Spinning straw into… mould? Sweet

Ordinary straw can be converted into the sweetener erythritol using a genetically modified mould fungi

Straw is often considered to be worthless and is therefore burned. But researchers have found a way to make it into erythritol, a sweetener that is 70 to 80 per cent as sweet as sugar.

Erythritol does not make you fat, does not cause tooth decay, has no effect on the blood sugar and, unlike other sweeteners, does not have a laxative effect.

In Asia it is already widely used and it is becoming more common in other parts of the world too.

Until now, erythritol could only be produced with the help of special kinds of yeast in highly concentrated molasses.

At the TU Vienna, a method has now been developed to produce the sweetener from ordinary straw with the help of a mould fungus.

First, the finely chopped straw has to be “opened up”: with the help of solvents, the cell walls are broken and the lignin is dissolved away. The remaining xylan and cellulose are then processed further.

“Usually the straw has to be treated with expensive enzymes to break it down into sugar,” said Professor Robert Mach (Vienna University of Technology). “In highly concentrated molasses, special strains of yeast can then turn the sugar into erythritol, if they are placed under extreme osmotic stress.”

However, a genetically modified mould fungus makes the intermediate step obsolete.

The enzymes opening up the straw can be obtained with the help of the mould fungus Trichoderma reesei. This kind of mould also plays the leading role in the new production process developed at the Vienna University of Technology.

Two big advantages have been achieved by genetically modifying the fungus: the process of obtaining the enzymes from mould cultures and chemically cleaning them used to be cumbersome – now the improved strain can be directly applied to the straw. Secondly, the mould can now produce erythritol directly from the straw.

The intermediate step of producing molasses is no longer necessary and no yeast is needed. “By genetically modifying it, we managed to stimulate the production of an enzyme, which enables the large-scale production of the sweetener.”

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