The high-humidity hot air impingement pretreatment (HHAIP) ,due to its advantages, is considered as an effective non-chemical pretreatment in grape drying. In the first phase of this research, a HHAIP system was designed, constructed, and eva‎luated to apply the pretreatment to the grape berries and study its efficacy on the texture and drying process of the berries. In the second phase, considering multi-scale modeling approach, the governing mechanisms of the heat transfer during the application of HHAIP on the grapes (macro-scale) were studied numerically. The simulation was performed at a temperature of 110 °C with different relative humidity values of 20%, 40%, 60%, and 80% and different velocities of 2, 6, and 10 m s-1. In this model, the heat transfer caused by the impinging jet and condensation on the surface of the berry was considered. Comparison of the predicted and measured values for the center temperature of the grape berry indicated an acceptable agreement with a determination coefficient of 0.993. Considering that the grape tissue is a multi-phase environment with heterogeneous characteristics, in the third phase, a geometric micro-scale model was developed by using the cell growth model based on the biomechanics of plant cells. The results showed the compatibility of the geometric parameters extracted from the grape tissue created by the cell growth model with those obtained from the real grape tissue with a coefficient of determination of 0.995. As the temperature increases, the permeability of the cell membrane increases. Destruction of the cell membrane leads to the loss of turgor pressure and, as a result, softening of the tissue. In addition, when applying the process, the heat causes the thermal degradation of the cell wall pectin through the beta elimination reaction, which leads to further loss of tissue. Therefore, in the fourth phase, a qualitative model was developed using the finite element method to study the changes in grape tissue under the influence of the HHAIP, which incorporates the geometric micro-scale features for the product. In this qualitative model, the elasticity coefficient of the grape tissue was considered as the tissue eva‎luative criterion. The developed micro-scale model included heat and moisture transfer at the micro-scale, pectin degradation in cell wall materials, and solid mechanical model at the micro-scale. It was used to predict the homogeneous elasticity coefficient of grape tissue during heating at different temperatures and durations. In order to apply boundary conditions on the representative volume element (micro-scale), the heat transfer simulation results in the grape during the HHAIP (macro-scale) were used. The simulated trends in the modulus changes were in good agreement with the experimental results. The performance of the developed model showed that the designed framework could be used to simulate the texture changes during the HHAIP. In the last phase, the effect of temperature and duration of the HHAIP at the air jet velocity of 10 m s-1 and relative humidity of 40% on the shade-drying process of the grapes was eva‎luated within a 3×5 factorial completely randomized design with three replications. The results showed a decrease in drying duration with increasing temperature and duration of the pretreatment. By increasing the duration of the pretreatment from 30 to 150 s at temperatures of 90, 100, and 110 °C, the drying rate increased by 31%, 37.5%, and 45%. Compared to the fresh grapes, the increase in the drying rate under the influence of the HHAIP varied from 8% for the duration of 30 s at 90 °C to 68% for the duration of 150 s at 110 °C. Based on the color analysis of the produced raisins, the color characteristics improved with increasing the duration and duration of the pretreatment. So that the temperature of 110 °C and the duration of 90-150 s were determined to be suitable conditions for applying the pretreatment.

مرتضي صادقي

ناصر همدمي , محمدحسين اهتمام

احمد ميره اي , ميلاد فتحي , بهرام حسين زاده ساماني