Sweetness, of D-fructose

As part of a study on the structural features responsible for the intense sweetness of D-fructose, 3-deoxy-D-fructose, prepared by a literature procedure... 

The taste of D-fructose has been widely studied, and numerous relativesweetness intensity-scores have been assigned to it. Shallenberger and Acree reported that the crystalline solid is 1.8 times as sweet as sucrose. Verstraeten claimed that it is 8 times as sweet, but this must have been a misinterpretation of a statement made by E. G. V. Percival. ... 

By application of first-order, kinetic equations, B. Anderson and Degn claimed that an equilibrated (25°) aqueous solution of D-fructose contains 31.56% of jS-D-fructofuranose and 68.44% of -D-fructopyranose. N.m.r. studies, however, showed that, at equilibrium, a solution of D-fructose contains /3-D-fructopyranose, -D-fructofuranose, a-D-fructofuranose, and a trace of a-D-fructopyranose the distribution of these isomers was shown by gas-liquid chromatography to be 76,19.5, and 4%, respectively. Based on Anderson and Degn s result, Shallenberger reasoned that, as 0.68 X 1.8 = 1.22 (which approximates the reported sweetness of mutarotated D-fructose ), the furanose form(s) must possess very little sweetness. 

L-Arabinose did taste just about as sweet as D-galactose, and L-sorb-ose, the 5-epimer of D-fructose, also tasted sweet although it was only about one-fifth as sweet as D-fructose. 

Pseudo-p-DL-fructopyranose 180 was found to be nearly as sweet as D-fructose. The D-enantiomer 196 and the L-antipode 207 were also as sweet as D-fructose, but 196 was somewhat sweeter than 207. The small but observable difference in sweetness of 196 and 207 might be due to a stereogeometrical deformation of interrelations between the sweetness eliciting tripartite and a sweet receptor. 

In recent years, crystalline D-fructose has been produced and offered not only as an unusually sweet sugar but as a beneficial sweetening agent for people with certain ailments or, because of its claimed ease of metabolism, for athletes in whom it is said to provide quick energy. A review of the voluminous literature on the metabolism of D-fructose presents relatively clear information, and an attempt is made in this article to set forth a useful summary. 

Preparation. The abundance and wide distribution of D-fructose in natural products, its sweetness, and its poor crystallizing properties have stimulated considerable experimental work on methods of preparation. Most methods for isolation of the sugar depend on the formation of a difficultly soluble calcium levulate or fructosate. ... 

When sucrose is hydrolyzed, it yields one molecule of D-fructose and one molecule of D-glucose. This hydrolysis is catalyzed by an enzyme, invertase, and produces a mixture known as invert sugar. Invert sugar derives its name from the fact that the mixture is levorotatory, whereas sucrose is dextrorotatory. Thus, the sign of rotation has been "inverted" in the course of hydrolysis. Invert sugar is somewhat sweeter than sucrose, owing to the presence of free fructose. Honey is composed mostly of invert sugar, which is the reason it has such a sweet taste. 

By use of the semi-empirical molecular orbital method AMI, the optimised geometries and energies of the pyranoid forms of D-fructose (14), L-sorbose (15), 5-deoxy-D-rhreo-hexulose (16), and carba-D-fmctose (17) have been determined. The findings that a low-energy conformation exists for each of the three sweet compounds (14), (16), and (17) [but not for the non-sweet analogue (15)] in which HO-2 is hydrogen-bonded to 0-3, is thought to provide an explanation for the relative sweetness of these compounds. ... 

The temperature dependence of the taste intensity is especially pronounced in the case of D-fructose (Fig. 4.4). It is based on the varying intensity of sweetness of the different isomers ... 

In contrast to glucose and galactose, D-fructose is a ketohexose. The structure of D-fructose differs from D-glucose at carbons 1 and 2 by the location of the carbonyl group. D-Fructose is the sweetest of the carbohydrates, twice as sweet as sucrose (table sugar). This makes D-fructose popular with dieters because less D-fructose and, therefore, fewer calories are needed to provide a pleasant taste. o-Fmctose is found in fmit jnices and honey. 

Besides the sweet taste, some sweet substances show other side-tastes. For example, maltose and D-glucitol have the flavour of syrups and the taste of D-fructose solutions is fruity and shghtly sour. Sucrose taste in threshold concentrations is even described as slightly bitter. Xyhtol shows a cooling effect on dissolution. 

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). 

B-512F Dextransucrase has also been used in conjunction with endodextranase to produce a product with a high content of isomaltose and D-fructose (24). The D-fructose can be removed by a Ca-cation exchange resin, giving an isomaltose syrup. The isomaltose can then be catalytically reduced to isomaltitol, which is sweet and of low caloric value. 

A Taste of Honey The fructose in honey is mainly in the jS-D-pyranose form. This is one of the sweetest carbohydrates known, about twice as sweet as glucose. The jS-D-furanose form of fructose is much less sweet. The sweetness of honey gradually decreases at a high temperature. Also, high-fructose corn syrup (a commercial product in which much of the glucose in corn syrup is converted to fructose) is used for sweetening cold but not hot drinks. What chemical property of fructose could account for both these observations ... 

Enzymes are specific, however. For example, starch is depolymerized using enzymes to D-glucose (dextrose). The solution of glucose is then treated with glucose isomerase [9055-00-9] to give D-fructose in about 42% yield. No D-mannose is formed. Addition of isolated D-fructose to this solution gives the common 55% high fructose com syrup (HFCS) so widely used in soft drinks in the United States. HFCS is about 1.5 times as sweet as sucrose. 

D-Fructose is aimost twice as sweet as normal table sugar (sucrose) with about the same number of calories per gram. Lite food products use only half as much fructose as sucrose for the same level of sweetness, and so they have fewer calories. 

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