Hemp cryo-milling and the impact of alkaline pretreatment on biochemical conversion

Authors: Stoklosa, Ryan; Latona, Renee; BERGER, BRYAN - University Of Virginia; TIMKO, MICHAEL - University Of Virginia; SHLANTA, ANDREW - Air Products & Chemicals; HIMES, MICHAEL - Air Products & Chemicals

Submitted to: ACS Sustainable Resource Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2024
Publication Date: 4/22/2024
Citation: Stoklosa, R.J., Latona, R.J., Berger, B.W., Timko, M.P., Shlanta, A.V., Himes, M.R. 2024. Hemp cryo-milling and the impact of alkaline pretreatment on biochemical conversion. ACS Sustainable Resource Management. https://doi.org/10.1021/acssusresmgt.4c00005.
DOI: https://doi.org/10.1021/acssusresmgt.4c00005

Interpretive Summary: Producing bio-based fuels and chemicals requires cultivating greater amounts of plant biomass. Recent interests have focused on cultivating a well-known fiber crop, industrial hemp, for use as a bioenergy feedstock. Hemp has been utilized in the textile industry for centuries, but due to regulations it was restricted for growing in the United States. Most of these regulations have been lifted or are being eased, and this allows farmers to grow the crop as a commodity. Unlike other traditional bioenergy feedstocks such as switchgrass, hemp requires a pre-processing step that separates the fibrous portion of the stalk from the inner core of the plant, which is termed hurd. This process is known as decortication, and can be done by a variety of methods. In this work industrial hemp was subjected to cryo-milling with liquid nitrogen to separate the fiber and hurd portion, and to reduce particle size. The cryo-milled hemp was then pretreated with an alkaline chemical. Due to the effects of cryo-milling, a lower amount of chemical was required to pretreat the hemp. Following pretreatment the hemp was subjected to a process known as enzymatic hydrolysis, which breaks down the polysaccharides in the plant cell wall to sugars. The sugar yields reached 80-90% even with a lower chemical loading used during pretreatment. The sugars recovered from the hemp could then be fermented to a chemical known as 2,3-butanediol (2,3-BDO), a precursor chemical for various applications including sustainable aviation fuel. The fermentation process using sealed reaction vessels to limit oxygen content had improved production of 2,3-BDO reaching approximately 20 g/L while also limiting unwanted byproduct generation. Cryo-milling of hemp results in lower chemical requirements, and thus lower severity, for chemical pretreatment while still achieving high sugar yields during downstream processing that results in a suitable media for fermentation to produce bio-based chemicals.

Technical Abstract: Cannabis sativa, commonly known as hemp, is a fiber crop that has historically been used for manufacturing textiles. Hemp has gained more attention recently as a potential bioenergy crop as regulations on cultivation have been loosened in countries such as the United States. As a lignocellulosic feedstock hemp requires chemical pretreatment before biochemical conversion, but also an added processing step to separate bast fiber and hurd. A way to accomplish this is with cryo-milling that can also reduce chemical loadings and severity of the pretreatment process. This work used liquid nitrogen (LN2) to cryo-mill hemp (CMH) that was then chemically pretreated with NaOH. Increasing NaOH chemical loading removed more hemicellulose and lignin, but moderate NaOH loadings provided the best condition for sugar recovery following enzymatic hydrolysis with glucan yields approaching 90% and xylan yields at 80%. Pretreated CMH hydrolysate was then fermented by the bacteria strain Paenibacillius polymyxa to produce the platform chemical 2,3-butnaediol (2,3-BDO). Under anoxic conditions P. polymyxa generated approximately 19 g/L of 2,3-BDO with no observation of reversible conversion into acetoin. These results show the utility of cryo-milling hemp before chemical pretreatment to lower processing severity while maintaining favorable product yields during biochemical conversion.