Sugar Alcohols (Polyols) - Sweeteners: Nutritional Aspects, Applications, and Production Technology - page 56

to its anticariogenic properties (Mäkinen 1994). Moreover, it is used in the diet of diabetic subjects
because it is slowly absorbed, its initial metabolic steps are independent of insulin, and it does not
cause rapid changes in blood glucose concentration (Lang 1969; Förster 1974). According to Uhari
et al. (1996), xylitol-containing chewing gum has been shown to reduce the occurrence of acute
otitis media in day-care children.
3.3.3 Sorbitol
Sorbitol, (2S,3R,4R,5R)-hexane-1,2,3,4,5,6-hexol, belongs to the group of naturally occurring
hexitols. It (d-glycitol) was discovered initially in the fresh juice of mountain ash berries Sorbus
aucuparia L . in 1872. Sorbitol is a reduced form of dextrose and the most available polyol in nature.
The fruits of certain rosaceae are especially rich in sorbitol: plums 1.7-4.5 wt%, pears 1.2-2.8 wt%,
peaches 0.5-1.3 wt%, and apples 0.2-1 wt%. In fruits and leaves, sorbitol is formed as a biochemi-
cal intermediate in the synthesis of starch, cellulose, sorbose, or vitamin C. Small amounts are
found in the plane tree, the African snowdrop tree, and in various algae. Because sorbitol occurs to
a very small extent in grapes, assay of the sorbitol content of wine (qv) has been used to detect its
adulteration with other fruit wines or apple cider. An anhydride of sorbitol, polygallitol (1,5-sorbi-
tan), is found in the Polygala shrub (Takiura et al. 1974).
In animals, sorbitol can be detected as an intermediate in the absorption of glucose or the for-
mation of fructose via glucose (Boussingault 1872; Steuart 1955). It forms γ-polymorphous, inely
crystalline white crystals that are odorless, freely lowable, and slightly hygroscopic. Sorbitol is
also available in the form of water-clear syrup solutions. Sorbitol has a sweet, cooling taste and has
approximately half the sweetness of sucrose. Its physiological caloriic value corresponds to that of
other sugar alcohols (Schiweck et al. 2011).
Sorbitol is readily soluble in water, and 70% solutions of sorbitol in water are available commer-
cially, but tend to crystallize when cooled to <10°C for long periods. Sorbitol is also soluble in dilute
acetic acid, methanol, and warm ethanol; it is practically insoluble in organic solvents. The melting
range of sorbitol is 92°C-96°C. The optical rotation of aqueous solutions is increased greatly by the
addition of complexing salts (borax, ammonium molybdate). The enthalpy of solution is +111 kJ/
kg. The bulk density of instant grades is 40-50 g/100 mL, and that of crystallized grades is 60-70
g/100 mL (Burt 2006; Schiweck et al. 2011).
Depending on conditions (crystallization from solvents or from the melt), sorbitol crystallizes in
several modiications (Steuart 1955), with the most stable being the γ-form (mp 101°C). In neutral
media, sorbitol is temperature-resistant up to 150°C and, consequently, is bake and boil proof. As
a sugar alcohol, sorbitol does not undergo Maillard reactions with amino acids or proteins at high
temperature. Like other hexitols, sorbitol forms chelate complexes with some metal ions (e.g., Fe,
Cu, Co, and Ni) in aqueous solutions that are stable in the alkaline range. Thus, sorbitol inhibits the
prooxidative effect exerted by trace metals in autoxidation (e.g., of fats; Levin et al. 1995; Schiweck
et al. 2011; Suzuki et al. 1985).
Sorbitol cannot be fermented by yeast. It prevents the growth of bacteria by osmosis in highly
concentrated aqueous solutions (sorbitol content of about 50%). On the contrary, at low concentra-
tion, sorbitol solutions serve as a nutrient medium for bacteria. Acetobacter xylinium oxidizes sor-
bitol enzymatically to give l-sorbose (an intermediate in the synthesis of ascorbic acid; Schiweck
et al. 2011).
Sorbitol can be produced from electrochemical reduction of dextrose in an alkaline medium,
which leads to the production of a considerable amount of mannitol due to the alkali-catalyzed
epimerization of dextrose to fructose, which is more readily available than dextrose. Sorbitol is
industrially produced either by catalytic hydrogenation of dextrose (Figure 3.7), which produces less
than 2% mannitol, or by catalytic hydrogenation of sucrose as a mixture with mannitol (Schiweck
et al. 2011; Wang 2003).