## 3 Results and Discussion
### Optimization of the Microwave Vacuum Drying Technology
#### 3.3.3 Effects of the Interaction of Factors on Test Indexes
This conclusion is similar to the research results of [PERSON] [34]. (2) Effects of the test factors on the chewiness  
It can be seen from Figure 4 that the chewiness decreased gradually with increases in the material surface temperature, and this then increased with increases in the microwave power; however, it increased first and then decreased with increases in the vacuum degree. The reasons for this are as follows. Given the fixed microwave power and vacuum degree, the time for mulberry drying was shortened significantly with increases in the temperature, and the dehydration volume of the mulberry per unit time increased. The pulp cells shrank quickly and formed a layer of hard film on the mulberry surface. The internal structure of the products was tight [35], thus increasing the hardness significantly, as well as tightening the internal structure and decreasing chewiness [36]. At a fixed vacuum degree (0.07 MPa) and material surface temperature (52.0 \({}^{\circ}\)C), the chewiness of the mulberry increased with increases in the microwave power. At a fixed material surface temperature and microwave power, the vacuum degree had positive effects on the chewiness of the mulberry. Similarly, at a fixed material surface temperature and microwave power, the vacuum degree had negative linear effects on the chewiness of the mulberry. The maximum chewiness force was observed when the material surface temperature was 52.0 \({}^{\circ}\)C, the microwave power  
Figure 4: Effects of the two factors on chewiness. (**a**) Effect of the surface temperature and vacuum degree on the chewability of the materials. (**b**) Effect of the surface temperature and microwave power on the mastacibility of the materials. (**c**) Effect of the vacuum degree and microwave power on the mastication of the materials. Note: The response surface color in the figure is usually used to represent the numerical size or degree of the response variable, where darker areas represent higher response values and lighter areas represent lower response values. was 350 W, and the vacuum degree was 0.07 MPa. The variation found in the chewiness of the mulberry with microwave power and vacuum degree conformed to the research conclusions of [PERSON] [10]. (3) Effects of the test factors on the Vc content  
It can be seen from Figure 5 that the Vc content increased first and then declined with increases in the material surface temperature and power; however, it only increased with increases in the vacuum degree. The Vc is extremely sensitive to heat, and it can be lost at high temperatures and oxidization levels [37]. With increases in the temperature, the drying time of mulberry was shortened, decelerating the Vc degradation [38]. However, the Vc may be oxidized to a brown color at high temperatures; such browning pigments are furfural compounds [39]. When the drying temperature is relatively high, the Vc was significantly destroyed. This is consistent with the research results of [PERSON] et al. [40], where the Vc was damaged more seriously if the microwave power was higher because excessive microwave power can destroy the cell structure and release bioactive compounds to the external environment [41]. Hence, the thermal degradation of the Vc content at 400 W of microwave power was higher than that at 350 W. The Vc content in the mulberry increased first and then decreased with increases in the microwave temperature and power, which is consistent with research results of [PERSON] [42]. Moreover, it takes a longer time to dry if the vacuum degree is lower and the Vc is damaged more severely. [PERSON] et al. [43] found that Vc degradation is increased as the drying process is prolonged. It can prevent the degradation of easy-to-oxidize component substances effectively at low-temperature and vacuum conditions; however, the Vc retention degree remained relatively high. Therefore, the vacuum degree must be increased appropriately to maintain the Vc content.