Improvement of Enzymatic Hydrolysis of Steam-exploded Wheat Straw by Simultaneous Glucose and Xylose Liberation

Lignocellulosic biomass, for its large quantities and relatively low cost, is regarded as a potential renewable energy resource for cost-effective ethanol production.1 Wheat straw is one of the most abundant crop residues in European countries with a production of 170 million tonnes per year, and it seems to be one of the cheapest and the most useful raw material for ethanol production.2 Wheat straw is composed of a mixture of cellulose and hemicellulose (45 % and 30 %, respectively) that are bound to lignin (approx. 25 % w/w DM) by hydrogen and covalent bonds. One of the major limitations of the second generation (cellulosic and hemicellulosic) ethanol production is the sugar recovery step, where fermentable carbohydrates are released from biomass using enzymes.3 This process step has to be further improved before commercialization of the process. There are several approaches to increase the efficiency and decrease the cost of cellulose and hemicellulose hydrolysis, i.e. better pretreatment techniques,4,5 optimizing enzyme complex composition and operating variables of hydrolytic process6–10 or addition of proteins, surfactants and other chemicals to enhance cellulose/hemicellulose conversion.11 The enzymes catalyzing the degradation of lignocellulosic material into fermentable sugars are a mixture of endoglucanases (EG), cellobiohydralases (CHB) and β-glucosidases. They act in synergism by targeting lignocellulosic material differently; endoglucanases randomly attack cellulose chains and release cello-oligosaccharides, cellobiohydrolases attack the ends of the cellulose and cleave cellobiose units off, thereby ‘feeding’ the cellobiohydrolases with cellulose ends and, finally, the β-glucosidases catalyze the hydrolysis of cellobiose and short chain oligosaccharides into glucose.12 The addition of β-glucosidase greatly increases the rate and extent of hydrolysis (saccharification) by ensuring the efficient hydrolysis of cellobiose and reducing the influence of end-product inhibition.13 The hydrolytic efficiency of a multi-enzyme mixture in the process of lignocellulose saccharification depends both on the properties of individual enzymes and their ratio in the multi-enzyme cocktail.7,14,15 There are many factors affecting enzymatic hydrolysis, including substrate concentration, reaction time, enzymatic activity (that depends on the temperature and pH), etc. To improve the yield of the saccharification stage, an optimization of the different processing parameters must be carried out. The aim of the optimization process is to obtain a more efficient enzymatic hydrolysis of lignocellulose (higher sugar concentrations to be converted into ethanol).7 Improvement of Enzymatic Hydrolysis of Steam-exploded Wheat Straw by Simultaneous Glucose and Xylose Liberation