Performance of storage-degraded switchgrass in dilute-acid pretreatment and enzymatic hydrolysis

Feedstock storage is necessary in order to provide year-round supply to biorefineries. Switchgrass is typically harvested at low moisture contents and is generally considered stable in storage, but moisture accumulation during storage can lead to dry matter loss at the expense of structural sugars. Compositional analysis, dilute-acid pretreatment, and enzymatic hydrolysis were used to measure storage performance of switchgrass that entered storage at 13% moisture (wet basis) and was stored in uncovered stacks for 18 months. Top, middle, and bottom bales exited storage at 62%, 16%, and 27% moisture and experienced 37%, 10%, and 21% dry matter loss, respectively. Switchgrass stored in top bales was associated with lower glucan and xylan content (p<0.05) and lower glucose yields following combined dilute-acid pretreatment and enzymatic hydrolysis (p<0.05) when compared to switchgrass in the unstored, middle, and bottom bales. No significant differences were seen in the glucan and xylan content or in glucose yield between the unstored bales and the bottom and middle bales after storage. Xylose release from combined dilute-acid pretreatment and enzymatic hydrolysis was less affected by storage and not statistically different in any of the bale locations. A strong relationship existed between feedstock reactivity and dry matter loss in the switchgrass, with a 0.6% decrease in reactivity per every 1% dry matter loss suffered in storage (R²=0.81, p<0.0001). These findings stress the importance of moisture mitigation during outdoor storage of switchgrass.