Pretreatment lignocellulosic materials

Several definitive reviews on pretreatment of lignocellulosic materials for improving cellulose hydrolysis (1,2,3) appeared a few years ago. More recently, two pretreatment methods (the Purdue process and the Iotech process) have been announced that claim superior perform-... 

Many pretreatments have been employed to enhance the degradation of lignocellulosic materials to glucose. The treatments fall into two general areas (Ryu and Lee, 1983) ... 

The production of fuel ethanol from renewable lignocellulosic material ("bioethanol") has the potential to reduce world dependence on petroleum and to decrease net emissions of carbon dioxide. The lignin-hemicellulose network of biomass retards cellulose biodegradationby cellulolytic enzymes. To remove the protecting shield of lignin-hemicellulose and make the cellulose more readily available for enzymatic hydrolysis, biomass must be pretreated (1). 

Corn stover, like lignocellulosic materials in general, is resistant to enzymatic hydrolysis, because of both the tight network in the lignocellulose complex and the crystalline structure of the native cellulose. These difficulties can be overcome by employing a suitable pretreatment (7). 

A major problem in the commercialization of this potential is the inherent resistance of lignocellulosic materials toward conversion to fermentable sugars (4). To improve the efficiency of enzymatic hydrolysis, a pretreatment step is necessary to make the cellulose fraction accessible to cellulase enzymes. Delignification, removal of hemicellulose, and decreasing the crystallinity of cellulose produce more accessible surface area for cellulase enzymes to react with cellulose (5). 

In this article, we describe the production of xylanase by T. lanuginosus IOC-4145 in semisolid cultivation using corncob as raw material in optimized conditions. Furthermore, we describe the pretreatment effect on corncob and sugarcane bagasse and the enzymatic hydrolysis of these lignocellulosic materials using the produced thermophilic xylanase. 

Acid hydrolysis was used to determine the amount of xylose present in the lignocellulosic materials without pretreatment, after thermal pretreatment, and after both alkali and thermal pretreatments. 

The purposes of pretreatment of lignocellulosic materials are the removal of lignin and hemicellulose, reduction of cellulose crystallinity, and increase in the porosity of the materials. Among the physical methods, mechanical treatments, such as chipping, grinding and milling are used to reduce cellulose crystallinity. The... 

Ammonia fiber explosion (AFEX) is another physicochemical pretreatment in which lignocellulosic materials are exposed to liquid ammonia at high temperature and pressure for a period of time, and then the pressure is swiftly reduced. A typical dosage is 1-2 kg ammonia/kg dry biomass at 90 °C and residence time of 30 min. To reduce the cost and protect the environment, ammonia must be recycled after the pretreatment. 

Acid hydrolysis has been successfully employed for pretreatment of lignocellulosic materials. Dilute sulfuric acid, used at either low or high temperature, achieves high xylan to xylose conversion. This is favorable to the overall economics, as xylan accounts for a large part of the total carbohydrates in the lignocellulosic materials. 

Dekker, R. F. H. 1991. Steam explosion an effective pretreatment method for use in the bioconversion of lignocellulosic materials. In Focher, B., Marzetti, A., and Crescenzi, V. (Eds.), Steam Explosion Techniques Fundamental Principles and Industrial Applications (pp. 277-305). Philadlphia PA Gordon and Breach Scientific Publishers-. 

Chemical modification reactions continue to play a dominant role in improving the overall utilization of lignocellulosic materials [1,2]. The nature of modification may vary from mild pretreatment of wood with alkali or sulfite as used in the production of mechanical pulp fibers [3] to a variety of etherification, esterification, or copolymerization processes applied in the preparation of wood- [4], cellulose- [5] or lignin- [6] based materials. Since the modification of wood polymers is generally conducted in a heterogeneous system, the apparent reactivity would be influenced by both the chemical and the physical nature of the substrate as well as of the reactant molecules involved. 

Many factors influence the reactivity and digestibility of the cellulose fractions of lignocellulose materials. These factors include Hgnin and hemicellulose content, crystalhnity of cellulose, and the porosity of the biomass materials. Pretreatment of Hgnocellulosic materials prior to utiHzation is a necessary element in biomass-to-ethanol conversion processes. The objective of the pretreatment is to render biomass materials more accessible to either chemical or enzymatic hydrolysis for efficient product generation. The goals of the pretreatment are ... 

Because the hemicellulose fraction of biomass materials can be separated from lignin and cellulose by dilute acid treatment, cellulose becomes more reactive towards cellulase. Hemicellulose hydrolysis rates vary with acid concentration, temperature, and solid-to-liquid ratio. With most lignocellulosic materials, complete hemicellulose hydrolysis can be achieved in 5-10 min at 160°C or 30-60 min at 140 °C. Dilute acid hydrolysis forms the basis of many pretreatment processes for example, autohydrolysis and steam explosion are based on high-temperature dilute acid catalyzed hydrolysis of biomass. 

Another approach to improving cellulose filler dispersibility is described in U.S. Pat. No. 4,559,376 [37], according to which cellulose or lignocellulose material is subjected to a hydrolytic pretreatment using diluted hydrochloric or sulfuric acid. Essentially, the treatment converts cellulose or lignocellulose material to a fine... 

Keywords Lignocellulosic materials Sugarcane bagasse Pretreatment Lime Hydrogen peroxide Enzymatic hydrolysis Statistical analysis... 

In general, lignocellulosic materials are resistant to bioconversion and require pretreatment to increase their biodigestibility and make cellulose more accessible to the... 

Lynd et al. [3] has summarized the desireable properties for an ideal lignocellulosic material after chemical pretreatment that is, it should (a) produce reactive fibers, (b) yield pentoses in nondegraded form, (c) not release the compounds that significantly inhibit fermentation, (d) work in reactors of reasonable size with moderate cost, (f) produce no solid residues, (g) have a high degree of simplieity, and (h) be effeetive at low moisture contents. 

In cellulosic ethanol production processes, a pretreatment procedure is needed to disrupt the recalcitrant structure of the lignocellulosic materials so that the cellulose can be more efficiently hydrolyzed by cellulase enzymes [2], These pretreatments include physical, biological, and chemical ways, such as uncatalyzed steam explosion, liquid hot water, dilute acid, flow-through acid pretreatment, lime, ammonium fiber/freeze explosion, and ammonium recycle percolation [3, 4], Most of these methods involve a high temperature requirement, which is usually achieved through convection- or conduction-based heating. 

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