Hemicelluloses digestion

Keys, Jr., J.E. J.V. DeBarthe. Cellulose and hemicellulose digestibility in the stomach, small intestine and large intestine of swine./. Anim. Sci. 1974, 39, 53-56. 

The two intermediates of commercial furan resins are furfural and furfuryl alcohol. Furfural occurs in the free state in many plants but is obtained commercially by degradation of hemicellulose constituents present in these plants. There are a number of cheap sources of furfural, and theoretical yields of over 20% (on a dry basis) may be obtained from both com cobs and oat husks. In practice yields of slightly more than half these theoretical figures may be obtained. In the USA furfural is produced in large quantities by digestion of com cobs with steam and sulphuric acid. The furfural is removed by steam distillation. 

Consid - the Kraft pulping process shown in Fig. 8.8 (Dunn and El-Halwa, 1993). The first step in the process is digestion in which wood chips, containing primarily lignin, cellulose, and hemicellulose, are cooked" in white liquor (NaOH, Na2S, Na2C03 and... 

Little information is available on the enzymatic hydrolysis of xylan. The kinetics of breakdown of xylan are reported.128 Hemicellulose from linden wood is said to be extensively hydrolyzed124 by the digestive juices of Helix pomatia. Xylanase128 of Aspergillus niger, while active on xylan between pH 2-8, has its optimum at pH 4.0. 

Fourth lesson - combination of different compounds in unique macrostructure provides unique performance properties. Starch is used extensively in nature to store carbon and energy. Starch is readily digested and must be protected from degradation by a resistant coating, for example, a seed (e.g. com, wheat or rice) or a skin (e.g. potato). Woody materials such as trees, soft plants and grasses are composed of a complex combination of aliphatic and aromatic compounds (cellulose, hemicellulose and lignin). 

Most often, the rates for feedstock destruction in anaerobic digestion systems are based upon biogas production or reduction of total solids (TS) or volatile solids (VS) added to the system. Available data for analyses conducted on the specific polymers in the anaerobic digester feed are summarized in Table II. The information indicates a rapid rate of hydrolysis for hemicellulose and lipids. The rates and extent of cellulose degradation vary dramatically and are different with respect to the MSW feedstock based on the source and processing of the paper and cardboard products (42). Rates for protein hydrolysis are particularly difficult to accurately determine due the biotransformation of feed protein into microbial biomass, which is representative of protein in the effluent of the anaerobic digestion system. 

Xylanases act on the P-l,4-linked xylan, the most abundant of the hemicelluloses that constitutes over 30% of the dry weight of terrestrial plants.116 They resemble cellulases and cooperate with cellulases and xylosidases117 in digestion of plant cell walls.110116118-121 Galactanase digests the P-l,4-linked component of pectins.122... 

Corn stover, a well-known example of lignocellulosic biomass, is a potential renewable feed for bioethanol production. Dilute sulfuric acid pretreatment removes hemicellulose and makes the cellulose more susceptible to bacterial digestion. The rheologic properties of corn stover pretreated in such a manner were studied. The Power Law parameters were sensitive to corn stover suspension concentration becoming more non-Newtonian with slope n, ranging from 0.92 to 0.05 between 5 and 30% solids. The Casson and the Power Law models described the experimental data with correlation coefficients ranging from 0.90 to 0.99 and 0.85 to 0.99, respectively. The yield stress predicted by direct data extrapolation and by the Herschel-Bulkley model was similar for each concentration of corn stover tested. 

Many favor dilute sulfuric acid pretreatment because both high hemicellulose recovery and good cellulose digestibility can be achieved (6-8). Moreover, most of the soluble sugars from dilute-acid pretreatment are released as monomers that can be readily fermented to ethanol by recombinant organisms (9,10). Pretreatment with just hot water or steam, termed uncatalyzed hydrolysis or autohydrolysis, eliminates chemical additives, lowers the cost of materials of construction, and generates less waste, but hemicellulose and cellulose yields from batch systems are limited. 

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