Rehydration capacity

Deuteration of silica gel therefore shows to be governed by the rehydration capacity of the substrate. At low pretreatment temperature the extent of H/D exchange is limited by internal silanols, which may not be reached by (heavy) water molecules. At high pretreatment temperature the exchange is restricted by the low physisorption ability of the isolated silanols. 

With fresh activated-clay catalyst, endothermic peaks are observed at temperatures of about 300, 1200, and 1600°F. These three peaks are attributed to loss of physically adsorbed water, loss of chemically bound (hydroxyl) water, and collapse of the montmorillonite structure, respectively. The hydroxyl water originally present amounts to 3 or 4%. The magnitude of the peak at 1200°F. decreases if the sample is heated above 800°F. prior to thermal analysis, and disappears completely if the sample is calcined at 1100°F. The thermal-analysis curve for the dehydrated catalyst is flat up to the point at which the montmorillonite structure begins to disappear. If the catalyst has not been heated above 1450°F., it becomes rehydrated upon exposure to moisture and a new endothermic peak appears in the curve between 800 and 1000°F. The size of the new peak increases as that of the original hydroxyl-water peak decreases it corresponds to 1.5 to 2.0% sorbed water with catalyst that has been rehydrated after calcination at 1100°F. The rehydration capacity of the catalyst decreases as the catalyst becomes partially deactivated with use. 

Levi et al. [84] observed that pectin, one of the major cell wall and intercellular tissue components, played a significant role in the rehydration capacity of dehydrated fruits. 

Rehydration will usually not lead to recovery of the initial product, but to a different product. Drying creates irreversible transformations such as protein denaturation (insoluble), modified aroma and color, loss of firmness and shape. In order to comparethe rehydration capacities, different criteria and standards, like temperature or stirring, have been defined according to product specificity and final use (Lewicki, 1998 Pisecky, 1997). 

Taiwo et al. (2002) reported a substantial increase in rehydration ability with regards to rehydration time and rehydration capacity after the osmotic dehydration of PEF-pretreated apple slices. Osmotic dehydration itself showed a higher rate, with a reduction in process duration by up to 40%. 

Silica gel is a polar material. The presence of silanol groups is responsible for the acidic catalytic effect of this material (the pK of Si OH is comparable to that of phenol). The mode of action of silica gel is based on adsorption (Fig. 3.9), a phenomenon that leads to the accumulation of a compound at the interface between the stationary and mobile phases. In the simplest case, a monolayer is formed (known as a Langmuir isotherm) but there is also some attraction and interaction between molecules that are already adsorbed and those still in solution. This contributes to the asymmetry of the elution profile. Although it demonstrates good resolution and a high adsorption capacity, bare silica gel is seldom used for analytical purposes. For most applications, it must be deactivated by partial rehydration (in 3-8% water). 

Deparaffinized and rehydrated tissue sections on slides are immersed in jars containing 1 mM EDTA-NaOH (pH 8.0), and the jars are placed in boiling distilled water in a stainless steel 6-liter-capacity pressure cooker with an operating pressure of 103 kPa/15psi. The pressure cooker is sealed and brought to full pressure the duration of heating is 3 min. The cooker is depressurized and cooled under running tap water for 20 min. 

It has been demonstrated that mixed oxides obtained from calcined LDHs have the ability to act as sorbents for a variety of anionic compounds from aqueous solution. This ability is because of the propensity for the mixed oxide to hydrate and re-form an LDH in such conditions and is of particular interest for the decontamination of waste-water. Hermosin et al. have found, for example, that MgAl-LDHs calcined at 500 °C are potential sorbents for the pollutants trinitrophenol and trichlorophenol from water [208, 209]. The adsorption mechanism was shown, using PXRD, to involve reconstruction of the LDH, with the uptake of the phenolate anions into the interlayers. Similarly, the ability of calcined MgAl-LDHs to remove nitriloacetate anions from solution has been demonstrated [210]. Calcined LDHs have been utilized also for the sorption of radioactive anions, such as 111, from aqueous solution [211]. A particularly attractive feature of the use of calcined LDHs for the remediation of waste-water is that the sorption capacity of the material may be regenerated via calcination of the rehydrated LDH. 

Swelling capacity swelling properties are reversible. Magnesium aluminum silicate swells to many times its original volume in water to form colloidal dispersions and may be dried and rehydrated any number of times. 

A complication arises from one of the methods of measuring sorption isotherms for food. Food that has previously been dried and then is rehydrated will have a different sorption isotherm (adsorption isotherm) from that which is in the process of drying (desorption isotherm). This difference is due to a change in water-binding capacity in foods that have been previously dried. 

Though possessing a large capacity for adsorption, silica gel in its simplest state, as described previously, is used less and less for analysis since its qualities change with time, resulting in a lack of reproducibility of separations. For many applications it must be, at least, rehydrated (3—8 per cent water) in order to be deactivated. 

For dehydration of diced potatoes, a three level four factor four responses design was used. The four factors stayed the same as in the carrot study. The four responses were rehydration ratio, bulk density, non-enzymatic browning and water holding capacity. Optimal conditions were determined to be 1450C arid 10 min in HTST fluidized bed dryer with blanching time of 4.5 min and biopolymer... 

Grant (58), single-cell protein concentrates by McElwain et al. (59), and fish protein by Groninger and Miller (60) and Chen et al. (61). Succinylated fish myofibrillar protein had rapid rehydration and good dispersion characteristics at neutral pH (60). Succinylation of fish protein concentrate improved its emulsifying capacity and emulsion stability (61). 

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