Barley Straw
Barley (Hordeum vulgare L.) is a major cereal grain, ranked as the fourth in the world both in terms of quantity produced and in area of cultivation. Some of the major uses of barley are as animal fodder, as a source of fermentable material for beer and beverages and as a component of various health foods and breads. Barley straw has been used as an algae control substance in fresh water ponds, however, some research has indicated that straw has no effect on certain varieties of algae.
According to Kim and Dale’s survey, 58.5 Tg of barley straw is produced in the world in a year, with the potential to be converted to 18.1GL of ethanol [1]. Similar to wheat straw, barley straw is rich in cellulose and suitable for aqueous-phase hydrolysis fermentation ethanol production. A complete analysis report of barley straw is shown in Table 3.10.
Pretreatment of Barley Straw
Many of the common pretreatment techniques have been tested on barley straw, these include dilute acid pretreatment [107, 108] and steam [4, 109, 110]. Saha and Cotta have recently evaluated several pretreatment methods like dilute acid, lime, alkaline peroxide and enzymatic saccharification procedures for the conversion
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Table 3.10 Chemical composition of barley straw (wt% on dry basis) [106].
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of barley straw to monomeric sugars [111]. The efficiency of these pretreatments was measured by analysis of the sugars produced after saccharification with a cocktail of three commercial enzyme preparations (cellulase, в-glucosidase and hemicellulases). In this study, relatively mild conditions of pretreatment such as 10% solid loading, 0.75% (w/v) acid, or 0.1 g lime/g straw and a moderate temperature of 121°C were used. These acid pretreated and lime pretreated barley straw samples gave 88 and 91% sugar yields, respectively. Then, upon enzymatic hydrolysis, the sugar solutions were subjected to fermentation using a recombinant bacterium without any over liming or detoxification steps for dilute acid pretreated barley straw feedstock. Saha and Cotta reported that upon fermentation, both dilute acid, and lime-alkaline peroxide pretreated barley straw samples produced 0.48 g ethanol/g available sugars. Furthermore, they reported that lime pretreatment did not produce any significant amounts of fermentation inhibitors [111].
Increasing the hydrolyzability of barley straw by genetic modifications of barley is an interesting approach. These mutation
technologies are known to perturb the lignin synthetic pathway, and it has been reported that low phytic acid (LPA) mutation in barley interferes with the expression of genes in pathways of cell wall biosynthesis, resulting in lower acid detergent fiber and reduced lignin content. Chen and Zemetra have recently studied this type of mutation effect on ethanol production from barley straw [5]. Composition differences of mutant straws, effects of acid pretreatment, enzymatic hydrolysis and simultaneous saccharification and fermentation (SSF) were investigated in this study. The fiber analysis (neutral detergent fiber, acid detergent fiber, and acid detergent lignin) indicated that there were no significant differences between modified and wild-type straw lines in terms of straw compositions. However, the difference did exist among straw lines on fiber utilization. The data indicated that the phytic acid mutant straws changed the fiber structure, which significantly influences their hydrolyzability. These results may lead to a possible solution of mutant plant species that is capable of increasing the hydrolyzabil — ity of biomass without changing their compositions and sacrificing their agronomy performance [5].