Xylose production

The hydrolysis of acetylated xylo-oligomers from the steaming extract of birchwood using an enzyme mixture containing only xylanase and / -xylosidase was very limited (Table III). Chemical deacetylation showed that the substrate specificities of xylanase and / -xylosidase with respect to substrate DP overlapped, and that /J-xylosidase also hydrolyzed longer oligosaccharides than xylobiose to xylose. The addition of acetyl esterase to the hydrolysis of acetylated xylo-oligomers enhanced xylose production (Table III), but deacetylation was still incomplete (the acetyl content of... 

Xylose is not a by-product of furfural but its precursor. On account of this, its production is governed by the very kinetics of furfural formation, but with the aim of avoiding the latter as best as possible. However, the technically most important difference between xylose production and furfural production is the fact that furfural, because of its low-boiling azeotrope with water, is readily recovered as a vapor, whereas xylose, being nonvolatile, ends up dissolved in the liquid reaction medium, together with many other unwanted by-products, from where a recovery in a sufficiently pure form is not as easy as in the case of separating a product from a vapor mixture. Consequently, xylose plants are far more complicated, and therefore more costly, than furfural plants. 

A feed concentration of 15 g glucose and 15 g xylose per litre was used over a feed rate of 20-200 ml/hr. Samples were taken at successive points along the reactor length, and the usual analysis for glucose and xylose consumption, organic acid production and cell density were done. A kinetic model for the growth and fermentation of P. acidipropionici was obtained from these data. 

The ICR flow rate was five to eight times faster than the CSTR. The overall conversion of sugars in the ICR at a 12 hour retention time was 60%, At this retention time, the ICR was eight times faster than CSTR, but in the CSTR an overall conversion rate of 89% was obtained. At the washout rate for the chemostat, the ICR resulted in a 38% conversion of total sugars. Also, the organic acid production rate in the ICR was about four times that of the CSTR. At a higher retention time of 28 hours, the conversion of glucose in the ICR and CSTR are about the same, but the conversion of xylose reached 75% in the ICR and 86% in the CSTR. 

Enzymic syntheses are considered next. Xylitol is a substrate for sheep-liver L-iditol dehydrogenase, a NAD-linked enzyme. 1-Deoxy-D-xylitol, prepared by Raney nickel reduction of D-xylose diethyl dithioacetal in a 27% overall yield from D-xylose, was also reported31 to be a substrate, although with a higher Km and lower Vmax. The product was assumed to be l-deoxy-D-f/ireo-pentulose because of the appearance of a yellowish fluorescent spot when a chromatogram was sprayed with acidic 3,5-aminobenzoic acid, resembling that formed from 1 -deoxyfructose. There was no more-rigorous characterization. 

Figure 8.9 External mass transfer resistance—xylose hydrogenation and isomerization to xylitol and by-products on sponge Ni (based on the results of Mikkola et al. [22]).

Maloney, M.T., Chapman, T.W., and Baker, A.J. (1986) An engineering analysis of the production of xylose by dilute acid hydrolysis of hardwood hemicellulose. Biotechnol Progr., 2, 193. 

SPS could be degraded by the cruder, aaikalus preparation Pectinex Ultra-SP, giving a product consisting mostly of the monomeric sugars xylose, galactose, fucose, arabinose and... 

The 1,4-addition of ZnEt2 to 2-cyclohexenone was also performed by these workers in the presence of thioether-phosphite D-xylose-derived ligands, depicted in Scheme 2.11. In all cases, the chemoselectivities in the 1,4-product were higher than 97% but the enantioselectivities were modest (<41% ee). It was noted that changing the substituent in the thioether moiety produced an elfect on both the reactivity and the enantioselectivity, as shown in Scheme 2.11. 

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