Kinetics, rehydration

Kaymak-Ertekin, R, Drying and rehydrating kinetics of green and red peppers, /. Food Sci., 67 (2002) 168-175. 

Smith, J.A.C., and Nobel, P.S. 1986. Water movement and storage in a desert succulent Anatomy and rehydration kinetics for leaves of Agave deserti. J. Exp. Bot. 37 1044-1053. 

While it is clear that many proteins may be freeze-dried and reconstituted with little or no loss in activity, it is usually not obvious as to whether the freeze-dried protein is basically native in conformation or whether the solid-state conformation is distinctly nonnative, with the native and active conformation quickly forming during rehydration. The observation that lysozyme regains enzymatic activity in the solid state above about 20% water [56] demonstrates that a protein need not be in a predominantly aqueous system to maintain activity and presumably possess native structure. However, this observation does not necessarily imply that the structure is nonnative at lower water contents where the enzymatic activity disappears. The loss of activity at lower water contents [56] could simply be a consequence of greatly slowed kinetic processes (i.e., greatly restricted molecular mobility as the system passes into the glassy state). 

The thermodynamic instability of all freeze-drying products at normal pressure and normal humidity (40 to 80%) results in their interaction with atmospheric moisture after freeze-drying, although the results of this interaction can be rather different. A large number of equilibrium lower hydrates demonstrate significant kinetic stability to rehydration, so that a short contact time of the product with... 

The dehydration and rehydration reactions of calcium sulfate dihydrate (gypsiun) are of considerable technological importance and have been the subject of many studies. On heating, CaS04.2H20 may yield the hemihydrate or the anhydrous salt and both the product formed and the kinetics of the reaction are markedly dependent upon the temperature and the water vapour pressure. At low temperatures (i.e. < 383 K) the process fits the Avrami-Erofeev equation (n = 2) [75]. The apparent activation energy for nucleation varies between 250 and 140 kJ mol in 4.6 and 17.0 Torr water v our pressure, respectively. Reactions yielding the anhydrous salt (< 10 Torr) and the hemihydrate ( (HjO) >17 Torr) proceeded by an interface mechanism, for which the values of E, were 80 to 90 kJ mol. At temperatures > 383 K the reaction was controlled by diffusion with E, = 40 to 50 kJ mol. ... 

The rate of rehydration of the hemihydrate [84] in liquid water passes through a maximum, the position of which depends on the rate of reactant dissolution and of product precipitation. A general kinetic equation was derived which also described previously published data. 

The thermal dehyi-ation of Na2C03.H20 between 336 and 400 K fits the Avrami-Erofeev equation with = 2 (E, = 71.5 kJ mol and., 4 = 2.2 x 10 s [110]). The apparent reduction in rate resulting from an increase of /r(H20) is ascribed to competition from the rehydration reaction. Electron micrographs confirm the nucleation and growth mechanism indicated by the kinetic behaviour, nucleation develops from circular defects that may be occluded solution. 

Rastogi, N.K., Angersbach, A., Niranjan, K., and Knorr, D. 2000a. Rehydration kinetics of high pressure treated and osmotically dehydrated pineapple. Journal of Food Science 65 838-841. 

In conclusion, for thermodynamic and kinetic reasons aldolizations are difficult to perform with good yields. In several examples a spectacular increase in catalytic activity upon rehydration shows that this reaction is very specific for OH or F groups. The effect of substituents and the rate law both suggest that the mechanism can be described in terms of conventional organic chemistry. 

The diffraction patterns of the dehydrated samples dispersed in water for 24, 48, and 72 h are shown in Figs. 8.5c—8.5e, respectively. As can be verified, the intensity of the 001 diffraction peak of the hydrated samples enhances in intensity for largest contact times. However, even after 72 h, less than half of the sample was rehydrated. So, both data sets, calorimetric and XRD, show that the dehydrated vermiculite does not behave as a hydrophilic compound, with its total surface, that is, the external and internal space of the lamella, exhibiting a very low affinity toward water molecules. This indicates that besides being unfavorable from a thermodynamic point of view (endothermic), the rehydration process, that is, adsorption of water molecules on the surfiice, is kinetically slow. 

Fig. 17.4. Infrared (a). X-ray (b) and DSC (c) evidence of the gradual transformation of H3OUO2PO4. SHjO material in (U02HP04)(CH3C0CH3)H20 (for a phase, the interslab distance is 1.08 nm) by dipping for a few days in acetone with 1% HjO (reproduced with permission of Chapman Hall). A logarithmic time scale is used. If dry acetone is used the a phase (UO2HPO4XCH3COH3) with an interslab distance of 0.97 nm is obtained. The kinetics of the acetone de-intercalation and rehydration process as a function of exposure to moisture is illustrated by the X-ray intensity (height) plot of the interslab distances and by the plot of total enthalpy measured on DSC traces between 350 and 450...

For example, an aminofunctional silane can condense almost instantaneously after hydrolysis due to the high pH of the amine group, leading to a kinetically controlled structure which is thermodynamically unequilibrated. Provided that a sufficiently high concentration was used, the solution turns into a gel. With time, in the presence of water, the catalytic action of the amine reorganizes the siloxane structure by rehydration and condensadon reactions, and redissolves the silane. Thus, the time factor is also important when silane stmctures are to be studied. 

Giri, S. K., Prasad, S 2007a, Drying kinetics and rehydration characteristics of microwave-vacuum and convective hotair dried mushrooms, J. Food Eng. 78 512-521. 

The study of the decondensation kinetics of the silicates is not easy. The NMR of Na reveals that when a solid sodium silicate is put into solution, the first step is the integral rehydration of the sodium that passes into the aqueous solvent. Afterward, there is progressive decondensation of the silicic polymers or oligomers. The decondensation reactions take place in the washing liquor, which is a dilute aqueous medium. The reactions evolve very rapidly, and the kinetics are difficult to measure by NMR. That is why other techniques, like the silico-molybdic complex, are used [16]. With this method, the quantity of silicic monomers in a solution resulting from the decondensation of the larger silicic species can be measured. The monomers formed react with the molybdic acid in controlled conditions to give a yellow complex that can be easily titrated by spectrophotometry. 

Based on the preceding discussions of water adsorption on dehydroxylated surfaces, the most likely mechanism of rehydration of silicate surfaces dehydroxylated above 450°C is adsorption on acidic silicon sites contained in strained two- and three-membered rings, followed by dissociative chemisorption. Since two-membered rings comprise a small fraction of the silica surface, the rehydration kinetics will initially reflect the rate of dissociative chemisorption of three-membered rings which cover approximately one quarter of the dehydroxylated surface. Subsequent water adsorption occurs preferentially on silanols formed by hydrolysis of three-membered rings. 

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