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Modelling of sorption isotherms of corn kernels :Microwave densityindependent permittivity function correlated towater activity

Ashutosh Prasad, Zeesham Ahmad, Hariniwas Singh, Priyanka Das, Kamlesh Prasad

The present study aims at correlating water activity [aw] to density independent microwave permittivity function () for corn( zea mays, L.) kernels in the sorption isotherm corresponding to 250C. The data for Ø were derived fromthe experimental results of relative permittivity and loss factormeasured at 2.45 GHz and sixmoisture levels from10.3%to 33.4%, all taken from the literature. These values of relative permittivity for bulk materials were then converted to those for solids with the help of eight equations for effective permittivity of randommedia, such as the logarithmic law of mixing. With these data, quadratic model for variation of two dielectric properties, relative permittivity and loss factor, as functions of decimal moisture contents, have been proposed. Four [Equilibrium Relative Humidity (ERH)/Equilibrium moisture content (EMC)] sorption isotherms were converted to aw/type of sorption isotherms. Fromthe study, it transpired that small hysteresis was shown by almost all the models, except in HalseyÂ’s, which manifested a large amount of hysteresis. The comparative goodness of fit for the four different isotherms was quantified through sigmoidal curve -fitting technique. The evaluated parameters showed thatModified Chung-Pfost equation (MCPE) provided the best fit with minimal [chi-square/ degrees of freedom(DOF)]ï‚»9ï‚´10-9-9ï‚´10-6 and maximumcoefficient of determination (r2) ï‚»0.94-0.99. Consequently, itwas supposed to be the most appropriate model in predicting sorption processes of corn kernels for the chosen range of temperature and water activity.