Dehydration, heat treatment

The structural features of ceU wall polysaccharides of carrots have been studied by Stevens and Selvendran (1984) and Massiot et al.(1988). Plat et al.(1991), Ben Shalom et al.(1992) and Massiot et al.(1992) investigated the changes in pectic substances of carrots after blanching, dehydration and extended heat treatment. Data on the changes in ceU waU polysaccharides of canned carrots are lacking. This study aims to investigate the effect of preheating time at low temperature and the addition of CaCL on texture and on the composition of various pectin fractions of carrots canned by conventional and by a new process. 

The total emission In the commercial heat treatment of 5 to 8 hours at 170 to 160°C varied from 0.4 to 1.2% for CO2 and 0.05 to 0.2% for CO and 0.04 to 0.1% for total acids based on dry board. Some of this emission might emanate from pyrolysis of higher molecular weight material condensed and deposited on the walls of the heat treatment chamber. The heat of formation of this CO2 and CO Is about half the total heat release measured. Part of the oxidation products might remain in the solid phase within the board material, e.g. as bound carbonyl and carboxylic groups, partly followed by heat consuming dehydration reaction. 

Instead of dehydrating the tissue sections by using a series of alcohol solutions, one can simply air-dry the slides prior to a heat-treatment step. 

The heat treatment leads to whey proteins becoming adsorbed, altering the behavior of the micelle. Dehydration by ethanol, for example, leads to aggregation of the micelles. 

The possibility of synthesizing MgO powders from liquid precursors by a sol-gel route involving the hydrolysis and condensation of magnesium ethoxide has been examined by several researchers.[44,45] Excellents results were reported by Kla-bunde et a/.[46,47] using a method involving the formation of Mg(OH)2 gel from Mg(OCH3)2. Heat treatment of Mg(OH)2 precursor at 773 K under vacuum yielded the dehydrated MgO with a 500 m2 g 1 surface area and 4.5 nm crystallite size. 

An example of this approach is the co-precipitation of mixed oxides, hydroxides and carbonates from aqueous solution. Another approach is for the cations of interest to be complexed to form an organometallic compound, an alkoxide, for example. Subsequent hydrolysis and heat treatment of the precursors yields the required oxide. Cations can also often be incorporated into sols and gels and the mixed oxides produced by dehydration. 

Fig. 7. N2 adsorption isotherms, measured at 25° C, for Ag/LSX (a) after drying at room temperature followed by vacuum dehydration at 450° C, (b) after drying at room temperature followed by vacuum dehydration at 350° C, (c) after drying in air at 100° C followed by vacuum dehydration at 350°C and (d) after drying in air at 100°C in air, followed by heat treatment in air at 450° C and, finally, vacuum dehydration at 450° C (Hutson and Yang, 2000).

The method normally applied for the conversion of the organic raw material into activated carbon is carbonization, that is, pyrolysis under inert atmosphere. This procedure is followed by activation, explicit heat treatment with an oxidizing agent, or by simultaneous carbonization and activation with a dehydrating compound [171,178],... 

The hydrated cations that form when salts dissolve in water often remain hydrated if the solutions are evaporated to dryness thus, evaporation of an aqueous solution of ferric chloride yields solid Fe(H20)jj+(Cl )3. It is a mistake to consider the water merely as something extra thrown into ordinary solid FeCh. Not only does the hydrate have a different color and different crystal structure from the anhydrous chloride, but upon heat treatment, the Fe—O bonds survive while the chlorine will leave the lattice as IICL (Thus anhydrous FeCl3 cannot be made by simple dehydration of the hydrate.) Such water of hydration, fittingly called cation water, occurs frequently. 

Such chemical changes may lead to compounds that are not hydrolyzable by intestinal enzymes or to modifications of the peptide side chains that render certain amino acids unavailable. Mild heat treatments in the presence of water can significantly improve the protein s nutritional value in some cases. Sulfur-containing amino acids may become more available and certain antinutritional factors such as the trypsin inhibitors of soybeans may be deactivated. Excessive heat in the absence of water can be detrimental to protein quality for example, in fish proteins, tryptophan, arginine, methionine, and lysine may be damaged. A number of chemical reactions may take place during heat treatment including decomposition, dehydration of serine and threonine, loss of sulfur from cysteine, oxidation of cysteine and methio-... 

Place the filter in a scintillation vial and place the vial (uncapped) in a 100°C oven for exactly 10 min. This heat treatment will dehydrate the filter and cause it to curl up in the vial. Avoid excessive heating, which will cause the filter to turn yellow and possibly quench the 3H (i particles in the sample as they are being counted color quenching, see introduction to Experiment 3). 

Fig. 12.7. Glass in anhydrobiotic larvae and their recovery after heat treatments, (a) DSC thermograms for slowly and quickly dehydrated larvae, (b) Dependence of the recovery rate after rehydration on exposure to high temperatures in slowly (,filled symbols) and quickly (open symbols) dehydrated larvae. Circles and triangles show recovery after exposure to high temperature for 5 min and 1 h, respectively. Data from [73]...

The name activated alumina is generally applied to an adsorbent alumina (usually an industrial product) prepared by the heat treatment of some form of hydrated alumina (i.e. a crystalline hydroxide, oxide-hydroxide or hydrous alumina gel). It has been known for many years that certain forms of activated alumina can be used as powerful desiccants or for the recovery of various vapours. It was apparent at an early stage that the adsorbent activity was dependent on the conditions of heat treatment. For example, in 1934 Bayley reported that the adsorption of H2S by a commercial sample of activated alumina was affected by prior heating of the adsorbent at different temperatures, the maximum uptake being obtained after heat treatment at SS0°C. During an investigation of the catalytic dehydration of alcohols, Alekseevskii (1930) found that a calcination temperature of c. 400°C was required to optimize the adsorption of the alcohol reactants, whereas calcination at 600°C was preferable for the adsorption of the olefine products. 

The structural and textural changes involved in the thermal decomposition of the three trihydroxides have been studied in considerable detail (Aldcroft et al., 1968 Lippens and Steggerda, 1970 de Boer, 1972 Rouquerol etal., 1975,1979a Ramsay and Avery, 1979 Stacey, 1987). It is now clear that the dehydration sequence is dependent not only on the crystalline structure of the trihydroxide, but also on its texture and the conditions of heat treatment. 

Dehydration of rutile crystals involves the removal of hydrogen-bonded water, coordinately bonded water and surface hydroxyls (dehydroxylation). The temperature ranges corresponding to these three stages overlap and depend on the rutile sample and the conditions of heat treatment. Generally the removal of molecular water occurs at temperatures of up to about 300°C and progressive dehydroxylation... 

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