Monday, May 4, 2009
9-17
A simultaneous isomerization and fermentation (SIF) process for efficient co-fermentation of hexose and pentose sugars
Dawei Yuan, Bioengineering, The University of Toledo, 2801 W. Bancroft St., MS 303, Toledo, OH 43606, Sasidhar Varanasi, Department of Chemical & Environmental Engineering, The University of Toledo, 2801 W Bancroft street, W Central Ave, Apt # C10, Toledo, OH 43606, and Patricia Relue, Department of Bioengineering, The University of Toledo, 3062 Nitschke Hall, 2801 West Bancroft Street, Toledo, OH 43606.
Cost effective production of ethanol from lignocellulose requires full utilization of both hexose and pentose sugars. Glucose is readily fermented to ethanol by Saccharomyces cerevisiae, better known as baker’s yeast. Xylose, the major pentose sugar released from the hydrolysis of hemicellulose, is not fermented by this same yeast. However, isomerization of xylose to xylulose makes it possible for baker’s yeast to convert both the C6 and C5 sugars to ethanol. The xylose isomerization is accomplished using commercially available immobilized xylose isomerase (XI). To accomplish the isomerization under conditions suitable for sugar fermentation, we co-immobilize urease with the XI and add urea to the biomass hydrolysate. In order to reclaim and reuse our co-immobilized enzymes from the yeast fermentation, we have developed a system which incorporates a packed bed reactor for isomerization and a hollow fiber membrane fermentor (HFMF) for sugar fermentation by yeast. The co-immobilized enzyme pellets are placed in the packed bed reactor and yeast cells are loaded on the shell-side of the HFMF. By connecting the two units in series, SIF is achieved under fermentation conditions. This configuration avoids direct contact between the enzyme pellets and the yeast, facilitating recovery and reuse of the enzyme pellets. Furthermore, high yeast density on the shell side is achieved which is crucial for efficient xylulose fermentation.