9.2.1 Wheat Straw

Wheat (Triticum aestivum L.) is the world’s most widely grown crop, cultivated in over 115 countries under a wide range of environmental conditions. Over the past 100 years, the yields of wheat have been increased and the annual global pro­duction of dry wheat in 2008 was estimated to be over 650 Tg [10]. Assuming residue/crop ratio of 1.3, about 850 Tg of wheat residues are annually produced which include straw as the major waste. The straw produced is left on the field, plowed back into the soil, burnt, or even removed from the land depending on the convenience of the landowner. Disposal of wheat straw by burning is viewed as a serious problem due to the increased concern over the health hazards of smoke generated [93]. Burning of wheat straw also results in production of large amounts of air pollutants including particulate matter, CO, and NO2 [110]. Thus, finding an

alternative way for disposal of surplus wheat straw is of paramount interest and an immediate necessity.

Wheat straw like any other biomass of lignocellulosic nature is a complex mixture of cellulose, hemicellulose, and lignin as three main components (Table 9.4), and a small amount of soluble substrates (also known as extractives) and ash. The overall chemical composition of wheat straws could slightly differ depending on the wheat species, soil, and climate conditions. The cellulose strands are bundled together and tightly packed in such a way that neither water nor enzyme can penetrate through the structure [104, 179]. Hemicellulose serves as a connection between lignin and cellulose fibers, and it is readily hydrolyzed by dilute acid or base, as well as hemicellulase enzyme. Lignin is covalently linked to cellulose and xylan (predominant hemicellulose carbohydrate polymer in wheat straw) such that lignin-cellulose-xylan interactions exert a great influence on the digestibility of lignocellulosic materials [104]. Due to this structural complexity of the lignocellulosic matrix, ethanol production from wheat straw requires at least four major unit operations including pretreatment, hydrolysis, fermentation, and distillation. Unlike sucrose or starch, lignocellulosic biomass such as wheat straw need to be pretreated to make cellulose accessible for efficient enzymatic depolymerization.