Hydrolysis autohydrolysis

Aspirin is an ester, bnt it still contains a carboxylic acid fnnction (p/Ca 3.5). In aqueous solntion, there will thus be significant ionization. However, this ionization now provides an acid catalyst for ester hydrolysis and initiates autolysis (autohydrolysis). The hydrolysis product salicylic acid (pACa 3.0) is also acidic both aspirin and salicylic acid are aromatic acids and are rather stronger acids than aliphatic compounds such as acetic acid (pACa 4.8) (see Section 4.3.5). An aqueous solution of aspirin has a half-life of about 40 days at room temperature. In other words, after about 40 days, half of the material has been hydrolysed, and the biological activity will have deteriorated similarly. 

Single stage acid hydrolysis of the cellulosic residue of autohydrolysis and extraction at 190°C with 0.5 to 4.0% HaSOi on the weight of the cellulose gave rather unsatisfactory results. 

Figure 2. Enzymatic hydrolysis of autohydrolyzed aspen wood (continuous autohydrolysis)...

Alcohol recovery from the fermentation brews was less than complete in most cases, which may be attributable to less than ideal conditions. The best yields, 60 to 97% of theory, were obtained with sugars obtained by hydrolysis of cellulosic residues of the autohydrolysis-extraction process. Unextracted pulps, or the hemicellulose solutions, gave poor ethanol formation, which suggests inhibition. In the calculation of material and energy balances which follows, we have assumed 95% yields of ethanol from wood sugars, which is readily achieved in industrial practice and which we believe to be achievable with our wood sugars as well. 

Figure 5. Material and energy balance. Acid and enzyme hydrolysis following autohydrolysis and caustic extraction.

The data above enable us to make some rough estimates of costs associated with two processes (i) ACID, that is aspen wood-autohydrolysis-caustic extraction-acid hydrolysis-fermentation-distillation and (ii) ENZYME, that is aspen wood-autohydrolysis-caustic extraction-enzymatic hydrolysis-fermentation-distillation. For purposes of comparison, the product in both cases will be assumed to be 10 million gallons of 95% ethanol per year, a minimum economic size. 

Wood pretreated by autohydrolysis and extraction is necessary for successful enzymatic hydrolysis, and advantageous for acid hydrolysis. 

The determination of the structure of a plant gum involves the establishment of its homogeneity, its equivalent weight, and rotation and its uronic acid and pentosan content by the well-known methods. The nature of the constituent sugars and the uronic acid are then determined, after hydrolysis of the gums, by ciystallization or by conversion into characteristic crystalline derivatives. As an example the hexose mannose may be identified as the free sugar or in the form of its anilide, methyl glycoside or phenylhydrazone. Autohydrolysis of the acid... 

The novel type of disaccharide IX, consisting of a unit of n-galactose joined to a unit of L-arabinose, was obtained by autohydrolysis of gum arabic. The structure of this disaccharide was proved by the fact that methylation yielded the heptamethyl ether X and this gave on hydrolysis 2,3,4,6-tetramethyl-D-galactose (XI) and 2,4-dimethyl-L-arabinose (XII). 

Ten Canadian flaxseed cultivars were analyzed for total cyanide content (Chadha, 1995) and contents of individual cyanogenic glycosides (Oomah et al., 1992). Chadha et al. (1995) determined cyanide content in 10 cultivars of flaxseed using an autohydrolysis method that required up to 5 hours of hydrolysis time. The maximum cyanide values were typically obtained... 

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. 

Autohydrolysis enables the selective hydrolysis of hemicelluloses to a mixture mainly consisting of oligosaccharides and monosaccharides. The monosaccharide content can be increased under harsher reactor conditions, but then monosaccharides can undergo decomposition reactions, thereby increasing the content of potential fermentation inhibitors in hydrolysates. 

Unlike starch or cellulose, BSG hemicellulose has a complex structure, which is still present, in part, on its autohydrolysis products. Oligosaccharides from BSG hydrolysis consist mainly of branched arabino-xylo-glucurono oligosaccharides that are not highly acetylated when compared to other xylans, such as from Eucalyptus wood (31). The action of several enzymatic activities including endo-l,4-P-xylanase P-xylosidase and accessory activities such as acetyl xylanesterase, a-glucuronidase, and a-arabino-furanosidase is therefore required for the complete hydrolysis of OCL to monosaccharides. 

Similar conditions have been described for posthydrolysis of steam-exploded Douglas fir wood chips performed at 120°C, but longer hydrolysis time was required (15). Similar to our results, increasing catalyst also increased monosaccharide recovery and higher participation of degradation reactions. For corn cobs (36), a material similar to BSG, the posthydrolysis of OCL autohydrolysis was carried out at CS 1.66 (calculated from the reported operational conditions 125°C, 0.5% H2S04,165 min), a more severe condition than the optimal conditions determined in the present work for BSG. 

Under acidic conditions, however, the liberation of free acetic acid during hydrolysis of the esters can increase the acidity and enhance further hydrolysis of not only additional ester groups, but acetal linkages and lignin bonds as well. A prime example is the so-called autohydrolysis reaction (2) in which acetic acid liberated by steam can cause complete hydrolysis of the hemicelluloses and convert the lignin into soluble fragments. 

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. 

The pretreatment of any lignocellulosic biomass is cmcial before enzymatic hydrolysis. The objective of pretreatment is to decrease the crystallinity of cellulose which enhances the hydrolysis of cellulose by cellulases (17). Various pretreatment options are available to fractionate, solubilize, hydrolyze and separate cellulose, hemicellulose and lignin components (1,18-20). These include concentrated acid (27), dilute acid (22), SOj (25), alkali (24, 25), hydrogen peroxide (26), wet-oxidation (27), steam explosion (autohydrolysis) (28), ammonia fiber explosion (AFEX) (29), CO2 explosion (30), liquid hot water (31) and organic solvent treatments (52). In each option, the biomass is reduced in size and its physical structure is opened. Some methods of pretreatment of Lignocellulose is given in Table I. 

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