Loss factor and moisture diffusivity property estimation of lentil crop during microwave processing

Abstract

Characterization of loss factor and moisture diffusivity are required to understand materials' precise behavior during microwave processing. However, providing the processing facilities to measure these properties in a real or simulated situation directly can be complicated or unachievable. Hence, this study proposes an alternative procedure for modeling these properties according to their affecting factors including temperature, and moisture content. The basis of this method is to use an algorithm that combines the optimization approach and the numerical solution of the heat and mass transfer governing equations, including boundary conditions. For this aim, the coefficients of estimated models for loss factor and moisture diffusivity were obtained by minimizing the sum square error of the experimentally measured mean surface temperature and moisture content and the predicted values by solving the system of partial differential equations. The suggested models illustrated that during the microwave process, the moisture diffusivity grows arithmetically, and the loss factor generally raises, but transition points were observed in the trend for the samples tempered up to the 50% moisture content. These points have been attributed to the starch gelatinization and confirm how the bio-chemical reaction would have a noticeable effect on this property, determining the microwave energy absorbance. The results of differential scanning calorimetry thermograms and the Fourier transform mid-infrared spectra of flours obtained from microwave processed lentil seeds also confirmed the greatest intensity of starch structure alteration happened for the samples tempered to 50% moisture content by showing the highest shifts in the endothermic peak and lowest degree of order.

Keywords: Effective moisture diffusivity; FT-MIR spectroscopy; Heat and mass transfer; Loss factor; Thermal properties.

Graphical abstract

Fig. 1

Variation of loss factor according to the changes in the mean temperature and moisture content of lentil seeds tempered to various moisture contents processing in (a) microwave at 0.7 kW nominal power and (b) microwave at 0.35 kW nominal power.

Fig. 2

Variation of diffusivity according to the changes in the mean temperature and moisture content of lentil seeds tempered to various moisture contents processing in (a) microwave at 0.7 kW nominal power and (b) microwave at 0.35 kW nominal power.

Fig. 3

Variation in the experimental surface temperature and the prediction of mean, surface and center temperature changes of lentil seed tempered to various moisture contents (a) tempered to 20%; (b) tempered to 35%; (c) tempered to 50%, processing in microwave at 0.7 kW nominal power on the left side, and at 0.35 kW nominal power on the right side indicating by * superscript.

Fig. 4

Variation in the experimental mean moisture content and the prediction of mean, surface and center moisture content changes of lentil seed tempered to various moisture contents (a) tempered to 20%; (b) tempered to 35%; (c) tempered to 50%, processing in microwave at 0.7 kW nominal power on the left side, and at 0.35 kW nominal power on the right side indicating by * superscript.

Fig. 5

DSC thermograms of the raw and microwave-treated lentil flours.

Fig. 6

The average and normalized FT-MIR spectra of the raw and microwave-treated lentil flours.