Enzymatic Saccharification and Fermentation Technology for Ethanol Production from Woody Biomass

Use of ethanol as a substitute for gasoline may be an effective measure to reduce petroleum consumption and mitigate global warming. Ethanol is already used as an additive or total substitute for gasoline worldwide. In particular, quite large amounts of ethanol for fuel use are produced from corn in the USA and from sugarcane in Brazil. However, those crops are also cultivated for food consumption, so ethanol production from those resources may cause food shortages and price rises. Hence, the use of non-food resources such as woody biomass is expected to provide a new source for ethanol feedstock. Japan relies on imports for the majority of its food supply, so ethanol production from food resources is impractical. In contrast, woody biomass resources such as timber from tree thinning, sawdust, or construction wood wastes are abundant in Japan1),2), so could become a major feedstock for ethanol production. Efficient ethanol production from woody biomass or agricultural residues such as straw presents several technological barriers. These resources consist of three major components, cellulose, hemicellulose, and lignin, and such biomass is called lignocellulosic biomass. Cellulose and hemicellulose can be hydrolyzed to the sugars required as substrates for ethanol fermentation by either acids or enzymes. At present, hydrolysis by acids such as sulfuric acid is relatively cost-effective but involves several problems, such as generation of substances which inhibit fermentation and decrease sugar yields, due to the excessive decomposition of sugars3). Furthermore, corrosive-resistant equipment and the recovery of used acids are expensive3),4). In contrast, enzymatic hydrolysis, which is also called saccharification, does not require hazardous chemicals and causes no excessive decomposition. However, little enzymatic hydrolysis occurs during direct processing of woody biomass, because lignin and hemicellulose fill in the gaps between cellulose molecules and hinder the approach of the enzyme molecules. Furthermore, cellulose molecules have very rigid structures, which also prevent efficient enzymatic saccharification. Therefore, pretreatment is essential before enzyme application to remove the protection by lignin and hemicellulose and to loosen the rigid structures of the cellulose. Various appropriate pretreatment technologies have been developed for the efficient enzymatic saccharification of lignocellulosic biomass5),6). For example, wet disk milling7), hot-compressed water treatment8), or the combinations of them9) can enhance the digestibility of lignocellulosic biomass by cellulase. However, considerable amounts of enzymes are still required after pretreatment, and generally account for 10 to 20 % of the total ethanol production cost10). Since the ethanol 128 Journal of the Japan Petroleum Institute, 58, (3), 128-134 (2015)