Moisture sorption isotherms

Kapsalis, J.G. 1987. Influence of hysteresis and temperature on moisture sorption isotherms. In Water Activity Theory and Application to Foods (L.B. Rockland and L.R. Beuchat, eds), pp. 173-213. Dekker, New York. 

A recent study of wood modified with a variety of anhydride reagents has questioned the validity of the use of the H-H model when interpreting the moisture sorption isotherms of modified wood (Papadopoulos and Hill, 2003). This has shown that although modification with anhydride reagents leads to a reduction in both monolayer and... 

Alternatively, aw may be measured by isopiestic equilibration. In this method, a dehydrated sorbent (e.g. microcrystalline cellulose) with a known moisture sorption isotherm (section 7.4) is exposed to the atmosphere in contact with the sample in an enclosed vessel. After the sample and sorbent have reached equilibrium, the moisture content of the sorbent can be measured gravimetrically and related to the nw of the sample. 

Figure 7.11 Hysteresis of a moisture sorption isotherm (from Fennema, 1985).

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

Labuza, T.P., Kaanane, A., and Chen, J.Y. 1985. Effect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods../. Food Sci. 50 385. 

Baskaran, V. M., Balasubramanyam, N., Malleshi, N. G., and Lokesh, B. R. (2000). Moisture sorption isotherms of nutritious supplementary foods prepared from cereals and legumes for feeding rural mothers and children. Eur. Food Res. Technol. 211, 27-31. 

Humidity control The question of humidity control was examined in several ways. The moisture sorption isotherm showed moisture levels from 3-6% over a relative humidity range of 20-95%. Also, samples held at 65°C for over 2 weeks remained within this range of water content, which was within pharmacopeial specification. Finally, samples which were heated did not show a change in crystal form by X-ray powder diffraction, such as partial conversion to an anhydrate. Based on these results, humidity control was not deemed necessary for comparative stress studies. Samples to be compared were all stored under identical conditions, i.e., in the same oven. Any local change in humidity around each sample was therefore a function of the sample itself. 

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. 

Fio. 22. Moisture sorption isotherm (25°C) for kangaroo tail tendon. The locations a, b, 0, d, and e divide the data into the four successive intervals discu.ssed in the text. (From Rougvie and Bear, 1953.)... 

F. 11. Moisture sorption isotherm for phenolic foam y = 68 kg m ) W moisture content by w eight... 

Fig. 4 Moisture sorption isotherm for lactose. (Adapted from Ref. i°f)...

The influence of sorbed moisture on chemical stability and the flow and compaction of powders and granulations is well established. The moisture content and hygroscopicity of excipients is particularly important as total product processing as well as finished product stability can be affected. Hygroscopicity, moisture-sorption isotherms, and equilibrium moisture content can be determined by thermogravimetric analysis and Karl Fisher titration methods. 

Moisture Sorption Isotherms. Green wood loses moisture to the atmosphere and approaches a moisture content designated as the equilibrium moisture content (EMC) for the particular atmospheric conditions. The EMC is a function of relative humidity, temperature, previous exposure history (hysteresis), species, and other miscellaneous factors. 

Effect of Relative Humidity and Sorption History. An indirect method for estimating wood moisture content is to measure its equilibrium relative vapor pressure h. This is related to wood moisture content by a sorption isotherm. The percent relative humidity (H) or relative vapor pressure (h) (H = 100 h) is the most important factor in determining the EMC for wood. A curve showing EMC as a function of percent relative humidity or relative vapor pressure at constant temperature is called a moisture sorption isotherm. 

When wood below the fiber-saturation point interacts with water, heat is evolved, and there are changes in the free energy and entropy of the sorbed water. Furthermore, the wood exerts swelling forces that can be measured. These effects can be treated by classical thermodynamic methods although moisture sorption by wood is not a perfectly reversible process because sorption hysteresis is involved, as was pointed out in the section on Moisture Sorption Isotherms (p. 136). 

Stamm and Loughborough (46) first calculated and Ql as functions of wood moisture content M by applying the Clausius-Clapeyron equation to the moisture sorption isotherms for wood at several temperatures. For example, can be obtained by replotting sorption isotherms, such as are shown in Figure 9, into the form of isosteres of constant moisture content, of In h against the reciprocal of Kelvin temperature. These plots yield a family of essentially straight lines, each at a different moisture content. The magnitude of Qi at any moisture content is calculated from... 

Velazquez de la Cruz G, Torres J, Martin-Polo M. Temperature effects on the moisture sorption isotherms for methylcellulose and ethylcellulose films. / Food Engin 2001 48 91—94. 

Moisture content hypromellose phthalate is hygroscopic it takes up 2-5% of moisture at ambient temperature and humidity conditions. For the moisture sorption isotherm of HP-50 measured at 25°C, see Figure 1. 

E. Other Methods, IR or Raman spectroscopy. X-ray diffraction, solid-state NMR, dilatometry, and moisture sorption isotherm may be used as tools for the qualitative or quantitative study of polymorphism. Brief descriptions of these techniques may be found elsewhere in this chapter. 

Bronlund, J. and Paterson, T. Moisture sorption isotherms for crystalline, amorphous and predominantly crystalline lactose powders, Int. Dairy /., 14, 247, 2004. 

Water acts as a plasticizer for soy flour (Yildiz and Kokini, 2001). Therefore, increase in water content will plasticize the matrix causing an increase in available free volume for molecular transport. Moisture diffusion as a result will be effected from the water activity of the system. The relationship between moisture content and a can be established using the moisture sorption isotherm (MSI) of soy flour. The glass transition temperature is a very important concept in the diffusion process. At the vicinity of the glass-transition temperature the diffusion process increases at a higher rate. Figure 46.1 shows the plasticization effect of moisture on soy flour and Gordon-Taylor prediction of Tg vs. moisture content (Yildiz and Kokini, 2001). 

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