Chemistry Reference
In-Depth Information
CH
CH
2
O
C
H
C
O
H
O
H
2
H
C
H
H
C
H
H
C
O
H
C
O
H
C
O
H
C
O
CH
2
CH
2
O
Glucose
Sorbitol
Figure 3.7
Sorbitol formation from glucose.
It is synthesized by sorbitol-6-phosphate dehydrogenase and converted to fructose by succi-
nate dehydrogenase and sorbitol dehydrogenase (Petrash 2004; Figure 3.8). Sorbitol is synthesized
commercially by high-pressure hydrogenation of glucose, usually using a nickel catalyst. Catalyst
promoters include magnesium salts, nickel phosphate, and iron. Other heterogeneous catalysts
used for glucose hydrogenation include cobalt, platinum, palladium, and ruthenium (Boyers 1959).
Reduction of glucose to sorbitol can also be effected using ruthenium dichlorotriphenyl phosphine
as a homogenous hydrogenation catalyst, preferably in the presence of a strong acid such as HCl
(Kruse 1976). In order to form sorbitol, glucose is usually hydrogenated in the pH range of 4-8.
Under alkaline conditions, glucose isomerizes to fructose and mannose; hydrogenation of the
fructose and mannose yields mannitol as well as sorbitol. In addition, under alkaline conditions,
the Cannizzaro reaction occurs, and sorbitol and gluconic acid are formed. Gluconic acid forma-
tion during hydrogenation can be minimized if anion exchange resins in the basic form are the
source of alkalinity (Jacot-Guillarmod et al. 1963). Although aqueous solutions are customarily
used, the monomethyl ethers of ethylene glycol or diethylene glycol are satisfactory solvents (Jacot-
Guillarmod et al. 1963). Electrolytic reduction of glucose was used formerly for the manufacture
of sorbitol (Creighton 1939). Both the γ- and δ-lactones of d-gluconic acid may be reduced to sor-
bitol by sodium borohydride (Hough et al. 1962). Sorbitol occurs from simultaneous hydrolysis
and hydrogenation of starch (Kool et al. 1952), cotton cellulose (Vasyunina et al. 1964), or sucrose
(Montgomery and Wiggins 1947; Figure 3.9).
The use of sorbitol is approved by more than 40 countries in foods, cosmetics, and pharmaceu-
ticals. It has been acknowledged as a generally recognized as safe (GRAS) substance in the United
States for its use in foods (LSRO for FDA 1986). The powdered grades are employed as a sugar
NADPH NADP
+
NAD
+
NADH
Glucose
Sorbitol
Fructose
Aldose reductase
Sorbitol dehyrogenase
Glucose-6-P
NADP
+
NADPH
CO
2
Figure 3.8
Sorbitol synthesis pathway.
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