## 4 Materials and Methods  
### Equipment and Method  
In this study, a new experimental scheme was designed which included the tempering ratio in the control factor based on the previous experiment, and appropriately expanded the level range. The level coding table of each factor in this experiment is shown in Table 8. The calculation formula of the tempering ratio is as follows:  
\[TR\ =\ \frac{t_{T}}{t_{D}}, \tag{18}\]  
where \(TR\) is the tempering ratio, \(t_{T}\) is the tempering time (h), \(t_{D}\) is the drying time (h).  
The experiment was started in October 2020, with a total of 59 groups of experiments lasting 30 days. To avoid any influence on the test results from rice variety, only \"Huang Hua Zhan\" (a particular type of high-quality, late indica rice from China) was selected for this experiment. A multi-parameter controllable drying apparatus was used for the tests, as shown in Figure 3. The main function of this equipment was to accurately control the drying environment of the materials. The control accuracies were as follows: hot air temperature \(\pm\)0.5 \({}^{\circ}\)C, hot air relative humidity \(\pm\)1%, and velocity of hot air \(\pm\)0.1 m s\({}^{-1}\). Samples with different initial moisture contents were prepared prior to the start of the test. First, 1000 g of a test sample was inserted into a material sieve. The stainless-steel material sieve with a 12-mesh screen at the bottom was used to hold the rice grains while allowing hot air to pass through the grain layer. The samples were then inserted into the test chamber, the chamber door was closed, the time was recorded, and the test was started.  
Figure 3: Multi-parameter-controllable drying test device: 1. wind shunt, 2. axial flow fan, 3. material bin door, 4. material sieve, 5. sensor bin, 6. electric sealing valve, 7. heating pipe, 8. inner tank, 9. wet exhaust fan, 10. test-bed shell, 11. sensor group, and 12. humidifier.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Factor**} & \multicolumn{5}{c}{**Level**} \\ \cline{2-6}  & \(-\)**2** & \(-\)**1** & **0** & **1** & **2** \\ \hline Hot air temperature \(X_{1}/^{\circ}\)C & 32.99 & 38.5 & 42.5 & 46.5 & 52.01 \\ Relative humidity of hot air \(X_{2}/\%\) & 41.11 & 48 & 53 & 58 & 64.89 \\ Initial moisture content of rice \(X_{3}/\%\) & 16.03 & 19.2 & 21.5 & 23.8 & 26.97 \\ Velocity of hot air \(X_{4}/\)m s\({}^{-1}\) & 0.36 & 0.5 & 0.6 & 0.7 & 0.84 \\ Tempering ratio \(X_{5}\) & 0 & 1.45 & 2.5 & 3.55 & 5 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Experimental factors and their levels.  
In this experiment, the real-time moisture content in the drying process was determined by a weighing method. After the start of the test, the material sieve was removed and weighed every 15 min, and the time and weight were recorded and input into the computer. The formula for calculating the moisture (\(M_{t}\)) for the weighing method was as follows:  
\[M_{t}\ =\ \frac{m_{st}}{m_{t}}\ =\ \frac{M_{0}*m_{o}-(m_{0}-m_{t})}{m_{t}}\ =\ 1-\frac{m_{0}}{m_{t}}(1-M_{0}), \tag{19}\]  
where \(m_{st}\) is the moisture mass of the rice sample at time \(t\) (g), \(m_{t}\) is the net weight of the rice sample at time \(t\) (g), \(m_{0}\) is the initial mass of the rice sample (g), and \(M_{0}\) is the initial moisture content (wet basis) of the rice sample (%). The drying test ended when the moisture content of the rice sample decreased to about 13.5% (wet basis).