In addition to isolating grains for processing, cereal-milling plants also generate fiber — rich fractions as a coproduct stream. In the wet milling of corn, the fiber fraction has traditionally been added into a feed product (figure 1.20). The University of Illinois developed a modified dry milling procedure to recover fiber fractions before fermenta­tion: this quick fiber contained 65% by weight of total carbohydrate and 32% by weight of glucans, and dilute acid pretreatment was used before fermentation of the substrate to ethanol by either Escherichia coli or S. cervisiae.102 Destarched, cellulose-rich, and arabinoxylan-rich fractions of the corn fiber support the growth of strains of Hypocrea jecorina and their secretion of hydrolases for plant polysaccharides; these enzymes act synergistically with commercial cellulases on corn fiber hydrolysate and represent a valuable source of on-site enzymes for corn fiber product utilization.103 Similarly, large quantities of wheat bran are produced worldwide as a coproduct of wheat milling; residual starch in the bran material can be hydrolyzed to glucose and oligoglucans by amylolytic enzymes, and acid hydrolysis pretreatment followed by cellulase treatment gives a sugar (pentose and hexose) yield of 80% of the theoretical: 135, 228, and 167 g/kg of starch-free bran for arabinose, xylose, and glucose, respectively.104

Rice husks are approximately 36% by weight cellulose and 12% hemicellulose; as such, this agricultural by-product could be a major low-cost feedstock for ethanol production; 60% of the total sugars could be released by acid hydrolysis and treatment with a mixture of enzymes ф-glucosidase, xylanase, and esterase) with no formation of furfuraldehyde sugar degradation products.105 Recombinant E. coli could ferment the released sugars to ethanol; high-pH treatment of the hydrolysate reduced the time required for maximal production of ethanol substantially, from 64 to 39 hours. In a study from India, rice straw was pretreated with and without exogenous acid, and the released hemicellulose sugars fermented by a strain of C. shehatae; ethanol production was also demonstrated by yeast cells immobilized in calcium alginate beads — an example of an advanced fermentation technology discussed in more detail in the next section.106

Fast-growing willow trees are a major focus of research interest as a bioenergy crop in Scandinavia; high sugar recoveries were achieved from lignocellulosic mate­rial by steaming sulfuric acid-impregnated material for a brief period (4-8 minutes) at 200°C, and then digesting the cellulose enzymically, liberating glucose with 92% efficiency and xylose with 86% efficiency. The pretreated substrate could also be used for SSF with a S. cerevisiae strain.107

Many “exotic” plant materials have been included in surveys of potential bio­mass and bioenergy sources; example of these are considered in chapter 5, section 5.5.2, when sustainability issues are covered at the interfaces among agronomy, the cultivation of bioenergy crops, land use, and food production. As a lignocellulosic, straw from the grass species Paja brava, a Bolivian high-plains resident species, can be considered here. Steamed, acid-impregnated material gave hemicellulose frac­tions at 190°C that could be fermented by three pentose-utilizing yeasts, P. stipitis, C. shehatae, and Pachysolen tannophilus, while a higher temperature (230°C) was necessary for cellulose hydrolysis.108 Much more widely available worldwide is the mixed solid waste of lumber, paper, tree pruning, and others; this is a highly digest­ible resource for cellulase, the sugars being readily fermented by S. cerevisiae and the residual solids potentially usable for combustion in heat and power generation.109