Food Biochemistry and Food Processing

3 Recent Advances 55

Xylitol, also called wood sugar, is made from
xylose, which is found in the cell walls of most land
plants (Nigam and Singh 1995). Pure xylitol is a
white crystalline substance that looks and tastes like
sugar, making it important for the food industry as a
sweetener. One of the main advantages of xylitol
over other sweeteners is that it can be used by dia-
betic patients, since its utilization is not dependent
on insulin (Pepper and Olinger 1988). Xylitol is be-
lieved to reduce tooth decay rates by inhibiting
Streptococcus mutans, the main bacteria responsible
for cavities. Because xylitol is slowly absorbed and
only partially utilized in the human body, it contains
40% fewer calories than regular sugar and other car-
bohydrates. In the United States, xylitol has been
used since the 1960s, and it is approved as an addi-
tive for foods with special dietary purposes (Emodi
1978). Yeast(S. cerevisiae)is considered the ideal
microorganism for commercial production of xylitol
from xylose because of its well-established use in
the fermentation industry. South African researchers
(Govinden et al. 2001) isolated a xylose reductase
(EC 1.1.1.21) gene (XYL1) fromCandida shehatae
and introduced it intoS. cerevisiae. TheXYL1gene
fromCandidawas cloned into the yeast expression
vector pJC1, behind thePGK1promoter, and the
construct was transformed into yeast by electropo-
ration. Xylitol production from xylose by the trans-
formant was evaluated in the presence of different
cosubstrates including glucose, galactose, and malt-
ose. The highest xylitol yield (15 g/L from 50 g/L of
xylose) was obtained with glucose as cosubstrate.

PRODUCTION OFCAROTENOIDS IN
MICROORGANISMS

Carotenoids are structurally diverse pigments found
in microorganisms and plants. These pigments have a
variety of biological functions, such as coloration,
photo protection, light harvesting, and hormone pro-
duction (Campbell and Reece 2002). Carotenoids are
used as food colorants, animal feed supplements, and
more recently, as nutraceuticals in the pharmaceutical
industry. Recent studies have suggested many health
benefits from the consumption of carotenoids. Carot-
enoids such as astaxanthin,-carotene, and lycopene
have high antioxidant properties, which may protect
against many types of cancers, enhance the immune
system, and help relieve the pain and inflammation of
arthritis (Miki 1991, Jyocouchi et al. 1991, Giovan-
nucci et al. 1995). There is an increased interest in

extracting large amounts of carotenoids from natural

sources. The 1999 world market for carotenoids was

$800 million, with projections for $1 billion in 2005

(Business Communications Co. 2000). Although re-

searchers have found certain microalgae such as

Haematococcus pluvialisthat produce high amounts

of the carotenoid astaxanthin, extraction of carot-

enoids from these microalgae is difficult because of

their thick cell wall. For this reason, genetic engineer-

ing methods have been applied to produce carotenoids

in other microorganisms. The edible yeastCandida

utilisis a good candidate for commercial carotenoid

production. It is a “generally recognized as safe”

(GRAS) organism, and large-scale production of

peptides, such as glutathione, has already being suc-

cessfully achieved in C. utilis(Boze et al. 1992).

In microorganisms and plants, carotenoids are syn-

thesized from the precursor farnesyl pyrophosphate

(FPP) (Fig. 3.13). Miura et al. (1998) developed a

Figure 3.13.Biosynthetic pathway of the carotenoids

lycopene, -carotene, and astaxanthin.