Monday, April 30, 2007

Evaluating herbaceous perennials as energy crops for production of fuel ethanol

Bruce S. Dien1, Hans-Joachim G. Jung2, Michael A. Cotta3, Patricia J. O'Bryan3, and Loren B. Iten3. (1) National Center for Agricultural Utilization Research, USDA-ARS, Midwest Area, 1815 N. University Street, Peoria, IL 61604, (2) Plant Science Research Unit, ARS, 411 Borlaug Hall, 1991 Upper Burford Circle, St. Paul, MN 55108, (3) National Center for Agricultural Utilization Research, USDA, ARS, Midwest Area, 1815 N. University Street, Peoria, IL 61604

Herbaceous perennial forages have potential use as energy crops for producing ethanol.  We have examined three species for which production systems exist:  alfalfa, reed canarygrass, and switchgrass.  These plant species represent a diverse set of physiologies, and include a legume and warm and cool season grasses.  Samples were collected at two or more maturity stages of each plant species.  Alfalfa leaves have value as a protein feed and, therefore, only the stems were considered.  The forage samples were analyzed for composition including, soluble sugars, structural carbohydrates, Klason lignin, and ash.  Total carbohydrates and lignin increased with maturity for the 3 plant species.  The forage samples were pretreated using dilute-acid and the resulting hydrolysates evaluated for enzymatic digestibility and ethanol yield.  Ethanol yield was determined by simultaneously saccharifying and fermenting the neutralized hydrolysates with S. cerevisiae D5A.  The alfalfa stems were found to require greater acid loading and a more severe pretreatment to obtain similar sugar yields compared to the other samples.  However, for all three species later maturity samples yielded less glucose  than immature forages.  Fermenting the hydrolysates was found to be problematic because of the presence of inhibitors.  Specifically, it was found that fructose readily degraded to HMF during the pretreatment and appeared to stall the subsequent fermentation.  This suggests that either soluble sugars will need to be extracted prior to treating with dilute-acid or alternate pretreatments will need to be considered.