Air dehydration

The analysis of how differently soluble pectin fractions of strawberry slices are modified by air dehydration, combined osmotic-air dehydration,... 

The difference between the upper and the lower curves for phase composition and texture, at equal water activity, is the result of the solid gain after osmotic treatment. The higher the solid uptake, the higher the difference in texture. Compared to simple air dehydration, the combination of osmotic dehydration and air dehydration can produce a softer product at low water activity, which is more pleasant to eat by hand, or to incorporate into pastry, ice cream, cheese, yogurt (Giangiacomo et al., 1994), and so on. 

Alvarez, C.A., Aguerre, R., Gomez, R., Vidales, S., Alzamora, S.M., and Gerschenson, L.N. 1995. Air dehydration of strawberries Effects of blanching and osmotic pretreatments on the kinetics of moisture transport. J. Food Engineer. 25, 167-178. 

Collignan, A., Raoult-Wack, A.L., and Themelin, A. 1992a. Energy study of food processing by osmotic dehydration and air dehydration. Agricult. Engineer. J. 1, 125-135. 

Erba, M.L., Fomi, E., Colonnello, A., and Giangiacomo, R. 1994. Influence of sugar composition and air dehydration levels on the chemical-physical characteristics of osmodehydrofrozen fruit. Food... 

Riva, M., Campolongo, S., Avitabile Leva, A., Torreggiani, D., and Maestrelli, A. 2002. Structure-property relationships in osmo-air-dehydrated apricot cubes. In Book of Abstracts Symposium Drying, Processes, Structure and Functionality (IberDESH 2002) , pp. 103-104. Universidad Politecnica de Valencia, Valencia, Spain. 

Riva, M., Corteflino, G., Maestrelli, A., and Torreggiani, D. 2001. Structure collapse and color changes in osmo-air-dehydrated peach cubes. Food Sci. Biotechnol. 10, 598-601. 

Capillary membrane modules are not as inexpensive or compact as hollow fine fiber modules, but are still very economical. Their principal drawback is the limited pressure differential the fibers can support, typically not more than 10 to 15 bar. This limitation means capillary modules cannot be used at the high pressures needed for hydrogen or natural-gas processing applications. However, capillary modules are ideally suited to lower-pressure separations, such as nitrogen from air or air dehydration. In these applications, capillary modules have essentially the entire market. 

Counterflow modules are always more efficient than crossflow modules, but the advantage is most noticeable when the membrane selectivity is much higher than the pressure ratio across the membrane and a significant fraction of the most permeable component is being removed from the feed gas. This is the case for air-dehydration membrane modules, so counterflow capillary modules are almost always used. With most other gas-separation applications, the advantage offered by counterflow designs does not offset the extra cost of making the counterflow type of module, so they are not widely used. 

Fig. 1.1. Schematic of sulfur burning sulfuric acid plant, courtesy Outokumpu OYJ www.outokumpu.com The main components are the catalytic S02 + A02 — S03 converter (tall, back), twin H2S04 making ( absorption ) towers (middle distance) and large molten sulfur storage tank (front). The combustion air filter and air dehydration ( drying ) tower are on the right. The sulfur burning furnace is hidden behind. Catalytic converters are typically 12 m diameter.

Table 6.3. Details of two sulfur burning air dehydration plants.

Prior treatment usually none, 02 is occasionally added in dried air gas cleaning and cooling 02 addition in air dehydration gas cleaning and cooling 02 addition in air dehydration... 

Dehydration of Air. Asaeda et al. [19S4] evaluated gamma-alumina membranes on kaolin supports with a reported pore diameter of about 1 nm for air dehydration under a temperature of 31 to 7VC and a low TMP of 0.04 to 0.39 bar. A separation factor of 7 to 460 was obtained. The corresponding permeance is about 9.9x10 cm (STP)/s-cm -cmHg. 

The reactivity of manganese(II) oxalate is significantly influenced by the conditions during dehydration. Rate coefficients for decomposition of reactant previously held (2 h) at 460 K in a maintained vacuum were about 2.5 times greater than those found for a similarly treated sample heated at 460 K in air. This difference was apparent in both decomposition and oxidation reactions. These data, together with electron microscopic observations, showed that nucleation in the vacuum dehydrated porous salt occurred at internal surfaces, while in the air dehydrated salt some reorganization of the crystal structure was possible and there was more pronounced product formation at the external surfaces. 

In another process described by Leistner (1987) and Buckle et al (1988), the whole muscle is boiled for 40-45 min, after which it is cut into cubes or pieces (5 x 5 X 10 cm). Although beef is preferred because of its fibrous nature, pork and chicken are sometimes used (Lo, 1980). The cubes are added to the cure in a steam kettle and cooked until nearly all of the cure has evaporated. The meat is removed from the steam kettle and dried in a hot air dehydrator. The final a , is about 0.69. Leistner (1987) concluded that an a < 0.69 is critical for Chinese dried meats, although Ho and Koh (1984) suggested that an a, < 0. 61 is needed to prevent mold growth. 

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