Rapid Method of Determination of Sorption Isotherms and Water Apparent Diffusivity

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Abstract
In the context of a research on the quality of pasta made from soft wheat, reaction kinetics had to be measured at definite temperatures and water contents. It was chosen to realize such experiments in a pilot dryer. To be able to fix temperature and water content of product for a definite period of time, it was first necessary to determine sorption curves of pasta and a drying model including the values of apparent diffusivity of water at elevated temperatures. This paper presents a method to meet these objectives. It consists in realizing a sequence of plateaux of air relative humidities of decreasing values, at constant temperature, in the dryer. Air-product equilibrium at the end of each plateau would have been too long to attain, therefore equilibrium product water content was calcultated using an exponential model. Apparent water diffusivity was determined by adjustement of a diffusive model, taking in account that the air humidity change from one plateau to the next one was a linear ramp and not a step-wise change. The average relative repeatability of equilibrium water Content is 0.1% water/dry basis. There is more dispersion on apparent diffusivity.

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