## 3 Results
### Parameter Optimization and Model Validation  
A single experiment was conducted to evaluate the effectiveness of the optimization conditions provided by the model using the specific parameters suggested by the model. The results showed acceptable agreement between the experimental and predicted values, and, thus, the model effectively predicted the response [46]. In order to obtain the optimum combination of retarded drying parameters, the target values of total drying time were taken, cumulative temperature values were limited at a minimum, and the target ranges of values of protein content, fat content, straight-chain starch content and branched-chain starch content were taken. According to [PERSON] et al. [47], it was shown that the protein range of high-quality rice is 7.86-9.32%, and the fat content is generally between 1% and 2%. According to the Chinese national standard quality for japonica rice GB/T 19266, the straight-chain starch content range is 15-20 g/100 g and branched-chain starch accounted for the content range of 60-70 g/100 g; a straight-chain starch content of more than 20% results in rice that is viscous, small, with a hard texture, no luster and poor taste; so a straight-chain starch content of 13% to 20% gives a better taste. If the content of straight  
Figure 6: Interaction effect of amylopectin. Note: The red value of the response surface diagram is larger, and the blue value is smaller.  
chain starch is too low, the rice will be soft, sticky and greasy with poor elasticity. These ranges are applicable to most high-quality rice varieties and maintain their nutritional and flavor values [41; 47].  
The parameters were optimized using the central composite design of experiment in Design-Expert-13 software, and the optimal combination of process parameters within the experimental range was obtained as a hot-air temperature of 54.58 \({}^{\circ}\)C, relative humidity of 38.95%, initial moisture content of 20.54%, air velocity of 0.63 m/s, and tempering ratio of 3.74. Under this optimized parameter, the total time consumed by drying was 4.40 h, effective cumulative temperature was 216.40 \({}^{\circ}\)C-h, protein content was 8.19 g/100 g, fat content was 2.67 g/100 g, straight-chain starch content was 16.08 g/100 g and branched-chain starch content was 81.59 g/100 g. Validation experiments were carried out using the optimized parameters, and, in order to eliminate the random errors, three parallel experiments were carried out, and the results of the experiments are shown in Table 6. Analysis shows that the average error between the experimental values and optimized parameter values is 4.17%, indicating that the experimental results are basically consistent with the optimized results, which are all in line with the standard parameter range of high-quality Japonica rice, and, therefore, the choice of factors affecting the drying quality of rice grain is reasonable. Therefore, the selection of factors affecting the drying quality of rice is reasonable [47; 48; 49].  
In order to verify the fitting effect of the established regression model, three sets of values were randomly generated within its parameter range for validation testing. The predicted values were obtained according to the regression equation and the relative error between the experimental and predicted values was calculated. The results from Table 6 show that the relative average error between the experimental and model values is 5.92%; the predicted values are in acceptable agreement with the experimental values [46; 50; 51], which indicates that the model fitting is credible.