Moisture Desorption Isotherms Research Articles

A method of obtaining the equations of moisture adsorption and desorption isotherms

A new method is suggested which can be used to obtain the approximate equations of adsorption and desorption isotherms using the error functions.

Moisture sorption and tensile strength of some tableted direct compression excipients

Abstract Moisture sorption and desorption isotherms, at 25°C, have been measured for some direct compression (DC) excipients and analysed according to the Young and Nelson equations and the GAB equation, which distinguish different physical forms of moisture. The mechanical characteristics, such as tensile strength and derived tableting indices (brittle fracture propensity and tensile strength isotropy) of compacts produced from the same DC excipients, have been determined after storage at various environmental relative humidities. The tensile strength reaches a maximum value and then begins to decrease when the moisture content is about double that corresponding to a tightly bound monomolecular layer. The changes in the mechanical characteristics have been explained by the combined effect of moisture on the interparticle and intermolecular forces.

PHYSICAL PROPERTIES of BAKED PRODUCTS AS FUNCTIONS of MOISTURE and TEMPERATURE

Thermal and physical properties, including moisture desorption isotherms, density, heat capacity and thermal conductivity of a commercial white bread, muffin and biscuit were determined experimentally as functions of temperature (20°C to 85°C) and moisture content (0 to 0.6 gig sample). Moisture desorption data were fit to a two parameter empirical model. Density was not a strong function of either temperature or moisture content but depended on the method of achieving a final moisture content. Heat capacity as a function of temperature was determined by differential scanning calorimetry. A mass fraction model adequately correlated heat capacity to moisture content. Thermal conductivity was measured with a thermal conductivity probe. Correlations were developed for thermal conductivity as a function of moisture and temperature.

AN INTERPRETATION OF THE MOISTURE DESORPTION ISOTHERM OF WHEAT

It is shown that the moisture desorption isotherm of wheat can be interpreted in harmony with Cassie's theoretical assumptions regarding adsorption on localized sites by the equation[Formula: see text]where N is the number of adsorbed molecules per gram of wheat and S is the number of localized sites. Kf/(1 + Kf) is a Langmuir-type factor for adsorption of the low energy fraction of molecules, and K′fn is a factor for adsorption of the normally condensed fraction of molecules. In agreement with theory K′,n,and S are independent of temperature. The dependence of K on temperature gives a value of −6.27 kcal/mole for the partial molal enthalpy of dilution of the low energy fraction of molecules, which is in the order of magnitude of a hydrogen bond. The analysis also illuminates the practice of the drying of wheat. An exact correspondence appears to exist between the calculated values of S and the apparent surface (or equilibrium) moistures observed in studying the diffusion of moisture out of wheat. The theoretical relation[Formula: see text]is therefore given for predicting apparent surface moisture contents in the vacuum drying of wheat, where[Formula: see text]