Biomedical Engineering Reference
In-Depth Information
7.5
Xylitol Production
Xylitol is a five-carbon sugar alcohol. Its sweetness equals that of sucrose, and the
heat value equals that of glucose. Xylitol metabolically is not involved with insulin
and can adjust abnormal glucose metabolism in vivo, with use as a nutritional
and therapeutic agent for diabetes. The xylitol has strong antiketone ability and
is available to rescue patients with ketoacidosis. Xylitol can slow the speed of
fatty acid production in plasma but not cause a rise of blood sugar. Thus, it is
also used as a hepatoprotective drug for patients with hepatitis. Xylitol has good
thermal stability and does not react with amino acids in heat. It can be made into a
variety of preparations, as nutritional medicines, with amino acids. Xylitol also has
an exceptional anticaries function as food. Xylitol has excellent characteristics as
glycerin and other polyhydric alcohols do. Thus, it has broad application prospects
in the pharmaceutical and chemical industries.
Currently, the method of industrial production of xylitol can be summarized as
follows: First, to hemicellulose rich in xylan is hydrolyzed to obtain purified xylose,
and then catalytic hydrogenation, column chromatography, and recrystallization
are used to obtain xylitol. The entire process consists of a series of complicated
purification steps; from xylose to xylitol, the yield is only about 50-60 %, with
production costs that are about ten times that of sucrose. The relatively high costs
limit the scope of xylitol use.
Biological conversion processes to produce xylitol are likely to reduce the
production costs of the process route, for which the fermentation does not need
to purify xylose, and the separation step can also be simplified. The enzymatic
synthesis is likely to achieve continuous and efficient production.
7.5.1
Microorganisms Used to Produce Xylitol
According to previous studies, only a small part of bacteria can generate xylitol
in nature and the efficiency of filamentous fungi is also not high, but the yeast
relatively easily changes xylose into xylitol. A study showed that the superior
performance of the yeast strains for producing xylitol was mainly concentrated in
the genus
Candida
, such as
Candida guilliermondii
,
Candida tropicalis
,
Candida
mogii
,
Candida parasilosis
; and partly in
Debaryomyces
, such as
Debaryomyces
hansenii
, and in
Pachysolen,
such as
Pachysolen cannophilus
.
7.5.2
Regulation of Microbial Xylitol Synthesis In Vivo
Xylose can be converted into 5-phosphorylation xylulose, which is the intermediate
of the hexose monophosphate pathway (HMP), in two paths. One is that xylose
is changed to xylulose by xylose isomers and then phosphorylated by xylulose
