Environmental Engineering Reference
Xylitol is a naturally-occurring sugar with a wide spectrum of potential applica-
tions. It has a sweetening power matching that of sucrose (table sugar), and is
used as a sugar substitute in the food processing industry . Xylitol produces
a perceived sensation of coolness in the mouth as it comes in contact with saliva
because of its negative heat of solution . Xylitol can be produced through
microbial transformation reactions by yeast from D-xylose, or by both yeast and
bacteria from D-glucose ; D-xylose can also be directly converted into xylitol
by NADPH-dependent xylose reductase .
Xylitol can be produced by bacteria and filamentous fungi , but often
the best producers are yeasts, especially species of the genus Candida,
such as C. guilliermondii, C. pelliculosu, C. parapsilosis, and C. tropicalis
[47, 48]. Other yeast genera investigated for xylitol production from xylose
include Saccharomyces, Debaryomyces, Pichia, Hansenula, Torulopsis, Kloeckera,
Trichosporon, Cryptococcus, Rhodotorula, Monilia, Kluyveromyces, Pachysolen,
Ambrosiozyma, and To r u l a . Bacteria species such as Enterobacter liqufaciens ,
Corynebacterium sp., and Mycobacterium smegmatis  can also produce xylitol.
The conversion of D-xylose to xylitol by microorganisms is important for industrial
production and has been studied extensively in yeasts, as summarized in Table 4.
Batch fermentation has been explored extensively for the production of xylitol (47) .
Laboratory-based investigations in culture flasks did not show significant xylitol
production. A higher substrate concentration is mandatory to obtain the genuine
yield of xylitol in batch fermentation. Further studies will help to define the mech-
anism of xylitol fermentation under the desired set of fermentation reactions. The
higher level of end products like ethanol, biomass and carbon dioxide in the media
may also inhibit xylitol production .
In fed-batch operations, a constant substrate concentration can be maintained
during the course of fermentation . C. boidinii NRRL Y-17231 fermentations
showed 75% theoretical xylitol yield in a fed-batch process, compared to 53% the-
oretical yield in a batch process . Alternatively, continuous culture techniques
have shown higher productivity with increased xylitol yields from several microor-
ganisms. Feeding of nutrient media with an optimized dilution rate is a critical
parameter in continuous cultures that helps achieve the higher rate of xylitol pro-
duction. Table 4 lists a variety of microbial strains producing xylitol using different