Info

Solubility (at 25°C), g/100-g solution

62 - 30.2 51.2°

30.2 ->• 51.2a'6

72 51.2°

Optical rotation, [a]D

112.2 52.7"

112.2 52.7a'6

18.7 -> 52.7°

Heat of solution (at 25°C), J/g''

+ 59.4

+ 105.1

+ 26.0

"Equilibrium value. 'Anhydrous basis.

To convert joules to calories, divide by 4.184.

"Equilibrium value. 'Anhydrous basis.

To convert joules to calories, divide by 4.184.

this temperature, crystallization of the hydrate occurs to its limit of solubility and the pattern then follows that of the monohydrate as described earlier. The rate of attainment of equilibrium is increased by heating or in the presence of acids or bases. Data for the solubility of the equilibrium mixtures, as interpolated from previous data (7), are listed in Table 3.

Dextrose in solution or in solid form exists in the pyr-anose structural conformation. In solution, a small amount of the open-chain aldehyde form exists in equilibrium with the two cyclic forms (Fig. 4) and is responsible for the reducing properties of dextrose (8).

Dextrose exhibits the reactions of an aldehyde, a primary alcohol, a secondary alcohol, and a polyhydric alcohol. In acid solution, either after standing for a prolonged time or after heating, dextrose undergoes polycondensa-tion, i.e., dehydration. This reaction yields a mixture of di-and oligosaccharides, most of which are the disaccharides gentiobiose and isomaltose. In acid solution and at high temperature, dehydration leads to formation of 5-hydroxymethylfurfural, which is a water-soluble, high-boiling, and relatively unstable compound. Polymerization of 5-hydroxymethylfurfural yields dark-colored compounds, and is an intermediate in the discoloration of sugar solutions (9). Dextrose decomposition under these conditions also yields levulinic and formic acids.

In mildly alkaline solution, the principal reaction of dextrose is partial transformation, i.e., isomerization, to fructose and other ketoses. D-Mannose, saccharinic acids, and other decomposition products are formed to a lesser extent. Highly alkaline solutions, particularly in the presence of atmospheric oxygen, can form a complex mixture of products of decomposition and rearrangement. Mild oxidation in slightly alkaline solution gives D-gluconic acid in quantitative yield. More vigorous oxidation with nitric acid yields glucaric acid, tartaric acid, oxalic acid, and other compounds resulting from fragmentation of the dex trose molecule. Alkaline Fehling's solution is reduced by dextrose with roughly 5 atoms of copper reduced per molecule of dextrose. Electrolytic reduction or catalytic hydrogénation of dextrose is practiced commercially to manufacture sorbitol.

When dextrose is heated with methanol containing a small amount of anhydrous hydrogen chloride, a-methyl-D-glucoside is obtained in good yield and can be isolated by crystallization. Similar reactions occur with higher alcohols, but the reaction products are more difficult to isolate by crystallization. Dextrose reacts with acid anhydrides in the presence of basic catalysts, yielding esters. The complete reaction gives the pentaacetylated derivative.

The reaction of dextrose with a nitrogen-containing compound, for example, amino acids or proteins, yields a series of intermediates that form pigments of varied molecular weight (Maillard reaction). The type of pigments produced is dependent on reaction conditions such as pH, temperature, and concentration of reactants (10).

Dextrose is the common intermediary metabolite in carbohydrate metabolism because other utilizable monosaccharides are converted to dextrose before they are further metabolized. Starch, glycogen, and the common monosaccharides are hydrolyzed enzymatically in the alimentary canal. Dextrose is normally absorbed into the portal-vein blood, by which it is transported first to the liver and then circulated to all parts of the body. Before dextrose or any other monosaccharide can be utilized metabolically, it must be phosphorylated and enter the glycolytic cycle. Other monosaccharides, such as galactose and fructose, eventually are converted to glucose-6-phosphate and metabolized like dextrose.

Manufacture

Until 1960, a commercial high dextrose content syrup was produced by acid-catalyzed hydrolysis of starch at elevated

Table 3. Solubility of Dextrose in Water

Temperature (°C)

Dextrose in solution (wt %)

Temperature (°C)

Dextrose in solution (wt %)

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