Researchers suggest an optimal method of pretreating wheat straw to maximise the yield of xylitol. Whether this is cost-effective or environmentally beneficial is less certain.
Some types of sugar contain fewer calories than others and are therefore attractive as food additives. This applies to xylitol, which can be produced from trees and plants and has numerous other health benefits.
Today xylitol is produced chemically, but in a new study, researchers showed how to optimise the processing of wheat straw so that yeast can then ferment the xylose to maximise the yield of xylitol.
The research has been published in the Chemical Engineering Journal.
“Instead of using a chemical process to make xylitol, we can use a biotechnological process that can potentially be cheaper and more environmentally sustainable. However, this requires determining all the costs associated with the process and how the process affects the environment. We calculated these parameters in this and two subsequent studies,” explains a researcher behind the study, Nikolaus Vollmer, PhD Fellow, Department of Chemical and Biochemical Engineering, Technical University of Denmark, Kongens Lyngby.
Pretreating wheat straw with dilute acid before fermentation
Three processes are required to convert wheat straw into xylitol.
- The wheat straw is pretreated to release xylose, the feedstock of xylitol.
- The xylose is fermented with yeast to convert it to xylitol.
- Finally, the xylitol is separated from the impurities resulting from the fermentation.
Nikolaus Vollmer and colleagues investigated how to optimally pretreat wheat straw to maximise the xylose fraction available to the yeast.
The plant biomass is broken down into smaller molecules in two ways.
- The wheat straw is pretreated under pressure and at high temperature.
- The wheat straw is pretreated under pressure and at a slightly lower temperature, adding dilute sulphuric acid as a biocatalyst.
The results show that adding dilute sulphuric acid maximises the yield of xylose.
The researchers obtained only 5–10% of the potential yield of xylose from the wheat straw by using the first method, whereas the second method yielded more than 95%.
“In addition to the high yield, we observed a relatively low number of side reactions, which produce molecules that may then be toxic to yeast or must be removed before fermentation for other reasons,” says Nikolaus Vollmer.
Not necessarily better at fermenting xylitol
In further steps of the research, Nikolaus Vollmer and colleagues calculated whether using yeast to ferment xylose into xylitol is viable.
The question is not just whether it can be done but also whether it has economic and environmental advantages compared with the current chemical method.
The researchers included all steps in the process in the calculations, and the preliminary answer to the question on environmental and economic advantages is “maybe”.
Nikolaus Vollmer explains that the calculations include many uncertainties regarding the process equipment and the product prices, and that the determination could go either way depending on the method used to produce xylitol from wheat straw.
The size of the tanks used to run the process is one factor. This affects the cost because producing substantial volumes of xylitol in large tanks can save resources compared with smaller tanks.
Another option is using biologically optimised yeast strains, which ferment more efficiently or tolerate the toxic molecules better.
Regarding environmental impact, the new process of converting wheat straw to xylitol emits the same quantity of carbon dioxide as the current chemical process. However, the overall environmental impact depends on which energy sources supply the process with steam, cooling and electricity, how the wastewater from the process is disposed of and how the large quantities of wheat straw are transported to the plant.
“We estimate that the probability of breaking even by using fermentation to make xylitol is 50%. In terms of the impact on global warming, for example, the fermentation process itself has very low impact, which is really good. However, considering such factors as wind turbines using copper extracted by mining via toxic chemicals makes the situation less clear, and fermentation can then potentially have certain negative environmental effects relative to the purely chemical processes,” concludes Nikolaus Vollmer.