Xylitol purification

Misra, S., Gupta, R, Raghuwanshi, S., Dutt, K., and Saxena, R.K. (2011) Comparative study on different strategies involved for xylitol purification from culture media fermented by Candida tropi-calis. Separation and Purification Technology, 78 (3), 266-273. 

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... 

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, a sugar alcohol, has potential use as a natural food sweetener, a dental caries reducer and a sugar substitute for diabetics. It is produced by chemical reduction in alkaline conditions of the xylose derived mainly from wood hydrolyzate (169). The recovery of xylitol from the xylan fraction is about 50-60% or 8-15% of the raw material employed. Drawbacks of the chemical process are the requirements of high pressure (up to 50 atm) and tenq>erature (80-140°C), use of an expensive catalyst (Raney-Nickel) and use of extensive separation and purification steps to remove the by-products that are mainly derived from the hemicellulose hydrolyzate (770). The bulk of xylitol produced is consumed in various food products such as chewing gum, candy, soft drinks and ice cream. It gives a pleasant cool and fresh sensation due to its high negative heat of solution. 

The chemical process is very expensive because of the high working temperature, application of pressme for the hydrogenation of xylose, and extensive steps for separation and purification. The industrial-scale production contains less xylose and other sugars such as arabinose, maimose, galatose, and glucose as major impurities (Sakakibara et al., 2009). From the economic viewpoint, the biotechnological production of xylitol seems to be very attractive, with the use of low-cost crude hemicellulosic hydrolysate as a potential substrate (Rao et al., 2006). 

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