Optimization and Experiment of Hot Air Drying Process of Cyperus esculentus Seeds

Abstract

To improve the drying efficiency, drying uniformity and germination rate after hot air drying of Cyperus esculentus seeds, this paper proposed a drying method that combines tempering and hot air drying. The drying curve of Cyperus esculentus was obtained by experiment. The influence of the drying method on the drying quality of Cyperus esculentus seeds and the trend of drying quality with the tempering process parameters (the tempering temperature, the moisture content of Cyperus esculentus at the beginning of tempering and the tempering duration) were analyzed by single-factor experiments. The regression models were established by star point design response surface methodology, and the relationships between the tempering process parameters and seed production quality indexes were analyzed. The results showed that the optimal combination of tempering process parameters was a tempering temperature of 24 °C, a moisture content of Cyperus esculentus at the beginning of tempering of 23%, and a tempering duration of 3 h. Under this combination, the tempering process increased the drying uniformity, germination rate and drying efficiency of Cyperus esculentus seeds by 21.122%, 4.205% and 22.832%, respectively. The error between the verification test value and the software optimization parameter value was acceptable. The study showed that the optimized tempering drying process significantly improved the drying quality of Cyperus esculentus seed production, and the results could provide a theoretical basis for production practices.

1. Introduction

Cyperus esculentus, as an emerging strategic crop, was adopted by the Chinese Ministry of Agriculture as one of the new oil crops to be vigorously promoted in 2016 [1,2,3,4]. Cyperus esculentus are easy to rot and mildew due to their high sugar and moisture content, which increase the difficulty of storage and transportation. Therefore, Cyperus esculentus must be dried in time after harvest [5,6,7]. Continuous hot air drying, as a traditional drying method, has many problems such as high energy consumption, uneven drying and low germination rate of seeds after drying [8,9,10]. Zhu et al. determined the drying characteristics of Cyperus esculentus under hot air drying conditions, and studied the effect of wind temperature and speed on its drying characteristics [11]. While this study identified the relationship between wind temperature and drying rate, there was a lack of targeted research on drying Cyperus esculentus seeds.

Tempering means stopping heating the dried material after a drying process and keeping the temperature unchanged for a period of time, which provides sufficient time for the dried material to diffuse moisture, so as to reduce the crackle ratio of dried materials [12,13,14]. In addition, tempering during the drying process can also avoid water concentration inside the dry product, improve water mobility, protect the activity of the dry product, and improve the drying uniformity [15,16,17]. The effect of tempering drying is mainly determined by the process parameters of drying and tempering and the matching level between them [18,19,20]. Duan et al. pointed out that tempering drying ensured the uniform heating of the rice, and avoided the deterioration of the drying quality due to the rapid temperature rise [21]. Wang et al. explored the effect of tempering process parameters on the drying quality index of rice, and pointed out that tempering drying significantly improved the drying quality of rice [22,23,24].

Based on the above research, this paper investigated the drying of Cyperus esculentus seeds. The drying curve of Cyperus esculentus was obtained by experiment. The effectiveness of the tempering technology in improving the drying quality of Cyperus esculentus seeds was determined through single-factor experiments. The star point design response surface methodology was used to comprehensively analyze the effects of the tempering process parameters (tempering temperature, moisture content of Cyperus esculentus at the beginning of tempering and tempering duration) on the drying quality and efficiency for Cyperus esculentus seeds, and established a prediction model for tempering process parameters and the drying quality. The combination of tempering process parameters was optimized and the results of the validation tests were consistent with the model predictions. This paper provides a reference for drying Cyperus esculentus seeds with high efficiency and quality.

2. Materials and Methods

2.1. Material

The variety of Cyperus esculentus used in this paper was Jisha No. 1, taken from the Cyperus esculentus planting demonstration base in Nong’an County, Changchun, Jilin Province, China.

Samples were taken from the fields using a random sampling method and stored at low temperatures to maintain sample activity. The Cyperus esculentus samples were classified according to physical characteristics such as mass, diameter and shape, as shown in Figure 1. The samples were set up according to the proportion of beans as commodities and beans as seeds in the existing Cyperus esculentus, and then the proportion in the subsequent Cyperus esculentus samples was counted. The results are shown in

Table 1. Classification and proportion of Cyperus esculentus.

Sample Type	Quality Range (g)	Actual Proportion (%)	Proportion of Beans as Seeds (%)	Proportion of Beans as Commodities (%)
Large beans	>1.3	20	40	20
Medium beans	0.8~1.3	45	60	50
Small beans	0.5~0.8	30	0	30
Broken beans	<0.5	5	0	0

.

