<br/>Plant cell wall structure consisting of several carbohydrate polymers encased in hydrophobic lignin polymer to protect against degradation, is an intricate and complex structure. This complex structure of the plants was designed to naturally restrict access to the carbohydrate polymers. Consequently, production of second-generation bioethanol from such a system in a commercially viable manner is not simple and thermochemical approaches are required to open this complex plant cell wall structure to improve enzyme access. However, the recalcitrant nature of lignin negatively affects efficient enzymatic access. Several different thermochemical pretreatments have been extensively developed and employed, but the exact nature of plant cell wall recalcitrance and the most efficient and economical approach to alter plant cell wall structure via pretreatment remains elusive. To understand the role of noncellulosic switchgrass polymers on the overall efficiency of pretreatment, the structural evolution of the noncellulosic polymers of the plant cell wall were investigated during dilute acid pretreatment (DAP) by employing in-situ small-angle neutron scattering (in-situ SANS). In this study, we observed real-time structural changes not possible to observe by any other technique. To deconvolute the structural evolution of lignin and hemicellulose polymers during DAP, native switchgrass (NATV), and isolated holocellulose (HOLO) and cellulose (CELL) fractions from NATV were studied. Our results show that aggregate particles first appear around 80 °C for NATV and HOLO samples. Due to the low temperature and pretreatment severity condition, these particles are likely derived from hemicellulose. The formations of much larger aggregate particles, only observed in the NATV sample, were attributed to lignin. For the HOLO sample, as the temperature and pretreatment severity condition increased, hemicellulose-derived aggregate particle sizes increased, suggesting this process was the nucleation and early stage formation of pseudolignin particles. Consistent with our interpretation of structural evolutions in NATV and HOLO samples, no formation of aggregate particles was observed in CELL samples for the entire duration of the pretreatment. These results suggest that not only lignin but also hemicellulose can form aggregate particles within plant cell walls during pretreatment.

Time-resolved in-situ reaction SANS study details structural changes to noncellulosic biopolymer in switchgrass plant cell wall

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