Freezing-point depression crystallization

Sweetness is primarily a function of the levels of dextrose and maltose present and therefore is related to DE. Other properties that increase with increasing DE value are flavor enhancement, flavor transfer, freezing-point depression, and osmotic pressure. Properties that increase with decreasing DE value are bodying contribution, cohesiveness, foam stabilization, and prevention of sugar crystallization. Com symp functional properties have been described in detail (52). 

The filtrate, obtained after the crystals of bis-(4-iminopentane-2-ono) copper (II) were filtered off, was evaporated under reduced pressure. The residue was extracted with water, the extracts were filtered, and the filtrate was treated with sodium hydroxide and p-toluenesulfonyl chloride (18). In this way, about 30% of the 1,3-propanediamine was accounted for as the crude ditosylate (identity confirmed by lack of freezing-point depression with authentic sample), but only a trace of diamine could be detected in the distillate. 

Physical properties of glycerol are shown in Table 1. Glycerol is completely soluble in water and alcohol, slighdy soluble in diethyl ether, ethyl acetate, and dioxane, and insoluble in hydrocarbons (1). Glycerol is seldom seen in the crystallized state because of its tendency to supercool and its pronounced freezing point depression when mixed with water. A mixture of 66.7% glycerol, 33.3% water forms a eutectic mixture with a freezing point of —46.5°C. 

Figure 1 Time profile of temperature, concentration, and viscosity during freezing of 3% sucrose. Data are calculated assuming that ice crystallization begins at -15°C and that the solution composition follows the equilibrium freezing point depression curve. Viscosities are estimated from a fit of viscosity data over a wide range of composition and temperature to a VTF-type equation (i.e., see [4]). (Data from [7,8].)...

You may be familiar with the property illustrated in the first demonstration, freezing-point depression, if you have ever used an ice cream maker. In ice cream making, the cream mixture is poured into a container and the container is placed in a bath of salted ice. The salt-ice mixture cools to a point below the freezing point of water, so when the water in the cream comes into contact with the sides of the container, it freezes. A paddle to scrape ice crystals off the sides of the container is turned constantly until the cream and ice form a uniform semisolid. The fi eezing point of the salt-ice mixture is lowered because of the presence of a dissolved solute (salt) in the solvent (water). Antifreeze works on the same principle. Chosen to be noncorrosive, antifreeze is a material added to radiator water to lower the freezing point so the radiator water will not readily freeze. 

Corn syrup is used in almost every type of confection to control sucrose and dextrose crystallization, which may lead to crumbling. Corn syrup in appropriate proportion with sucrose and dextrose allows the formation of an amorphous glass and produces a candy with the desirable appearance. The following physical properties of corn syrup are extremely important in the preparation of medicated candies density, dextrose equivalent (DE), hygroscopicity, sugar crystallization, viscosity, freezing-point depression, and osmotic pressure. 

In other instances, the values did not correlate with those provided from other sources and in at least one instance no freezing point depression was noted. These effects have not been well explained, although similar anomalies have been observed with other high polymers. For example, in the case of cellulose derivatives, Freudenberg has shown that the freezing point method is very susceptible to error, due both to a tendency of the solute to crystallize and to the probable presence of small amounts of impurities of low molecular weight. 

The phenomenon of freezing-point depression is analogous to that of boiling-point elevation. Here, we consider only cases in which the first solid that crystallizes from solution is the pure solvent. If solute crystallizes out with solvent, the situation is more complicated. 

Pure solid solvent coexists at equilibrium with its characteristic vapor pressure, determined by the temperature (Section 10.4). Solvent in solution likewise coexists with a certain vapor pressure of solvent. If solid solvent and the solvent in solution are to coexist, they must have the same vapor pressure. This means that the freezing temperature of a solution can be identified as the temperature at which the vapor-pressure curve of the pure solid solvent intersects that of the solution (Fig. 11.12). As solute is added to the solution, the vapor pressure of the solvent falls and the freezing point, the temperature at which the first crystals of pure solvent begin to appear, drops. The difference ATf = T/ — Tf is therefore negative, and a freezing-point depression is observed. 

Melting point also changes when a solute is added, but it is not related to the vapor pressure. Instead, it is a factor of crystallization. Impurities (the solute) interrupt the crystal lattice and lower the freezing point. Freezing point depression for an ideally dilute solution is given by the equation ... 

The starting point for the analysis of the freezing-point depression phenomenon is the observation that when a solid freezes out from a mixture, it generally is pure and it is the component that is concentrated in the solution, that is, the pure solvent. [Indeed, this is the basis for zone refining, in which melting and resolidification (or recrystallization) are used to purify metals.] The equilibrium condition when the first crystal of pure solvent forms is... 

The determination of the molecular weight has also often led to difficulties. Freezing-point depression, diffusion and sedimentation in the ultracentrifuge have sometimes led to discrepancies owing to association or solvents of crystallization. The partial substitution method of Battersby and Craig , the thermoelectric method of Simon and Tomlinson , mass-spectrometric measurements and x-ray analysis have been used recently to overcome such difficulties. 

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