Continuous fermentation of feed streams containingD-glucose andD-xylose in a two-stage process utilizing immobilizedSaccharomyces cerevisiae andPachysolen tannophilus

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Abstract
In the U.S., forest and crop residues contain enough glucose and xylose to supply 10 times the country's usage of ethanol and ethylene, but an efficient fermentation scheme is lacking,1,2,3 To develop a strategy for process design, specific ethanol productivities and yields of Pachysolen tannophilus NRRL Y‐2460 and Saccharomyces cerevisiae NRRL Y‐2235 were compared. Batch cultures and continuous stirred reactors (CSTR) loaded with immobilized cells were fed glucose and xylose. As expected from previous reports, Y‐2235 fermented glucose but not xylose. Y‐2460 consumed both sugars but fermented glucose inefficiently relative to Y‐2235, and it suffered a diauxic lag lasting 10‐20 h when given a sugar mixture. Immobilized Y‐2235 exhibited increasing productivity but constant yield with in creasing glucose concentration. In contrast, Y‐2460 exhibited an optimum productivity at 30–40 g/L xylose and a declining yield with increasing xylose concentration. Immobilized Y‐2235 tolerated more than 100 g/L ethanol while the productivity and yield of Y‐2460 fell by 80 and 58%, respectively, as ethanol reached 50 g/L. A 38.8‐g/L ethanol stream could be produced as 103 g/L xylose was continuously fed to Y‐2460. If it was blended with a 274 g/L glucose stream to give a composite of 23.7 g/L ethanol and 107 g/L glucose, Y‐2235 could en rich the ethanol to 75 g/L. Taken together these results suggest use of a two‐stage continuous reactor for pro cessing xylose and glucose from lignocellulose. An immobilized Y‐2460 CSTR (or cascade) would convert the hemicellulose hydrolyzate. Then downstream, an immobilized Y‐2235 plug flow reactor would enrich the hemicellulose‐derived ethanol to more than 70 g/L upon addition of cellulose hydrolyzate.