Xylitol crystallization

Meso configurations can have enantiomeric conformers in crystals. Ribitol and xylitol have meso configurations and are optically inactive in solution. In the crystalline state, they have the bent-chain enantiomorphic conformations. Xylitol crystallizes in space group P212121, so each crystal contains either only the optically active left-handed or the optically active right-handed conformers. There are, of course, an equal number of left- and right-handed crystals in any one batch, which on dissolution give no optical activity.1... 

The xylitol produced is discolored, crystallized, and dried in the same equipment and conditions as the one described in the catalytic process. The productivity achieved for the biotechnological process is 0.73kg xylitol crystals per 1 kg of xylose. Figure 12.7 presents the process flow diagram for the biotechnological production of xylitol. 

Sampaio, F.C., Passes, F.M.L., Passes, F.J.V., De Faveri, D., Perego, P., and Convert , A. (2006) Xylitol crystallization from culture media fermented by yeasts. Chemical Engineering and Processing Process Intensification, 45 (12), 1041-1046. 

The most important pentose is xylose which can be produced from hardwoods by mild acid hydrolysis. Reduction of xylose gives xylitol, an interesting sweetener because of its ability to prevent dental caries. Industrial production of xylitol from birch wood hydrolyzates started in the 1970s in Finland. An interesting process was developed in which an ion exclusion separation technique is applied for purification and separation of xylose and xylitol from other impurities. Among the corresponding reduction products of hexoses, mannitol, which is also a natural product, has found some use. It can be separated from other alditols by crystallization. Under more drastic... 

Xylitol occurs as white crystals or as a crystalline powder. One gram dissolves in about 0.65 mL of water. It is sparingly soluble in ethanol. 

If not utilized in the pulp industry, hemicelluloses are hydrolyzed in the acid-catalyzed process, mainly to monosaccharides and to furan-2-aldehyde (pen-tosanes) and 5-hydroxymethylfuran-2-aldehyde (5.62) (hexosanes). Monosaccharide-containing syrups, after purification, are either fermented or utilized as wood molasses for feeding ruminants. In another approach, xylose, the least soluble component of syrup, is allowed to crystallize. Separated xylose is then hydrogenated over an Ni/Al catalyst at 120°C under 6 x 106 Pa into xylitol. Hemicelluloses, together with proteins, are capable of the Maillard reaction and may contribute to the overall secondary aroma of processed foodstuffs (Tomasik and Zawadzki, 1998). 

XylitoL jcyfo-Pentane-l,2,3,4,5-pentol xylite Eutrit Kannit Klinit Kylit Newtol Torch Xyliton. C5-Hl20 mol wt 152.15. C 39,47%, H 7.95%, O 52.38%. Intermediate in metabolism of d-glucose through glucuronate cycle in livers. Prepd by reduction of xylose G. Bertrand Bull Soc Chim. France [3] 5, 555 (1891) E, Fischer R-Stahel, Ber 24, 538 (1891). Prepn of metastable crystals M. L, Wolfrom, E, J, Kotin, /, Am. Chem, Soc, 64, 1739... 

The first observations clearly show that polyols modify the equilibrium morphology of boehmite particles, as it has been reported in the case of gibbsite crystallization [20, 21]. The use of polyols allows to obtain departures of the diamond shaped morphology observed at pH ll. If boehmite particles synthesized in presence of C2 to C4 polyols are always diamond shaped, the proportion of (101) and (010) planes is modified (table 1). The highest proportion of (101) face is reached for boehmite synthesis in presence of mesoerythritol (C4). Xylitol (C5) causes much important changes as the particle morphology is isotropic in this case. 

Dunitz and Bernstein [5] have recently documented several cases of vanishing polymorphs. These are usually metastable forms which, despite their thermodynamic instability, may have crystallized preferentially due to more rapid nucleation. Such metastable forms may persist and be used for many years before being displaced , when a thermodynamically more stable form is prepared. Attempts to regenerate the original polymorph are frequently met with failure. Specific compounds with such a history include e.g. 1,2,3,5-tetra-0-acetyl- -D-ribofiiranose, benzocaine picrate and xylitoL This disturbing phenomenon extends to pseudopolymorphs. A previously known monohydrate of the antibiotic ampicillin has not been obtained since the appearance of the trihydrate [24]. A possible explanation for this behaviour is that after minute particles of the stable polymorph enter the environment, they eventually become widely disseminated ( planetary seeding [5]) and serve as nuclei promoting crystallization of their own kind exclusively. 

