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Figure 3. Velocity curves of combined convection-IR drying of mulberries at different convection drying temperatures (a) and drying capacities (b)
As can be seen from figure 3, the power was constant and the drying speed increased significantly (p<0.05) with increasing temperature. Possible reasons for this phenomenon in the test:
1) the higher temperature increased the effective diffusion coefficient of moisture in the material;
2) the infrared radiation time was lengthened at high temperature, the net convection time in the su-vide stage was reduced, the drying efficiency was increased and the drying speed was accelerated.

Figure 3 also shows that the drying speed decreases significantly (p<0.05) with increasing infrared power in the early stages of drying. This may be due to the fact that the infrared lamp emits infrared light for a longer period of time when a constant convection temperature is maintained at a lower power and the drying speed is correspondingly accelerated when the material is dried under infrared radiation for a shorter period of time. And in the later stage of drying, when the material has dried to a certain degree, the surface layer appears dry and does not contribute to infrared penetration, the infrared power to the drying process is no longer significant.

Activation energy of combined convection-IR drying of mulberries. The relationship between the drying temperature (IR power recorded at 230 W) and the effective moisture diffusion coefficient for combined convection-IR drying of mulberry fruit is shown in Fig. 4.

From the slope of the linear regression in equations (7) it follows that the activation energy of drying of mulberry is 53.97 kJ/mol. A review of the literature shows that the activation energy for combined convection-infrared drying of mulberry is higher than that for hawthorn (27.90 kJ/mol) and apple-tree (36.58 kJ/mol), lower than that of date (54.51 kJ/mol), grape (67.29 kJ/mol) and pumpkin-tree (78.93 kJ/mol) etc [8].