Chemistry Reference
In-Depth Information
Table 5.13.
Physico-chemical properties of oligosaccharides
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Taste - sweetness decreases as molecular size increases
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Mouth-feel - larger-sized oligosaccharides have applications as fat replacers
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Viscosity - increases with increasing molecular size
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Solubility - decreases with increasing molecular weight
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Hygroscopicity - oligosaccharides can be used to control the moisture level in foods
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Colour reactions - decreases as molecular weight increases, because there are relatively fewer
available reducing moieties. Thus, reduced Maillard reaction in heat-processed food.
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Freezing point - depression of freezing point is inversely proportional to molecular weight
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Osmolality - decreases as molecular weight increases
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Coarse crystal formation - reduced occurrence with increased molecular weight
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Hydrolysis of oligosaccharides in acid conditions not related to molecular weight, but depends
on chemical composition
The susceptibility to hydrolysis of oligosaccharides during passage
through the gastrointestinal tract is an important characteristic. This has
been determined in a range of in vitro model systems of the stomach and
small intestine of humans. Different types of oligosaccharides have been
subjected to regimes such as (a) hydrolysis by human saliva (50%, v/v, saliva,
30 min exposure, pH 7.0, 378C); (b) hydrolysis by gastric acid (4 h exposure,
pH 2.0, 378C in acid/pepsin); (c) hydrolysis by pancreatic and brush border
carbohydrases (17%, v/v, porcine pancreatic and duodenal homegenates for
1 h exposure, pH 7.0, 378C). Carbohydrate analysis before and after exposure
by HPLC then determines the degree of hydrolysis. Tests such as these have
shown that xylo-oligosaccharide mixtures, palatinose condensates, commer-
cial galacto-oligosaccharide mixtures and lactulose are very resistant to
hydrolysis, whereas lactosucrose, along with gentio-oligosaccharides, soy-
bean oligosaccharides, fructo-oligosaccharide mixtures and inulin oligosac-
charides are hydrolysed slightly by such conditions (Crittenden and Playne,
1997, unpublished data).
5.7.2.
Properties of Lactulose
Lactulose solution is a yellowish, odourless clear syrup with a sweet
taste. Dry lactulose is a white, odourless crystalline powder. Lactulose is
soluble in water, but poorly soluble in methanol and insoluble in ether. Its
solubility in water is 76.4% (w/w) at 308C, rising to 86% at 908C. The melting
point is 168.5-170.08C. Its sweetness is classified as 0.48-0.62 that of sucrose.
It is 1.5 times sweeter than lactose. Acid hydrolysis of lactulose yields galac-
tose and fructose. Unlike lactose, lactulose cannot be hydrolysed by human
intestinal enzymes. Lactulose can be fermented by some human colonic
bacteria and acts as a prebiotic ingredient (Mizota et al., 1987; Tamura