The configurations of a series of 2- and 5-substituted derivatives of 1,4 3,6-dianhydro-hexitols (4) have been established from their C n.m.r. spectra. Crystal structure measurements have shown thatl,5-anhydro-2,3,4-tri-0-benzoyl-xylitol and -ribitol both adopt the expected Ci(d) conformation, whereas l,2,3,4-tetra-0-benzoyl-/3-D-xylopyranoside adopts the alternative C4 form, emphasizing the importance of the anomeric effect in the latter case. Unsaturated alditol derivatives are mentioned in Chapter 12. 

A study of the relaxational transitions and related heat capacity anomalies for galactose and fructose has been described which employs calorimetric methods. The kinetics of solution oxidation of L-ascorbic acid have been studied using an isothermal microcalorimeter. Differential scanning calorimetry (DSC) has been used to measure solid state co-crystallization of sugar alcohols (xylitol, o-sorbitol and D-mannitol), and the thermal behaviour of anticoagulant heparins. Thermal measurements indicate a role for the structural transition from hydrated P-CD to dehydrated P-CD. Calorimetry was used to establish thermodynamic parameters for (1 1) complexation equilibrium of citric acid and P-CD in water. Several thermal techniques were used to study the decomposition of p-CD inclusion complexes of ferrocene and derivatives. DSC and derivative thermogravimetric measurements have been reported for crystalline cytidine and deoxycytidine. Heats of formation have been determined for a-D-glucose esters and compared with semiempirical quantum mechanical calculations. ... 

C). They concluded that xylitol separation by crystallization from fermented hemicellulose hydrolyzate is feasible. 

The technical literature abounds with tales, some dating back over 150 years, of problems caused by the perverse behaviour of crystallizing systems (e.g., Buckley, 1952 Woodward and McCrone, 1975 Dunitz and Bernstein, 1995). Xylitol, for example, first prepared in 1891 was considered to be a liquid until... 

The X-ray irradiation of xylitol, sorbitol, and dulcitol single crystals has been studied. ... 

Thermal parameters affecting the crystallization of sorbitol have been studied depending on conditions, two crystalline modifications can arise, and a process for the rapid crystallization of molten sorbitol was proposed. Compression parameters of crystalline xylitol have also been reported. The viscosity of concentrated aqueous solutions of sodium and potassium halides in presence of D-mannitol has been studied the viscosity was related to the structure-making or structure-breaking effect of the salt in solution. ... 

The hemicelluloses are potentially very usdiil. Studies on utilization of hemicelluloses from cer straws have demonstrated to be a potential fermentation feedstodc in inoduction of dhanol, acdone, butanol, and rgiitol. The current wes of lan on an industrial scale involve then conversion to Qrlose, xylitol and fotfritaL Xylitol is produced by hydrofysis tii x lan, crystallization j lose, and iQrdrc enation. This has bemi tested in a variety of... 

Figure 12.6 presents the process flow diagram of the catalytic production of xylitol. Three main sections are (i) pretreatment, (ii) reaction-condensation, and (iii) crystallization. The pretreatment section deals with the pH adjustment and the removal of the impurities contained in the feed stream, soluble and insoluble solids, and remnants of the hydrolysis of biomass. But the main goal is the removal of the alkalis, whose presence catalyzes the conversion of xylose to xylonic acid (Cannizzaro mechanism). The first step of the downstream processes (unit PI) involves a series of treatment by ionic resins, activated carbon column, and chromatographic separations. Then, the pH is adjusted (unit Rl) in a range of 5-6 through the neutralization of acids (mainly acetic acid) by 1M solution of calcium hydroxide (Ca(OH)2) ... 

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