The low cost and purity of sucrose make it an appealing starting material for chemical modification into other useful products, such as the fat replacer sucrose polyester. Although the trivial name implies a polymer, olestra is only a modified disaccharide. Sucrose polyester (olestra, Olean) is a mixture of the hexa-, hepta-, and octa-esters of sucrose obtained by esterification with natural long-chain fatty acids. Esterification of at least six of the eight hydroxyls of sucrose renders it stable to iipase action, and therefore it is not digested and not absorbed. The water-insoluble hydrophobic character of olestra gives it oily laxative properties, a potential problem if it is consumed in excessive amounts. Olestra is excreted intact without any metabolism by the colonic microflora.
A small fraction of dietary fat-soluble vitamins (A, D, E, and K) is excreted along with the olestra because of mutual solubility. Supplemental vitamins are recommended when this is a concern. The more water-soluble vitamins are not partitioned into the olestra and are not lost. Soveral types of beneficial phytochemicals have been evaluated also, but only members of the lipophilic phytosterols and carotenoids show losses via excretion with olestra. Olestra showed no toxicity in human and animal studies. Because olestra has organoleptic and functional properties similar to those of natural fats and oils as well as good heat stability, it offers advantagns over other fat replacers for certain foods.
copyranosyI-(l—>4)-D-glucopyranose) are consumed as such in the diet. Maltose occurs naturally in the seeds of starch-producing plants, and small amounts are used in processed foods. Isomaltose (a-D-glucopyranosyl-(1—>6)-D-glucopyranose) probably does not occur naturally. Both maltose and isomaltose are formed by acidic hydrolysis of starch; isomaltose results from the structural branch points of amylopectin. a,«-Trehalose (a-D-glu-copyranosyl-(l<->l)-a-D-glucopyranoside) is a nonreducing disaccharide found in fungi (particularly in young mushrooms), yeasts, and insects.
These three a-linked glucose disaccha-rides are readily digested by intestinal a-glu-cosidases (glucoamylase, sucrase/a-dextri-nase, and trehalase). Digestion of starch by a-amylases in the lumen of the gastrointestinal tract yields maltose, maltotriose, and a-dex-trins containing the isomaltose moiety; glucoamylase and sucrase/a-dextrinase also complete the digestion of these products. It is curious that trehalase has persisted in the brush border of the human small intestine, because trehalose is a rather insignificant dietary disaccharide. Digestion of carbohydrates is discussed in detail in Chapter 5.
Raffinose and stachyose are oligosaccha rides carrying an a-D-galactopyranosyl and an a-D-galactopyranosyl-(l—»6)-a-D-galacto-pyranosyl unit, respectively attached to C-6 of the glucose unit of sucrose. They occur in relatively large amounts in soybeans, lentils, and other legume seeds. Because humans do not have a digestive a-galactosidase, these oligosaccharides pass into the lower gut to be metabolized by the anaerobic bacteria. Excessive flatulence may result from fermentation of these oligosaccharides, as well as of pectic polysaccharides.
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