Bioethanol is produced from the anaerobic fermentation of carbohydrates (e.g., simple sugars, starch, cellulose, and lignin) by yeast. Basic metabolic by-products include ethanol, carbon dioxide, and water. The carbohydrate-rich substrate used for bioethanol production is called the feedstock. Thanks to government subsidies, corn grain is the most commonly used feedstock in the United States (Pimentel et al. 2002). Spent corn feedstock is called distillers grain and can be used as a high-protein feed for cattle. Corn-based bioethanol is highly criticized because the massive amounts of fossil fuels and land resources used to produce it outweigh the return. Sugarcane is a slightly more efficient feedstock, but still, some estimates suggest that 70 percent more energy is used in the production of corn or sugarcane-based bioethanol than is returned (Pimentel et al. 2002).
Perhaps a more practical feedstock is cellulose, an abundant structural carbohydrate found in plants. Farm waste products such as corn stalks and husks contain high levels of cellulose, and could be used in lieu of the grain itself. Paper products and wood chips are cellulosic. Dedicated crops of fast growing poplar trees, planted in marginal, or contaminated soils hold promise. Even salvaged food waste such as potato peels, rejected bananas and apples, or instant noodle by-products could be excellent feedstock materials (Zhang et al. 2016).
The main drawback to cellulosic bioethanol production is the relative difficulty of cellulose degradation. Not only is cellulose inherently difficult to breakdown into its simple sugar subunits, it is also often protected by a rigid polymer called lignin. Degradation of lignin is required for the release of cellulose for bioethanol production or paper manufacturing. To overcome this hurdle, work is being done to metabolically engineer yeast strains with enhanced catabolic activities. For example, a recent study outlines a high throughput method for the identification and gene cloning of novel cellu- lases from diverse genomic sources (Yang et al. 2016a). Other strategies include the genetic engineering of feedstock plants to contain less abundant or more tractable lignin molecules. For instance, a new transgenic poplar makes modified lignin enriched with ester linkages, a structural change that makes the molecule easier to digest (Wilkerson et al. 2014).
Box 5.4. Trash to treasure
Use of household food waste for biofuel feedstock is an intriguing idea, since this zero-value resource is almost always landfill-bound. Discarded food makes up 20 percent of U.S. landfill waste (Gerlock 2014). A shocking 40 percent of the U.S. food supply is discarded (Gerlock 2014). Food waste is chemically complex; containing lipids, carbohydrates, proteins, and vitamins. Lipids can be extracted and refined into biodiesel. Carbohydrates can be fermented into bioethanol. Thorough techno-economic analyses have not yet been conducted on the use of food waste as feedstock, but rough estimates suggest that it would dramatically decrease the cost of biofuels. As it stands, feedstock production expenses make up 80 to 90 percent of the cost of biofuels (Karmee 2016). Replacement with a zero-value material would mitigate these expenses. One challenge with a waste-to-fuel program would be the widespread collection of discarded household foods. That said, the process could piggyback existing collection systems such as neighborhood garbage pick-up and community compost drop-offs.17