Mathematical Modeling of Moisture Sorption Isotherms and Determination of Isosteric Heats of Sorption ofZiziphusLeaves

Abstract

Desorption and adsorption equilibrium moisture isotherms ofZiziphus spina-christileaves were determined using the gravimetric-static method at 30, 40, and 50°C for water activity(aw)ranging from 0.057 to 0.898. At a givenaw, the results show that the moisture content decreases with increasing temperature. A hysteresis effect was observed. The experimental data of sorption were fitted by eight models (GAB, BET, Henderson-Thompson, modified-Chung Pfost, Halsey, Oswin, Peleg, and Adam and Shove). After evaluating the models according to several criteria, the Peleg and Oswin models were found to be the most suitable for describing the sorption curves. The net isosteric heats of desorption and adsorption ofZiziphus spina-christileaves were calculated by applying the Clausius-Clapeyron equation to the sorption isotherms and an expression for predicting these thermodynamic properties was given.

Drying of yam with solar adsorption system

“…One of the ways by which yam can be preserved to extend its shelf life is by drying, but not much is known about the parameters related to drying characteristics of yam, especially regarding desorption and sorption isotherms. Sorption isotherms of food materials have been reported by amongst others Gálvez et al (2006), Montes (2009) and Saad et al (2014). The majority of the work pertains to constant temperatures in the range of 15°C -60°C on samples from a variety of crops such as potato, carrots, green pepper, onions and dates (Samaniego-Esquerra et al, 1991;Kiranoudis et al, 1993;Myhara et al, 1998a,b).…”

“…Models used to describe sorption isotherms of foods include: Guggenheim-Andersonde Boer (GAB), Henderson, Halsey, Oswin, Smith, Brunauer-Emmett-Teller (BET) and Peleg. Most studies are either restricted to desorption or adsorption (Andrade et al, 2011;Montes et al, 2009 andSaad et al, 2014). The method of assessing the parameters have differed from one researcher to the other, whereas limited attention has been given to the uncertainty in the parameters.…”

“…The GAB model is subsequently considered for describing the desorption and adsorption data at temperatures 25°C and 50°C. Montes et al (2009) mentioned that the Peleg model was best for describing the desorption isotherm of two varieties of yam (D. rotundata), while Peleg and Oswin models were found suitable by Saad et al (2014). For graphical presentation, the GAB and Peleg modeled graphs are shown in…”

“…The various models for describing the equilibrium moisture content in relation to water activity at different temperatures were extensively considered. Some works in the literature have been restricted to desorption or adsorption alone as reported in the work of Andrade et al (2011), Montes et al (2009) and Saad et al (2014)). In the current work (Chapter 2) experimental equilibrium moisture isotherms for yam at temperatures of 25°C and 50°C and over a range between 0 -0.85 water activity, both for adsorption and desorption were observed.…”

“…While others have reported a decrease in EMC for increased temperature at the same water activity, Montes et al (2009) showed no temperature effect on the desorption isotherms of yam (Dioscorea rotundata) at temperatures 45°C and 55°C. Moreover, some works were restricted to desorption or adsorption alone as reported in the work of Andrade et al (2011), Montes et al (2009) and Saad et al (2014). The reported investigations also differ in the method of assessing the parameters, whereas limited attention has been given to the uncertainty in the parameters (a noteworthy exception is e.g.…”