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. 

Batch autohydrolysis was carried out in 300 ml pressure vessels in a preheated silicone bath, with due allowance for temperature equilibration to achieve as nearly as possible 195 C for 30 minutes. Chip moisture was adjusted to 50% for the batch autohydrolysis. 

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. 

We acknowledge with thanks the co-operation of Stake Technology, especially Mr. Robert Bender, in the continuous autohydrolysis, and of the Natick group under Dr. Leo Spano in providing cellulase. We have received financial assistance from the National Research Council of Canada and from the University of Toronto. 

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). 

Since the most recent in-depth reviews, the Iotech and Purdue processes have been proclaimed by their advocates as breakthroughs. Waymans discovery of means to prevent lignin self-condensation during autohydrolysis also is a most significant finding. 

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... 

Chadha, R.K., Lawrence, J.F., and Ratnayake, W.M.N. 1995. Ion chromatographic determination of cyanide released from flaxseed under autohydrolysis conditions. Food Add. Cont. 12, 527-533. 

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. 

Comparison of Two Posthydrolysis Processes of Brewery s Spent Grain Autohydrolysis Liquor to Produce a Pentose-Containing Culture Medium+... 

The present work deals with a two-step procedure of autohydrolysis followed by posthydrolysis to obtain a fermentable, monosaccharide-rich liquor from the hemicellulose fraction of BSG. The autohydrolysis step was carried out under preoptimized conditions, andboth enzymatic and chemical posthydrolysis were evaluated. Preliminary studies are also reported on the production of polyols from such liquor by Debaryomyces hansenii, a polyol-overproducing yeast (20-25). 

Starch-free BSG was subjected to reaction with water (autohydrolysis) in a 2-L stainless steel Parr reactor model 4532 (Moline, IL), to cause the hydrolytic degradation of hemicelluloses, operating under optimized conditions (liquid-to-solid ratio of 8 1 [w/w], standard heating temperature profile up to 190°C, isothermal reaction at 190°C for 2.5 min) (2). After the reactor was cooled down, the oligosaccharide-containing liquor (OCL) was separated from the residual solid by filtration (Whatman no. 1 filter paper). 

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. 

Under the BSG autohydrolysis conditions used herein, most of the hemicelluloses are converted mainly into oligosaccharides. To obtain a monosaccharide-rich hydrolysate, the OCLs have to be further hydrolyzed. 

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. 

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