Crystallization of D-fructose

The seed crystals of D-fructose were obtained by ball milling crystals produced by spontaneous nucleatlon from an aqueous ethanolIc solution of fructose and allowing them to stand at room temperature in slightly supersaturated ethanolIc solution until the desired crystal size (20-40 microns) was achieved. They were then stored at 30 C in saturated, anhydrous ethanol to prevent further crystal growth. A portion of this slurry was added to the crystallizer as the seeds. 

Very concentrated, aqueous solutions containing 95% or more of D-fructose on a dry basis can be converted into a powder of the same (dry basis) composition by adding seed crystals of D-fructose and kneading the mixture into a crystalline mass which, on being cooled, solidifies to a stable, dry, high-D-fructose product.61... 

Crystallization of D-fructose is usually effected at ice-box temperature from an ethanol solution, and further purification and recrystallization is performed at room temperature. The spherulitic aggregates of fine needles obtained by this technique are those of the hemihydrate, as shown by Young and coworkers. This possibility had been put forward by Honig and. lesser, but sufficient proof was lacking at that time. The anomalous x-ray diffraction data reported by Wolfrom and Thompson for their preparation of i fructose, in comparison with those of the normal form of D-fructose, may be explained by hemihydrate formation. Indeed, purification with ethanol at 25° results in dehydration, with formation of the... 

Figure 2.5 Forms of D-fructose in solution. In water, the major conformers are the (3-pyranose 0-p 73% at 25°C) and (3-furanose ((3-/, 20%) forms. On crystallization in water, D-fructose exclusively adopts the chair conformation, as shown by X-ray analysis. ...

Batch crystallization studies of D-fructose from aqueous ethanolic solutions demonstrate that crystal growth rate is dependent on supersaturation (possibly to the 1.25 power), ethanol content and temperature. It appears that solution viscosity also has an effect. Growth rates of up to 1 pm/mln were measured. 

The separation of mixtures of hexitols has long been a difficult problem. The removal of sorbitol from L-iditol by bacterial action is a classical example. Destruction of one component as a means of separation is drastic and is applicable to only a few mixtures. Even from an analytical point of view, separation has been difficult. The proportions of D-mannitol and sorbitol in the reduction products of D-fructose may be determined approximately by crystallizing and weighing the D-mannitol, but the amount of D-mannitol still in solution remains an unknown quantity. 

Reaction of D-fructose with two molecular proportions of trityl chloride in pyridine for two days at room temperature gave a ditrityl-D-fructose (30% yield), which crystallized (m. p. 96-97°) with two molecules of pyridine. Removal of the pyridine left amorphous ditrityl-D-fructose, [a]D +17.5° in chloroform. This gave a crystalline oxime identical with that obtained by tritylating D-fructose oxime. Helferich148 designates this ditrityl derivative as 1,6-ditrityl-D-fructose without further proof. 

In addition to enzymatic hydrolysis (see Section 5.8.1), inulin can be hydrolyzed chemically and subsequently dehydrated to yield hydroxymethylfurfural (van Dam et al., 1986), a key industrial chemical (Gretz et al., 1993 Fuchs, 1987 Kunz, 1993 Makkee et al., 1985 van Dam et al., 1986). Likewise, catalytic hydrogenation of D-fructose yields D-mannitol and D-sorbitol mixtures from which mannitol can be removed via crystallization (Fuchs, 1987). 

D-Fructose is crystallized from water-alcohol on a commercial scale.60 The solubility of D-fructose in the alcohol—water mixtures is less than in pure water, so that systems saturated and supersaturated with respect to D-fructose can be much lower in concentration of D-fructose than at corresponding states of D-fructose-water systems. 

D-fructose the most readily water-soluble sugar. It does not crystallize from stored juices. Because of its hygroscopicity, it retains moisture in sugar-preserved food and intensifies its flavor and aroma. Metabolism of D-fructose delivers less energy than sucrose. This saccharide neither causes nor accelerates tooth cavities. It accelerates ethanol metabolism. 

P- pyranose form of D- fructose (this form is present in crystals of this sugar) to the a- pyranose, a, P- furanose and to the non-cyclic fructose. In ehemistry this reaction is known as a mutarotation. 

Gel formation was observed during the phase-diagram studies of Yoimg and coworkers this occurred between —20° (62.5% of D-fructose) and -f 10° (78% of D-fructose). Another translucent gel formed dining crystallization from a cold solution in absolute methanol containing some calcium chloride. ... 

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. ... 

The best source of D-fructose for large-scale purposes is probably the inversion of sucrose by acids or invertase. The separation of the ketose from the concomitant D-glucose may then be accomplished by direct crystallization, by removal of the d-glucose after oxidation with bromine to D-gluconic acid (the ketose is not affected), or by precipitation of the calcium fructosate. Hydrolysis of the natural inulins already mentioned may also serve for the preparation of D-fructose, which may be isolated from the hydrolyzate by precipitation of the lime complex.141 D-Fructose is fermented by yeast. 

The keto tautomer has been isolated in crystalline form for a number of D-fruc-tose-A -alkylanilines and the structure was unequivocally confirmed by IR and solid-state NMR spectroscopic data, " as well as by X-ray diffraction studies. " The ability of D-fructose-A -alkylanilines to crystallize spontaneously in the acyclic keto form is unprecedented among six-carbon reducing carbohydrates, and is matehed only by ribulos- and xylulos-amines, " which themselves are the Amadori rearrangement products. This phenomenon may be explained on the basis of an interplay of the hydrophobic effect and crystal-packing forces, given that many D-fruetose-A -alkylanilines still crystallize in the p-pyranose form, and for the majority of those which crystalhze in the keto form, the acyclic tautomer constitotes a minor eonstituent in equilibrated solutions. 

D-fructose, C HijOo. Crystallizes in large needles m.p. 102-104 C. The most eommon ketose sugar. Combined with glucose it occurs as sucrose and rafftnose mixed with glucose it is present in fruit juices, honey and other products inulin and levan are built of fructose residues only. In natural products it is always in the furanose form, but it crystallizes in the pyranose form. It is very soluble in... 

The raw materials from which di-D-fructose dianhydrides can be obtained in appreciable yield are readily available from comparatively inexpensive agricultural feedstocks. Thus, these compounds are attractive as chiral-starting materials for chemical synthesis. Their stability to acid and heat, and their relative rigidity, because of the conformational constraints covered here, are also features that might be exploited during syntheses.119 A series of variously substituted di-D-fructose dianhydrides has been prepared,119 starting from 6,6 -dideoxy-6,6 -di-halosucroses. The properties of these and other derivatives of di-D-fructose dianhydrides are summarized in Tables XIV-XX. Two of these derivatives, 48 and 56, exhibit thermotropic liquid-crystal properties.119... 

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