Healthy sweetness from sour waste
Some waste materials are just too precious to be simply thrown out. The pulp and paper industry produces large quantities of the by-product hemicellulose, a substance which potential to date received hardly any recognition. By means of new procedures, however, this hemicellulose can be turned into valuable products - for example xylitol, a sweetener, which has an intense taste, helps to prevent tooth decay, and only causes low insulin release. Researchers at the TU Wien succeeded in combining chemical, enzymatic and biological process steps. With the help of specific designer microorganisms, it is now possible to produce xylitol from hemicellulose in an environmentally friendly way in only one procedural step.
In contemporary biotechnology, certain microorganisms are being used purposefully. “Living cells are always extremely complex, which is why to date they have been regarded simply as a black box. And researchers have been using trial and error to find a way how to improve the production process”, says Prof. Christoph Herwig (Institute of Chemical Engineering). “In contrast, we are taking a different route: For us, it is important to understand what exactly happens in the cells, to improve them purposefully.”
To do so, the species Sulfolobus has been deployed technologically for the first time. These simple microorganisms from the archaebacteria domain appreciate an environment in which other living beings cannot survive: At 70-80 °C and pH values slightly above 2.5 (which is almost as sour as lemon juice), they do their best job. The CRISPR/Cas method was used to produce new Sulfolobus stems, to improve the productivity and yield of this process.
“Thereby, we managed to combine the advantages of living cells, enzymatic and chemical catalysts”, explains Oliver Spadiut (TU Wien). “In our new hybrid process we use microorganisms, but we also add certain enzymes and chemical catalysts. As a result, we can produce the desired product in our bioreactor in only one process step - and this in an energy-efficient and accurately verifiable manner, which can easily be scaled up to industrial standards.”
Already today, xylitol is being produced from hemicellulose - however, in a significantly more complicated way. “Normally, the hemicellulose has to be extracted with acid at high temperatures. Using microorganisms, however, the pre-manufacturing steps of the hemicellulose can be milder and shorter, which saves cost. Moreover, the formation of inhibitors - unwanted substances which must be discharged later in a cumbersome way - can be drastically reduced”, explains Oliver Spadiut. “The conversion into a valuable product, which usually takes place at 50 bar pressure, is expensive and requires a lot of energy.” The procedure from the TU Wien is clearly much less expensive and more environmentally friendly.
Future vision biorefinery
The available research results not only allow the production of the sweetener xylitol. The vision is much greater: “Similar to oil refineries, where several different substances are being produced, the future will see biorefineries”, believe Christoph Herwig and Oliver Spadiut. “From materials, which were thought to be useless up to now, many sought-after substances will be obtained - even expensive pharmaceuticals.”