Modified yeast cells can process paste from the paper industry and produce xylitol and cellulose nanofibers, according to research by a team in Japan. Such products have unique characteristics with respect to their mechanical properties, film-forming properties, and viscosity and could be used in food, hygiene, medical, cosmetic, and pharmaceutical products. [Guirimand-Tanaka, G. et al., Green Chem. (2019); DOI: 10.1039/C8GC03864C]

Demand for these materials is high, but their manufacture comes at a price, economically and environmentally. At the moment, industrial production from purified D-xylose and cellulose fibers involve costly and polluting processes that uses catalytic hydrogenation. In order to solve these issues and realize a sustainable and environmentally-conscious society, there is an urgent need to make use of renewable biomass such as paper paste (Kraft pulp) and to develop innovative processes that avoid the use of catalysts and solvents.

In new work by Gregory Guirimand-Tanaka of Kobe University and colleagues could make the production process a little more sustainable and environmentally friendly. Their approach involves using renewable biomass such as paper paste (Kraft pulp). This material is 17% D-xylose. Using enzymes to release this sugar from the paste might be the obvious way forward, but enzymes are costly so the team has turned to a whole organism, the much cheaper yeast, instead. With a little genetic modification yeasts can produce the requisite enzymes on the cell surface, so the team refers to their technology as "cell surface display".

In their experiments the researchers used a modified strain of baker's yeast (Saccharomyces cerevisiae YPH499 strain) to express three enzymes - beta-D-glucosidase (BGL), xylosidase (XYL), and xylanase (XYN) - which are co-displayed on the yeast cell surface. Using this strategy, not only were they able to produce xylitol and cellulose nanofibers, but also to considerably increase the purity of the cellulose itself and the cost efficiency of the process by reducing the amount of commercial enzymes initially required by the process.

In a proof-of principle setup, the researchers were able to carry out production on a relatively large volume, for a laboratory setup, with two-liter fermenting jars. This, they suggest, points the way to the plausibility of bio-refinery industrial scale-up.