferment lignocellulosic-hydrolyzates at high solids loading, even in the presence of toxic levels of inhibitory compounds. Lignocellulosic-hydrolyzate is the fermentable material after the pretreatment of lignocellulosic biomass (to "deconstruct" the plant cell walls) and subsequent saccharification (to convert carbohydrate polymers in the deconstructed plant cell walls, into simple sugars).

Saccharomyces cerevisiae is the common microorganism (a type of yeast) for the fermentation of lignocellulosic-hydrolyzates into ethanol, but these organisms do not perform well at high solids loading, and are often inhibited by pretreatment by-products (substances such as fufural and acetic acid, which are present in lignocellulosic-hydrolyzates). The scientists attempted to developer an improved strain of Saccharomyces cerevisiae by subjecting the yeast to selective pressure (i.e.,lignocellulosic (softwood pine) hydrolyzate at high solids loading (17.5% w/v solids) with inhibitory compounds) in continuous culture (technically called a "chemostat").

The chemostat was repeatedly operated for many cycles. During the long operation at many cycles, the yeasts are considered to "adaptively evolve" under these conditions of selective pressure. The scientists reported that they obtained an evolved strain of Saccharomyces cerevisiae (AJP50), which possessed "greater fermentation capability than its parent in both rich liquid media supplemented with various combinations of inhibitory compounds, and in pretreated pine fermentations at high solids loadings." The full paper can be accessed in the open-access journal, Biotechnology for Biofuels (URL above).