Temperature versus water content

This method involves preparation of a standard curve by equilibration of a specific amount of dry standard material in duplicate or triplicate over different saturated salt solutions. The standard curve is a plot of aw versus water content of the standard material. The standard must be stable during reuse of the material, and the chamber used should be exactly the same as will be used later. For the measurement, the size of each standard should be in a controlled narrow range, (e.g., —1.6 0.1 g). Once the standard curve is made, dry aliquots of the same mass of standard are equilibrated over a large quantity of the food material (-10 to 20 g). The moisture content of the standard material is then measured (e.g., by mass gain) and aw is estimated from the standard curve (Vos and Labuza, 1974). This method avoids preparation or storing of saturated salts for each determination moreover, use of a standard shortens the equilibration, thus requiring less time for measurement, and less abuse of temperature for sample as well as standard. 

Glass Transition Temperature (Tg) of Amorphous Sucrose versus Water Content... 

Fig. 19 Separation characteristics of a PVA/PVSu membrane (60 bilayers) for various alcohol/water feed mixtures. Plot of water content in permeate (a), total flux (b), separation factor a (c) and separation efficiency (d) versus water content in feed mixture. Per-vaporation temperature 58.5 °C (from Ref. [73])...

Fig. 7.7 Glass-rubber transition temperatures associated with the midpoints of the transitions versus water content for glycerol plasticized waxy maize starch filled with 0 (filled circle), 3.2 (circle), 6.2 (filled square), 16.7 (square), and 25 wt% (filled diamorul) tnnicin whiskers. Solid lines serve to guide the eye (Angles and Dulresne 2000). Copyright (2000) American Chcanical...

The hydration shell is formed with the increasing of the water content of the sample and the NA transforms from the unordered to A- and then to B form, in the case of DNA and DNA-like polynucleotides and salt concentrations similar to in vivo conditions. The reverse process, dehydration of NA, results in the reverse conformational transitions but they take place at the values of relative humidity (r.h.) less than the forward direction [12]. Thus, there is a conformational hysteresis over the hydration-dehydration loop. The adsorption isotherms of the NAs, i.e. the plots of the number of the adsorbed water molecules versus the r.h. of the sample at constant temperature, also demonstrate the hysteresis phenomena [13]. The hysteresis is i( producible and its value does not decrease for at least a week. 

Fig. 6.20 Plots for yields of hydrogen from supercritical water extraction of shell samples versus cellulose content at different temperatures...

Oil content versus moisture content in dry basis for control slices fried at 120,150, and 180°C is shown in Figure 11.2. There is a clear effect of the frying temperature on oil uptake at moisture contents < 1 g water/g dry solid the higher the fiying temperature the lower the oil content -average values of 0.39, 0.35, and 0.30 g/g dry basis for 120, 150 and 180°C, respectively. Figure 11.3 shows kinetics of total oil (TO) uptake and their different fractions (PSO, STO,... 

Pfeifer et al. (263) conclude from their measurements of T, and T2 versus temperature in samples with controlled water contents that the lifetime of sorption complexes of water is 3.5 x 10-9 sec at 50°C with nonlocalized cations and at - 10°C with localized ones. Water was found to be bound more strongly in faujasites with higher Si/Al ratios, which agrees with model calculations by Dempsey (282) of the electrostatic fields around cations. At higher coverages the mobility of H20 is independent of the Si/Al ratio and is two orders of magnitude lower than in bulk water. 

The water content of the vapor phase in (H-V) equilibrium is very small (typically less than 0.001 mole fraction) and therefore difficult to measure accurately. As a consequence, in the history of gas processing, semilogarithmic straight lines (gas water content versus reciprocal absolute temperature) from the Lw-V region were extrapolated into the H-V region with limited justification. 

Fig. 4.7 shows a temperature versus composition diagram for an ethanol-water mixture. It is slightly idealized to make a pedagogical point. If one starts with initial composition Q in the liquid (point A in Fig. 4.7), the vapor mixture will have the composition B. If this vapor is condensed, the liquid mixture will have composition C2 at point C the vapor phase will have the composition C3 at point D. Further cycles will achieve ever smaller increases in ethanol liquid content until the azeotrope (constant-boiling) composition is reached at 95.6 mass% and 78.2°C... 

Temperature versus enthalpy profiles similar to those shown in Figure 4-15 are obtained when starches are heated either in excess water, that is, when water stareh ratios are equal to two or greater, or under intermediate water content. The start of the endothermic peak at To corresponds to loss of birefringence, in the form of the typical Maltese cross, when the starch granule is viewed under polarized light. A single endotherm, such as that obtained under excess water conditions, is referred to as the... 

Figure 1 Schematic evolution of the freeze-drying process. Temperatures (upper curve) and water content (lower curve) are indicated versus time. In the temperature diagram cs = maximum temperature of complete solidification 7 = minimum temperature of incipient melting = absolute limit for fast process = maximum allowed temperature for the dry product RMF, final requested residual moisture.

FIGURE 27.5 Water content required for membrane saturation versus temperature for different pressures of air. The dashed lines indicate the water produced for different air stoichiometries (ZSW (Zentmm fiir Sonnenenergie- und Wasserstoff-Forschung) measurements Internal publication). 

Another interesting result is given in fig. 13 in which is plotted the amount of desorbed water versus the annealing temperature for the 15 % H2O - acid Nafion system. From this figure it is shown that around 60 % of the total water content can be desorbed. We therefore have around 9 % desorbable water and around 7 % fixed water. These values are in close agreement with the two absorption regimes and also with the fact that no DSC peak has been observed for water content velow 8 % water. 

---