Enzymes turn inedible waste plant material into food and fuel

Ian Steadman - arstechnica.com - 04/17/2013
Food Knowledge

Fuel is perfect.

Starch? Do we need more starch?


Researchers have managed to turn indigestible cellulose into starch, a process that could render billions of tons of agricultural waste into food and fuel.

Plants grow more than 160 billion tons of cellulose—the material that makes up the walls of plant cells—every year, but only a tiny fraction of that is useful to humans in the crops we grow. This is frustrating, as cellulose is made up of glucose chains that are almost, but not quite, the same as those that make up the starch that constitutes 20 to 40 percent of most peoples' daily calorie intake.

With the world's population forecast to reach nine billion by 2050, working out how to alter cellulose glucose into something more practical could be vital for preventing starvation. There's also an extra benefit in that some could be used for biofuels.

Biological systems engineers from Virginia Polytechnic Institute and State University investigated ways of breaking cellulose down into more basic glucose blocks, and how to combine them back together into more complex starches. To do this, the team needed enzymes generated by a genetically modified E.coli bacteria (the genes were taken from fungi, potatoes and other bacteria).

That gave the team a collection of enzymes. The first breaks cellulose down into the slightly simpler cellobiose, which another enzyme then splits into two individual glucose molecules—one on its own, the other with one phosphate molecule attached. That phosphate molecule allows the glucose to combine together into amylose, a starch powder that's edible but not digestible. It is present in many foods, is a good source of fiber, and has been shown to improve the digestive health of people who eat it regularly.

It also has other practical uses. The study's coauthor, Percival Zhang, said: "Besides serving as a food source, the starch can be used in the manufacture of edible, clear films for biodegradable food packaging. It can even serve as a high-density hydrogen storage carrier that could solve problems related to hydrogen storage and distribution."

The process renders a third of cellulose into edible starch, with the other two-thirds rendered into a useable biofuel—none of the cellulose is wasted. For every ton of grain harvested by farmers, there can be two or three times as much plant material discarded, and that "grain stover" is what was used in this research. However, future research could mean that cellulose found in other plants that are not considered edible at all, such as trees, might prove to have a use.

However, it is unclear how economical this process is. Zhang told Science that it could cost roughly $1 million to turn 200kg of cellulose into 20kg of starch, which is a lot of money for what is apparently only enough for one person across 80 days. Further research will be needed to see if the process has a commercial future.

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