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Title: Pre-senescence Harvest of Switchgrass Inhibits Xylose Utilization by Engineered Yeast

Proper timing of switchgrass harvest for bioenergy is important to maximize yield and optimize end use conversion. Proposed windows range from peak biomass to the following spring after overwintering in the field. There are various pros and cons associated with harvest timing: earlier harvests maximize yield but can remove nutrients from the field that may require replacement, while later harvests have reduced biomass yields due to weathering but maximize nutrient resorption in belowground tissues. Switchgrass composition changes during the harvest period, with losses of potential fermentation nutrients (amino acids and minerals), and sources of pretreatment-derived inhibitors (soluble sugars), which could affect downstream conversion by microorganisms. For this work we investigated whether switchgrass harvest could be timed to maximize beneficial impacts on fermentation. Switchgrass samples were harvested from five replicate field plots in Wisconsin, roughly every 2–3 weeks from peak biomass (Aug. 20) until after the killing frost (Nov. 7). Cell wall composition showed little consistent variation with harvest date while bulk biomass analysis showed a relative increase in cell wall content (lignin and structural sugars) and loss of extractives (minerals, protein, soluble sugars, and others). Following high or low severity AFEX pretreatment and high solids enzymatic hydrolysis (6% glucan loading),more » two field replicates were fermented using Saccharomyces cerevisiae 424A, a strain engineered to utilize xylose in addition to glucose. For both pretreatment severities, S. cerevisiae 424A grown in hydrolysates from the three earlier harvests utilized only a small fraction of available xylose, while almost complete utilization occurred within 96 hr for the last three harvest dates. Detailed analysis of the hydrolysate low molecular weight aromatics did not indicate any compounds potentially responsible for the inhibition, with most of the observed variation in their concentration due to pretreatment severity. Amino acid composition also did not appear to be limiting. Current indications point to a plant-generated compound that degrades during senescence, which future work will attempt to identify. Ultimately this work demonstrates that, although an attractive option to maximize yield, harvesting switchgrass before it begins senescing could have a negative effect on downstream conversion processes.« less
 [1] ;  [2] ;  [3] ;  [4]
  1. Michigan Technological Univ., Houghton, MI (United States). Department of Chemical Engineering and DOE Great Lakes Bioenergy Research Center
  2. Univ. of Tennessee, Knoxville, TN (United States). Department of Chemical and Biomolecular Engineering; Michigan State Univ., East Lansing, MI (United States). Department of Chemical Engineering
  3. Michigan State Univ., East Lansing, MI (United States). Department of Chemical Engineering
  4. University of Wisconsin-Madison, Madison, WI (United States). Department of Agronomy and DOE Great Lakes Bioenergy Research Center
Publication Date:
Grant/Contract Number:
SC0018409; FC02-07ER64494
Published Article
Journal Name:
Frontiers in Energy Research
Additional Journal Information:
Journal Volume: 6; Journal ID: ISSN 2296-598X
Frontiers Research Foundation
Research Org:
Great Lakes Bioenergy Research Center, Houghton, MI (United States)
Sponsoring Org:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23)
Country of Publication:
United States
09 BIOMASS FUELS; Biomass composition; Fermentation inhibition; Harvest timing; lignocellulosic biofuel; Switchgrass; Xylose utilization
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1459517