Sugars hydrolysis kinetics

Traditional hemicellulose hydrolysis kinetic models cannot account for a change in hemicellulose sugar yields with solids concentration and suffer from inconsistencies that bring into question their mechanistic accuracy. Thus, although current models can be usefiil for a given flow regime, their ability to describe different systems such as flowthrough reactors on a consistent basis is unproven (29). 

In classical examples of kinetics, such as the hydrolysis of cane sugar by acids in water solution, the reaction takes hours to approach completion. Therefore Whilhelmy (1850) could study it successfially one and a half centuries ago. Gone are those days. What is left to study now are the fast and strongly exothermic or endothermic reactions. These frequently require pressure equipment, some products are toxic, and some conditions are explosive, so the problems to be solved will be more difficult. All of them require better experimental equipment and techniques. 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

It remains unclear whether or not the rate of cleavage of the gly-cosyl bond in sugar nucleotides depends upon the structure of the nucleotide residue, and the same uncertainty is true for other reactions that affect the glycosyl group no systematic kinetic studies have been reported. However, it may be noted that there appears to be no essential difference between the rates of acidic hydrolysis of the 5 -(a-D-glucopyranosyl pyrophosphates) of uridine and N3-methyluridine, 331... 

This model has been reinforced by kinetic studies. The five-carbon sugar xylose, stereochemically similar to glucose but one carbon shorter, binds to hexokinase but in a position where it cannot be phosphorylated. Nevertheless, addition of xylose to the reaction mixture increases the rate of ATP hydrolysis. Evidently, the binding of xylose is sufficient to induce a change in... 

Other kinetically allowed mechanistic models, i.e. hydroxide ion attack on the monoanion, can be rejected on the grounds that the required rate coefficients far exceed that found for alkaline hydrolysis of phosphate triesters. At pH > 9 two new reactions appear, one yielding a 1,6-a.nhydro sugar by nucleophilic attack through a five-membered transition state of the 1-alkoxide ion upon C-6 with expulsion of phosphate trianion. The second is apparently general-base catalysis by 1-alkoxide of water attack on C-6 or phosphorus through greater than six-membered cyclic transition states. 

In a typical enzymatic hydrolysis of a 5% suspension of ball-milled newsprint, a sugar syrup containing 1.6% glucose, 1.4% cellobiose, and 0.2% xylose is readily obtained. Figure 3 is illustrative of kinetic results obtained with T. viride cellulase at 3.5-filter-paper-units/mL (FP units/ mL) strength in a 5% suspension of ball-milled newsprint. 

Kinetics studies were conducted at 65 1°C in a jacketed batch reactor. Five hundred milliliters or 1 L of buffer was added to the reactor and heated to the assay temperature. The buffer pH was chosen according to the optima specified by the enzyme manufacturers. Corn flour (100-300 g/L) was then added to the reactor, along with a specified quantity of either soluble or immobilized amylase to initiate hydrolysis. Samples were collected at regular intervals over 30-60 min, and centrifuged to separate solids. The supernatant was analyzed for sugar content by measuring the %Brix with an optical refractometer. 

The present study investigated the inhibition of Saccharomyces cerevisiae by the liquid hydrolysate and the kinetics of enzymatic hydrolysis of the solid components produced by the pretreatment of aspen wood and corn stover by liquid hot water and hot carbonic acid. Inhibition of yeast was determined by measuring the rate of glucose consumption by yeast growing in hydrolysates produced at various reaction severities. The enzymatic hydrolysis rates of pretreated solids was determined by measuring rates of sugar accumulation of enzyme-digested pretreated solids. 

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