Study of the Moisture Sorption Isotherms and Isosteric Heat of Sorption of the Medicinal Plant Launeae Nudicaulis from A
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 4, July, 2020
STUDY OF THE MOISTURE SORPTION ISOTHERMS AND ISOSTERIC HEAT OF SORPTION OF THE MEDICINAL PLANT Launeae Nudicaulis FROM ALGERIAN SAHARA A. Fedol,a A. Cheriti,a and M. R. Ouahranib
UDC 544.31
The aim of this work is to determine the sorption isotherms of Launeae nudicaulis, a medicinal plant growing in south-west of Algeria. The equilibrium moisture content was measured, using the static gravimetric method, with the water activity from 6.3 to 89.8% at 30 and 40oC. It was shown that at higher water activities the moisture content increases sharply with the temperature, resulting in crossing of the isotherm curves. Four models were applied for analyzing experimental data (namely, the Peleg, GAB, Henderson–Thompson, and the BET modified ones). The desorption and adsorption data showed the best correlation with the Peleg model. The isosteric sorption heat of water was determined from the equilibrium data at different temperatures. This heat was shown to decrease as the moisture content increases and to be a polynomial function of the moisture content. Keywords: Launeae nudicaulis, equilibrium moisture content, modeling of sorption isotherm, isosteric heat of sorption. Introduction. Water profoundly influences the product properties, such as quality and safety. Thus, it is important to know the water amount (moisture content) in the product that can be maintained at a given energy state (the water activity). The moisture sorption isotherms describe the relationship between the water activity and moisture content at constant temperature. The nature of this relationship depends on the interaction between water and other ingredients. The amount of water vapor that can be absorbed by a product depends on its chemical composition, physicalchemical state, and physical structure. Consequently, the isotherm shape is unique for each product type due to the differences in the capillary, surface, and colligative effects. Products with low water activity are often recognized as dry, those in the activity range from 0.60 to 0.90 are intermediate-moisture products, and those with the water activities higher than 0.90 are high-water activity products [1] Sorption Isotherms. The desorption isotherms are appropriate to the equilibrium process which enables wet samples to be equilibrated with the outer air due to moisture loss, and the adsorption isotherms correspond to the equilibrium process that begins with dry samples. When the hygroscopic equilibrium is reached, water exchange between air and product achieves equilibrium, and the water activity becomes equal to its equilibrium value. Depending on whether the adsorption or desorption process takes place, the water amount at a given water activity may be different. Thus, hysteresis is observed (Fig. 1) [2] that is characterized by the difference between the values of the moisture contents for these processes. To control the drying process and moisture storage conditions, it is necessary to know not only the
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