Along with all the coffee we drink every day, over 6 million tons of spent coffee grounds are produced annually worldwide. Some of these grounds are reused as biofuel but the rest are disposed of in landfills. Over the last decade, research has focused on how to reuse these grounds. The primary focus has been on the polysaccharides from the cellulose and hemicellulose in the ground up coffee bean's cell walls. Polysaccharides are used in composites, biopolymers, food packaging, construction materials and cellulose nanofibers (CNFs). CNFs specifically, which are cellulose reduced to nanoparticle size, 3 to 5 nm, have many uses in the food, cosmetic, and coating industries.
Japanese researchers from Yokohama National University pioneered a method that used spent coffee grounds as a new waste material to isolate CNFs using TEMPO-mediated oxidation in 2020. However, that left up to ~40% of the coffee grounds' hemicellulose unused. So, they turned their attention to holocellulose, the combination of hemicellulose and cellulose, to extract holocellulose nanofibers (HCNFs).
"Chemically unmodified and uniform quality HCNFs from agricultural/food waste are highly desirable for food additives such as emulsifiers. We hypothesized that the high hemicellulose contents in the holocellulose from spent coffee grounds and their unique structure could achieve completed nanofibrillation down to 3–5 nm wide and 1–3 μm long by mechanical disintegration," said Izuru Kawamura, a professor at the Faculty of Engineering at Yokohama National University. In fact, they not only formed HCNF, but they also discovered a method of preservative-free long-term storage of the HCNF with added benefits for transport and handling, thereby significantly increasing its utility for the food and cosmetic industries.
Their research was published in Carbohydrate Polymer Technologies and Applications on June 25.
To form HCNF out of the spent coffee grounds, the researchers removed lignin and lipids and then reduced the rest of the holocellulose fibrils to the nanoscale via nanofibrillation, the process of disintegrating fibril bundles into nanofibrils. The researchers used a jet mill with ultrahigh water pressure to mechanically nanofibrillate the holocellulose to form the HCNF.
The least degraded hemicellulose left in the spent coffee grounds after roasting is mannan. In the grounds, mannan has been shown to form a network between cellulose fibrils. This association is strong enough that even undergoing chemical treatments may not break it and, in some circumstances, mannan may recrystallize. The presence of mannan was essential in the ease of reconstituting the HCNFs after they had been freeze-dried. Generally during dehydration, the physical properties of nanocellulose change and they lose the ability to redisperse in water. However, when freeze-dried HCNFs were placed in room temperature water, a simple shake caused them to redisperse back into the nanoscale.
"The spent coffee grounds-derived HCNFs were completely nanofibrillated to 2–3 nm wide and 0.7–1 μm long, which was finer in width and shorter in length than general CNFs or HCNFs obtained by mechanical nanofibrillation, and desirable morphologies for food additives," said Noriko Kanai, assistant professor, Faculty of Environment and Information Sciences, Yokohama National University. Not only did they form finer and shorter HCNFs, but the discovery of the distinctive behavior of the HCNF in its freeze-dried state has many benefits. "The advantages of the once-freeze-dried HCNFs from spent coffee grounds are 1) preservative-free for long-term storage, 2) volume reduction during transportation, and 3) easy handling with only handshaking without solvent change or additional refinement process," said Kanai.
The research teams next project will move forward with the work they have done with HCNFs. "Dried HCNFs have some advantages for commercial use, such as long-term storage without preservatives and volume reduction for transportation. As a next step, we are exploring the possibility of upcycling spent coffee grounds-derived HCNFs as cosmetic and food additives," Kawamura said.
Other contributors include Kohei Yamada, Chika Sumida, Miyu Tanzawa, Yuto Ito, Toshiki Saito, Risa Kimura, and Toshiyuki Oyama from the Graduate School of Engineering Science, Yokohama National University; Miwako Saito-Yamazaki from GRACE Co., Ltd, Yokohama; Akira Isogai from the Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo.
This work was supported in part by JSPS KAKENHI and JST COI-NEXT program.
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