Differential scanning calorimetry was employed to determine the amount of bound (nonfreezable) water in several model systems as a function of water activity (aw). Water activity was controlled by varying total moisture content or by adding a solute, urea, to the aqueous phase of the model system. Since the amount of bound water is dependent on the nature of the components, correlations between bound water and a, are meaningful only for specific systems. In every case studied, bound water, as g H,O/g solids, decreased with decreasing a,, along what might be called a bound water isotherm. The results indicate that measurements of bound water should refer to a specified value of a,. In addition, a, of the solution phase appears to be a major contributor to the driving force for water binding by macromolecules.
1. When ovotransferrin is partially saturated with iron, endotherms for apo-ovotransferrin, two monoferric ovotransferrins and Fe2-ovotransferrin are observed by differential scanning calorimetry. The relative sizes of the endotherms are changed in the presence of the iron-chelating agents nitrilotriacetic acid and ATP. 2. When iron is added as Fe(III)-nitrilotriacetate, at Fe-nitrilotriacetate: ovotransferrin ratios less than unity, the endotherm for Fe2-ovotransferrin is essentially absent. At Fe-nitrilotriacetate: ovotransferrin ratios of unity the only species present in solution in appreciable concentration as evidenced by their differential-scanning-calorimetry endotherms, are two monoferric ovotransferrins in approximately equal amounts. At Fe-nitrilotriacetate: ovotransferrin ratios greater than unity, the apo-ovotransferrin endotherm is absent, and the endotherms for the two monoferric ovotransferrins decrease in size as the endotherm for Fe2-ovotransferrin increases. 3. In the presence of nitrilotriacetate, binding of iron to the two sites of ovotransferrin is highly anti-co-operative, but essentially indiscriminate. When monoferric ovotransferrin is formed from apo-ovotransferrin, binding at one site is slightly favoured compared with binding at the other site, but once iron has been bound at either site, the binding affinity for iron at the unoccupied site is much decreased.
The sweetness of lactulose over the range of concentration 5-35% (W/V), measured by a trained panel using paired comparison with standard reference solutions of sucrose of varying concentrations, is 48% to 62% of that of sucrose. In addition, sensitivity thresholds and recognition thresholds for sweetness of lactulose and sucrose were determined by a rating-scale method. The sweetness of a mixture containing 10% (W/V) lactulose and 5% (W/V) sucrose showed a svnergistic effect of 22%. and a mixture of 5% (W/V) lactulose and 2.5% (W/V) sucrose showed 12% synergism. Partial' hydrolysis of lactulose to give a mixture containing 5% (W/v) lactulose, 2.5% (W/V) galactose, and 2.5% (W/V) fructose caused a 6% synergistic effect on sweetness.
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