Sweet tasting noncarbohydrates

Synthetic noncarbohydrate compounds can also produce a sweet taste. Saccharin, a synthetic compound, tastes 400 times as sweet as sucrose and has the following structure ... 

Monosaccharides are sweet-tasting solids that are very soluble in water. Noncarbohydrate low-calorie sweeteners such as aspartame have been developed as sugar substitutes. Pentoses and hexoses form cyclic hemiacetals or hemiketals whose structures can be represented by Haworth structures. Two isomers referred to as anomers (the a and p forms) are produced in the cyclization reaction. All monosaccharides are oxidized by Benedict s reagent and are called reducing sugars. Monosaccharides can react with alcohols to produce acetals or ketals that are called glycosides. 

Although many of the monosaccharides and disaccharides taste sweet, they differ considerably in their degree of sweetness. Dietetic foods contain sweeteners that are noncarbohydrate or carbohydrates that are sweeter than sucrose. Some examples of sweeteners compared with sucrose are shown in Table 18.1. 

The relatively low degrees of sweetness of carbohydrates when compared with the sweetness of some noncarbohydrate compounds such as cyclamates, saccharin, certain aminoacids, and so on (see Fig. 5.3) can be explained by the relatively weak hydrophobic character of the C-6 hydroxymethyl group found in many pyranoses. However, the presence of a hydrophobic sweetness intensifier can explain why certain carbohydrates are much sweeter than other carbohydrates for example, D-fructose and D-xylose are much sweeter than D-glucose and sucrose (see Table 5.1). Both D-fructopyranose and D-xylopyranose (the predominant forms of D-fructose and D-xylose in solution and in the crystalline state) have methylene groups that are not substituted by a hydroxyl group, and hence are more hydrophobic and produce a sweeter taste (see Fig. 5.5). 

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