Grape pomace, the mashed up skins and stems left over from making wine and grape juice, could serve as a good starting point for ethanol production, according to a new study.
Due to growing interest in biofuels, researchers have started looking for cheap and environmentally sustainable ways to produce such fuels, especially ethanol. Biological engineer Jean VanderGheynst at the University of California, Davis, turned to grape pomace, because winemakers in California alone produce over 100,000 tons of the fruit scraps each year, with much of it going to waste.
To determine how much ethanol they could produce from pomace, VanderGheynst and her team processed pomace from the Sutter Home Winery in St. Helena, Calif., under various fermentation conditions. The researchers found that pomace from white grapes yielded the most ethanol. Winemakers only squeeze the juice out of these grapes and don’t ferment the pomace, so much of the fruits’ sugar remains. Meanwhile, red grape pomace has been fermented over long periods, so less sugar remains for ethanol production. But the scientists found that adding dilute acid to the red grape pomace boosted ethanol yields.
On average, the researchers found, grape pomace produces less than half as much ethanol as corn does by dry weight. To squeeze the maximum ethanol out of the grape waste, researchers would need to develop techniques to convert the grape’s cellulose into ethanol, says lead author Yi Zheng, a chemist at the biotechnology company Novozymes, in Denmark. But, he thinks pomace could still be a feasible feedstock because the material is readily available. Ethanol producers could make grape pomace more economically viable if they combined ethanol production with manufacture of other pomace-based products, such as fertilizers or animal feed, he says.