True Density Prediction of Garlic Slices Dehydrated by Convection

Physiochemical parameters with constant values are employed for the mass-heat transfer modeling of the air drying process. However, structural properties are not constant under drying conditions. Empirical, semi-theoretical, and theoretical models have been proposed to describe true density (ρ_p). These models only consider the ideal behavior and assume a linear relationship between ρ_p and moisture content (X); nevertheless, some materials exhibit a nonlinear behavior of ρ_p as a function of X with a tendency toward being concave-down. This comportment, which can be observed in garlic and carrots, has been difficult to model mathematically. This work proposes a semi-theoretical model for predicting ρ_p values, taking into account the concave-down comportment that occurs at the end of the drying process. The model includes the ρ_s dependency on external conditions (air drying temperature (T_a)), the inside temperature of the garlic slices (T_i), and the moisture content (X) obtained from experimental data on the drying process. Calculations show that the dry solid density (ρ_s) is not a linear function of T_a, X, and T_i. An empirical correlation for ρ_s is proposed as a function of T_i and X. The adjustment equation for T_i is proposed as a function of T_a and X. The proposed model for ρ_p was validated using experimental data on the sliced garlic and was compared with theoretical and empirical models that are available in the scientific literature. Deviation between the experimental and predicted data was determined. An explanation of the nonlinear behavior of ρ_s and ρ_p in the function of X, taking into account second-order phase changes, are then presented.