## 3 Results and Discussion
### Optimization of the Microwave Vacuum Drying Technology
#### 3.3.3 Effects of the Interaction of Factors on Test Indexes  
The experimental data were analyzed and processed using Design-Expert12. According to the established regression models, the surface diagrams of the effects of the factor interaction on the indexes was obtained [30] (Figures 3-5). (1) Effects of the test factors on the soluble solid content  
It can be seen from Figure 3 that the soluble solid content increased and then decreased with increases in the material surface temperature, and it only increased with increases in the microwave power and vacuum degree. At a fixed microwave power (350 W), the soluble solid content changes in the mulberry during microwave vacuum drying were mainly divided into two classes in the temperature range of 48.0-56.0 \({}^{\circ}\)C. One was caused by dehydration and the other was caused by the cleavage reaction of the polysaccharide structure. The water content in the mulberry decreased owing to drying and heating. As \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Index**} & **Source of Variation** & **Quadratic Sum** & **Degree of Freedom** & **Mean Square** & **F-Value** & _p-Value_ \\ \hline \multirow{4}{*}{Chewiness} & Model & 150.06 & 9 & 16.67 & 26.91 & \textless{}0.0001 ** \\  & Residual & 6.20 & 10 & 0.6197 & \\  & Lack of fit & 6.16 & 5 & 1.23 & 169.78 & \textless{}0.0001 ** \\  & Error & 0.0363 & 5 & 0.0073 & \\  & Model & 7.20 & 9 & 0.800 & 4.25 & 0.0169 * \\  & Residual & 1.88 & 10 & 0.1881 & \\  & Lack of fit & 1.80 & 5 & 0.3591 & 21.18 & 0.0022 ** \\  & Error & 0.0848 & 5 & 0.0170 & \\  & Model & 22.34 & 9 & 2.48 & 65.58 & \textless{}0.0001 ** \\ Vc & Residual & 0.3785 & 10 & 0.0379 & \\  & Lack of fit & 0.3605 & 5 & 0.0721 & 20.03 & 0.0025 ** \\  & Error & 0.0010 & 5 & 0.0002 & \\ \hline \hline \end{tabular} Notes: ** indicates extremely significant (_p_ < 0.01), and * indicates significant (_p_ < 0.05).  
\end{table} 
Table 13: Variance analysis results of the regression models. Figure 3: Effects of two factors on the soluble solid content. (**a**) Effect of the surface temperature and vacuum degree on the soluble solid content. (**b**) Effect of the material surface temperature and microwave power on the soluble solid content. (**c**) Effect of the vacuum degree and microwave power on the soluble solid content. 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. the concentration of polysaccharide molecules increased, the soluble solid content of the mulberry increased. When the drying temperature exceeded 52.0 \({}^{\circ}\)C, the spatial structures of the internal polysaccharides changed. However, the interaction of the polysaccharide molecules was enhanced after cooling, thus inducing aggregation. At high temperatures, hydrogen bonds break and some heteropolysaccharides are decomposed into monosaccharides, thus decreasing the heteropolysaccharide content [31]. This is consistent with research results of [PERSON] [32] and [PERSON] et al. [33]. At a constant temperature, the soluble solid content of mulberry increases gradually with increases in the vacuum degree and microwave power in the threshold intervals; the retention rate of polysaccharides was also increased to a certain extent. This might be because the corresponding drying temperature increases with decreases in the vacuum degree. Due to microwave radiation, the degradation of polysaccharides in mulberry increases, thus increasing the degree of the Maillard reaction at the same time. Hence, the vacuum degree should be increased appropriately to decrease the loss of polysaccharides in the products. Given the small microwave intensity, it takes a longer time to dry products, and it also takes longer time for the heating-induced degradation of polysaccharides and the Maillard reaction of proteins to take place. As a result, the soluble solid content decreased.