Dried sorption isotherms

Figures 4.2.1 and 4.2.2 [4.9] show sorption isotherms 1, for such products which become less hygroscopic with increased temperature, or 2, for glucose, fats and oils which become more hygroscopic with increased temperature. Since freeze drying lowers aw values, the growth of bacteria, fungus and yeast below aw = 0.8 is reduced, or impossible. On the other hand, the Maillard reaction increases with decreasing aw up to a maximum at aw 0,6-0,7,...

Fig. 4.2.1. Typical sorption isotherms of many food products. n = mass of water/mass of dried product, aw at approx. +25 °C.

Arabosse, P., Rodier, E., Ferrasse, J.H., Chavez, S., and Lecomte, D. 2003. Comparison between static and dynamic methods for sorption isotherm measurements. Drying Technol. 21, 479-497. 

Despite some conflicting evidence (Kinsella and Fox, 1986), it appears that denaturation has little influence on the amount of water bound by whey proteins. However, other factors which may accompany denaturation (e.g. Maillard browning, association or aggregation of proteins) may alter protein sorption behaviour. Drying technique affects the water sorption characteristics of WPC. Freeze-dried and spray-dried WPC preparations bind more water at the monolayer level than do roller-, air- or vacuum-dried samples, apparently due to larger surface areas in the former. As discussed above, temperature also influences water sorption by whey protein preparations. The sorption isotherm for /Mactoglobulin is typical of many globular proteins. 

Figure 6. Moisture sorption isotherms of three ENCAPSULATED ORANGE OILS SPRAY DRIED AT DIFFERENT VOLTAGES a 200v b 150v c 75v.

Fio. II. Nitrogen sorption isotherm of two catalysts made from the same hydrogel, i.e., one very active, the other completely dead. The active sample was specially dried by extraction with an organic solvent of low surface tension to protect the larger pores. 

Sorption isotherm curves are graphical relationships showing the partitioning between solid and liquid form where mass adsorbed per unit mass of dry solids (S) is plotted against the concentration (C) of the constituent in solution. K is the sorption equilibrium constant N is a constant describing the intensity of sorption. The linear sorption isotherm can be expressed as follows ... 

Hammami et al. [4.27] determined 0.5 mbar and rSh = 55 °C as the optimum (for quality and operational data) drying conditions for the freeze-drying of apple slices, loaded with 17 kg/m2 and dried in 48-50 h. The rehydration ratio was -0.55 g/g of water removed and the texture loss of rehydrated apples was estimated to be more than 85%. Sa et al. [4.28] used DSC to measure the Tg of freeze-dried Golden Delicious apples after exposure to aw from 0.12 to 0.93. The sorption isotherms are reported. 

Water activity (a,) and water content have a profound influence on textural properties of foods. The three regions of the sorption isotherm can be used to classify foods on the basis of their textural properties (Figure 8-45). Region 3 is the high moisture area, which includes many soft foods. Foods in the intermediate moisture area (region 2) appear dry and firm. At lowest values of a, (region 1), most products are hard and crisp (Bourne 1987). 

Kapsalis et al. (1970) reported on a study of the textural properties of freeze-dried beef at different points of the moisture sorption isotherm over the complete range of water activity. Important changes in textural properties were observed at aw values of 0.85 and at 0.15 to 0.30. 

The content of non-evaporable water, relative to that in a fully hydrated paste of the same cement, was used as a measure of the degree of hydration. Portland cement paste takes up additional water during wet curing, so that its total water content in a saturated, surface dry condition exceeds the initial w/c ratio. Evidence from water vapour sorption isotherms indicated that the properties of the hydration product that were treated by the model were substantially independent of w/c and degree of hydration, and only slightly dependent on the characteristics of the individual cement. The hydration product was thus considered to have a fixed content of non-evaporable water and a fixed volume fraction, around 0.28, of gel pores. 

Brunauer and co-workers (B55,BI08) considered that the gel particles of the Powers-Brownyard model consisted of either two or three layers of C S-H, which could roll into fibres. D-drying caused irreversible loss of interlayer water, and the specific surface area could be calculated from water vapour sorption isotherms, which gave values in the region of 200m g for cement paste. Sorption isotherms using N2 give lower values of the specific surface area this was attributed to failure of this sorbate to enter all the pore spaces. 

Sorption isotherms, using water, Nj or other sorbates, have been widely used to study the pore structure of hep. The results obtained by dififerent investigators show considerable variations, which arise in part from dififer-ences in the conditions of preliminary drying. 

In spite of the composite nature of the stratum corneum, its water sorption isotherm is qualitatively identical to those of the more simple protein systems shown, suggesting that water interacts predominately with the protein components of the corneum. This conclusion is supported further by the results of chloroform-methanol (3/1 by volume) extraction which removed as much as 25% of the original dry weight (lipids and low molecular weight water-soluble components) but did not quantitatively alter the isotherm in the low relative humidities (18). The application of the Zimm-Lundberg cluster theory (56, 57) to the isotherm yields additional information as to the state of the sorbed water in the corneum. The tendency of water to cluster is expressed in this theory by the cluster function CiGn ... 

A sorption isotherm on excised human abdominal stratum corneum (female, age 68) is presented in Figure 9. The data were obtained on separate pieces of skin which were initially dried and then exposed to an air stream with a given RH uptake was followed until equilibrium was reached. Values of D calculated from half-time data for each humidity interval were 1.37-5.10 X 10 cm /sec. Scheuplein and Blank (16) reported a value of 5 X 10 cm /sec for human stratum corneum. The range of our values would indicate that D is concentration dependent. Full details will be reported in the future. 

If measurements are made at atmospheric pressure the maximum relative humidities that can be attained decrease with increasing temperature (Figure 10). The maximum relative humidity possible at any temperature is equivalent to the ratio of the prevailing atmospheric pressure to the vapor pressure of water at that temperature, expressed in percent. The practice of drying lumber at high temperatures (above 100 °C) has created a renewed interest in the sorption isotherms of wood at these temperatures (23). 

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