Newswise — Generating biofuels from agricultural sources is nothing new; the United States has been using corn to generate ethanol for decades. Until now, corn has been a passable biofuel source, but it is an annual crop requiring yearly planting, costly fertilization and irrigation as well as a narrow harvest window dependent on grain maturity. The benefits of using dedicated energy crops, crops grown with the sole purpose of being used to create fuels, give plants like switchgrass a hefty leg up over corn.
The current viable harvest window for switchgrass extends from peak biomass, when the grass is at its biggest before it begins to degrade from weathering, to the first killing frost, when the plant shuts down completely for the winter. The longer the plant stays in the field, the more nitrogen returns to the soil—a clear environmental benefit—but this extra time reduces the amount of biomass harvested from the crop. Future biofuel manufacturers may want to harvest multiple times per season to efficiently use storage facilities, while farmers must consider weather and the harvest times of their other crops. A conundrum.
Nailing down a definitive answer to the switchgrass harvest debate is something Rebecca Ong, assistant professor of chemical engineering, is exploring in her research. Working with researchers from the University of Tennessee, the University of Wisconsin-Madison and Michigan State University, Ong monitored and sampled several stands of Wisconsin switchgrass to better understand the costs and benefits to harvesting at various points in the season.
“Previously, people had been saying ‘Well, you can harvest any time. Earlier is better [for processing], but you can really harvest any time.’” Ong says. “And what we’re finding is that this may not be true. So, you may have a more limited harvest window than we were expecting.”
Biofuels require fermentation. The types of sugars present in each energy crop vary, and the microbes often need some persuading to use certain sugars like xylose, one of those found in switchgrass. Previous researchers at Purdue University have engineered yeast, the microbe involved in switchgrass fermentation, to break down xylose, but the process can be fickle and sensitive to inhibition.
“Something we’re trying to do in our lab,” Ong says, “is break up that polymer, break all the links so that we have those individual sugars and then convince the microbes that they want to actually eat them.”
In the later samples from Ong’s study, the yeast didn’t need convincing. The xylose had become consumable.
Ong believes this change is due to the plants senescing, when they begin to go dormant and dismantle compounds in their stems and leaves for storage and reuse the following growing season. When the switchgrass was harvested before senescence, one or more of these compounds might have been interfering with the microbes’ ability to digest the xylose. Giving the plants more time in the field may have made the switchgrass sugar more appealing to the yeast.
“Most of the time,” Ong says, “people say ‘harvest early, get the highest conversions,’ but no one has taken it through fermentation to see what the microbes do in the process.”
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