NCSU researchers develop 'self-processing' sweet potato for efficient ethanol production

Sweet potatoes are being re-engineered by North Carolina State University (NCSU) scientists as source of ethanol and bioplastics to help the U.S. bioproducts industry’s reliance on corn. The researchers' goal is to embed enzymes straight into the starch-rich tuber, so that it grows its own bioconversion enzymes and processes itself into biofuels. This would be yet another example of 'third generation' energy crops, which are being developed by several biotech firms and science teams (pevious post, here and here).

The industrial sweet potato can produce twice the starch content of corn – the leading source of ethanol in the U.S. Using plants from China, Africa, and South America, the NCSU scientists have created hybrids with starch contents over 50 percent higher than the sweet potatoes most Americans eat. These industrial sweet potatoes are capable of producing 'tremendous amounts of biomass', mostly starch-based. More starch means more sugars that can be fermented into ethanol.

Dr. Craig Yencho, an NC State associate professor of Horticultural Science, who is leading a project to develop alternative uses for the vegetable says the industrial sweet potato is edible, but not palatable. While the table version is orange inside and becomes sweet during baking as enzymes break down starch into sugar, the industrial sweet potato typically has a purple or white skin and white inside with a much higher starch content that limits its sweet taste.

North Carolina produces about 40 percent of the U.S. sweet potato crop. The industrial sweet potato could help diversify the state’s farm income. NCSU has several Potato and Sweetpotato Breeding and Genetics Programs running to research the use of the crop for the production of energy and bioproducts.

The biggest challenge is lowering production costs to take advantage of that higher starch content. Sweet potatoes traditionally are planted by hand using transplants, a process that costs up to 10 times as much as planting corn. But if a technique is developed to plant them the same way Irish potatoes are planted – by planting cut 'seed' pieces and mechanically planting them into the ground - planting costs could be cut in half.

In that case, ethanol production from sweet potatoes then becomes much more cost effective and feasible. Not only would these sweet potatoes be a much more viable ethanol source than corn, but because they are industrial sweet potatoes, farmers wouldn’t be taking away from a food source, says Yencho, who is currently in China helping the world’s number one producer of sweet potatoes tap the crop’s biofuel potential.

'Self-processing' crop
While the best of conventional breeding techniques have been used to develop NC State’s industrial sweet potato, Yencho is also teaming with colleague Bryon Sosinski, an associate professor of horticulture and the director of the Genome Research Lab, on an unconventional approach to further boost sugar – and thus ethanol – yield. Sosinski is trying to insert genes from bacteria that live in the hot waters around thermal vents on the ocean floor into sweet potato plants. The genes are active only at high temperatures, producing enzymes that break starch chains apart into much smaller sugars.

The goal is to produce what Yencho calls a 'self-processing' sweet potato that doesn’t need additives to be prepared for fermentation. The harvested roots could be thrown into a vat, and when the heat is turned up, the internal enzymes would digest the starch to a point where the resulting sugars could be fermented into fuel. Sosinski is now growing genetically modified sweet potato seedlings in the lab, and he hopes to move into greenhouse trials next year and into field plantings within three years:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: ethanol :: bioplastics :: starch :: sweet potato :: bioconversion :: biotechnology :: bioeconomy ::

The special genes used to grow the self-processing tuber would reduce the cost of enzymes that are used by biofuel processors to break down the starch in corn to sugars which are then converted into alcohol by fermentation.

Ultimately, NC State scientists believe the industrial sweet potato can compete with corn – now much cheaper to produce – as a viable alternative source of ethanol. Corn is by far the leading source of ethanol, but corn-based biofuel has come under increasing attack by poverty-fighting and other groups who argue, among other things, that diversion of corn crops for biofuels aggravates world-hunger problems. At the same time, Congress and state legislative leaders concerned about dependence on imported oil are pushing for increased use of biofuels. The new Energy Bill has given the corn ethanol industry a major boost.

There isn’t one magical crop that will solve our energy problems, but the industrial sweet potato can play an important role, especially in the southeastern U.S. where the crop is grown. - Dr. Craig Yencho, NC State associate professor of Horticultural Science

Research into the sweet potato for biofuels has added advantages: it can further enhance its value as a nutritional food staple while simultaneously finding new ways the crop can help replace petroleum as source for industrial products ranging from plastics to natural colorants and high-value specialty chemicals.

And in their zeal to mine the tuber’s variability, Yencho and his team of NC State researchers have created a hybrid intended for neither food nor fuel – the non-bearing “Sweet Caroline” variety developed strictly for ornamental use.

References:
North Carolina State University News: NC State University Researchers Brewing Energy From Sweet Potatoes - November 30, 2007.

NCSU: Brewing Energy from Natural Resources [*.pdf].

North Carolina State University Potato and Sweetpotato Breeding and Genetics Website.

Biopact: Third generation biofuels: scientists patent corn variety with embedded cellulase enzymes - May 05, 2007

Biopact: Syngenta to trial third generation biofuel crop that grows its own bioconversion enzyme - November 12, 2007

Biopact: Agrivida and Codon Devices to partner on third-generation biofuels - August 03, 2007