Cellulose and lignocellulose nanofibrils and amphiphilic and wet-resilient aerogels with concurrent sugar extraction from almond hulls

Authors: Patterson, Gabriel; Orts, William; McManus, James; HSIEH, YOU-LO - University Of California, Davis

Submitted to: ACS Agricultural Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2022
Publication Date: 12/23/2022
Citation: Patterson, G.D., Orts, W.J., McManus, J.D., Hsieh, Y. 2022. Cellulose and lignocellulose nanofibrils and amphiphilic and wet-resilient aerogels with concurrent sugar extraction from almond hulls. ACS Agricultural Science and Technology. 3(1):140-151. https://doi.org/10.1021/acsagscitech.2c00264.
DOI: https://doi.org/10.1021/acsagscitech.2c00264

Interpretive Summary: Tree nuts are a major global agricultural commodity, and almonds specifically account for the largest share at nearly 2 million metric tons in 2020/2021. California contributes over 85% of the global almond production and, as a result, generates an increasing fraction of under-utilized coproducts – the shells and the hulls – at nearly three times the dry weight of the kernels. At approximately a 2:1 mass ratio to the kernel, the sugar-rich and fleshy almond hulls constitute the largest coproduct. Currently, almond hulls are being used in animal feed products, but the combination of increased almond production and the reduction in the number of dairy cattle in California has created a need to find additional uses for them. This research focused on finding a way to increase the value of almond hulls by developing a concurrent process to extract sugars and also make high value-added nanocellulose materials. Nanocelluloses are desired for their exceptional strength and thermal stability properties but the ability to make them on a profitable basis has eluded many companies. The process described in this paper helps open the door to create multiple value streams by a single process and prevent almond hulls from going to landfills.

Technical Abstract: With downward pressure on the value of almond hulls (AH), the major coproduct from the largest tree nut crop globally, a streamlined process was introduced that combines production of several grades of cellulose nanofibrils, CNFs, and aerogels with concurrent sugar extraction. Hot water extraction of almond hulls from a soft-shell variety produced 49% (by dry weight) water-soluble sugars with 51% lignocellulosic fraction used for CNF processing following three protocols of increasing complexity. A streamlined aqueous treatment of the AH in either 4% NaOH or NaClO2/KOH yielded 15% alkali cellulose or 12% cellulose. Coupled TEMPO and blending converted these into 88%, 91%, and 95% yields of lignin-containing micro/nanofibrillated lignocellulose (LCMNF), alkali cellulose nanofibrils (ACNFs), and cellulose nanofibrils (CNFs) with similar 4:1 width-to-thickness aspect ratio and ultra-high length-to-thickness aspect ratios of 1469, 1943, and 799, respectively. The LCMNF aerogel was the most wet-resilient, with superior dry and wet shape recovery and wet-stability, whereas the aerogel from the thinnest and long ACNF (1.05 nm thick, 2.08 µm long, 1.02 mmol/g) had the highest compressive Young’s Modulus (31.6 kPa/(mg/cm3). The isolation of nearly equal parts of free sugars and highly efficient production of CNFs with unique qualities provide a synergy that could help drive these combined processes toward commercial adoption.