2.2. Instruments and Equipment

The instruments and equipment used in this paper are shown in Figure 2. The self-made Cyperus esculentus hot air tempering dryer is composed of drying Section 1, drying Section 2 and a tempering section. The drying section is 15 m long, composed of 10 layers of conveyor belts with fans on the side, and uses a fuel furnace as a heat source. The Cyperus esculentus were evenly spread on a conveyor belt with a forward speed of 1 m/s, which could realize the turning of the Cyperus esculentus during the drying process and improve their drying uniformity. The vacuum drying oven (Model DZF, Shanghai Lichen Bangxi Instrument Technology Co., Ltd., Changchun, China) with stable temperature control was used for exploring the drying characteristics of Cyperus esculentus. The moisture detector was used with the traditional Chinese medicine slicer (Model DQ-101, Wenling Dade Traditional Chinese Medicine Machinery Co., Ltd., Changchun, China). The shell and pulp of the Cyperus esculentus are closely connected, which made it impossible to directly detect its moisture. Therefore, it is necessary to slice the Cyperus esculentus before measuring its water content. The electric drying oven (Model GZX-9070 MBE, Shanghai Boxun Industry Co., Ltd., Changchun, China) was used for the study of the Cyperus esculentus tempering process. It can provide fixed air speed for the Cyperus esculentus drying process, and its drying temperature adjustment range is 5~250 °C. The sample sieve (accuracy of 2.0 mm, Shangyu Huafeng Hardware Instrument Co., Ltd., Changchun, China) was used to clean the surface soil of Cyperus esculentus. The sample box (100 mm × 100 mm × 50 mm) had small holes with a uniform diameter of 2 mm at the bottom.

2.3. Experimental Method

2.3.1. Drying Characteristics Experiment Results and Analysis

To ensure the accuracy of the drying characteristics experiment, the vacuum drying oven needed to be calibrated so that the error did not exceed 1%. The drying oven needed to be preheated to maintain a temperature of 40 °C for the drying characteristics experiment of Cyperus esculentus. The experiment required Cyperus esculentus to be dried to a moisture content of less than 14% [25,26,27]. Each group of experiments was repeated three times to reduce the error.

2.3.2. Single-Factor Experiment

The single-factor experiment was used to investigate the effect of the drying method and tempering process parameters on the drying of Cyperus esculentus, and provide a reference for the selection of experimental factor levels in the multi-factor experiment. The experiment was conducted at the Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University. The temperature in the laboratory was 12~15 °C and the relative humidity was 24~28%. The drying temperature was 40 °C and the heating range of the electric drying oven was 15~150 °C. The experimental scheme is shown in

Table 2. Experimental scheme of single-factor experiment.

Levels	Drying Method	Tempering Temperature (°C)	Moisture Content of Cyperus esculentus at the Beginning of Tempering (%)	Tempering Duration (h)
1	Vacuum drying	20	30	1
2	Hot air drying	25	28	2
3	Single-stage tempering treatment during hot air drying	30	26	2.5
4		35	24	3
5			22	3.5
6			20	4
7			18	4.5
8			16	5

. The drying method, the tempering temperature, the moisture content of Cyperus esculentus at the beginning of tempering and the tempering duration were selected as experimental factors [28,29,30]. The drying uniformity, initial germination rate, and drying time were selected as evaluation indexes [31,32,33,34].

Two electric drying ovens and one vacuum drying oven were used for the experiment at the same time. One electric drying oven used the hot air drying process and the other used the tempering process. The air volume and air speed of the two electric drying ovens were kept the same, and the vacuum drying oven was used as the control group.

2.3.3. Multi-Factor Experiment

The Response Surface Methodology (RSM) is an experimental design method proposed by Box et al. [35]. It is an optimization method that integrates experimental design and mathematical modelling. This method allows regression fitting of the functional relationship between factors and results in the global scope by testing a representative number of local points, and obtaining the optimal value for each factor [36,37].

The multi-factor experiment was designed using the star point design response surface methodology to systematically investigate the influence of the tempering process parameters on the drying characteristics of Cyperus esculentus, and to obtain the optimal combination of tempering process parameters.

According to the results of the single-factor experiment, the tempering temperature (X₁), the moisture content of Cyperus esculentus at the beginning of tempering (X₂) and the tempering duration (X₃) were selected as experimental factors, and the growth rate of drying uniformity (Y₁), the growth rate of initial germination rate (Y₂), and the growth rate of drying efficiency(Y₃) were selected as evaluation indexes. The experimental factors and levels coding table is shown in

Table 3. Experimental factors and levels coding of multi-factor experiment.

$\mathbf{Levels} (\mathit{\gamma }=1.682)$	Factors
	Tempering Temperature X1 (°C)	Moisture Content of Cyperus esculentus at the Beginning of Tempering X2 (%)	Tempering Duration X3 (h)
$\gamma$	34	29	4.5
1	30	26	4
0	25	22	3
−1	20	18	2
$-\gamma$	16	15	1.5

.

2.4. Detection Method and Evaluation Index

2.4.1. Moisture Detection Method

In order to determine the degree of water evaporation during the drying process of Cyperus esculentus and improve the accuracy and efficiency of its moisture detection, the moisture detection method and the calculation method of Cyperus esculentus moisture ratio were stipulated for all moisture detection and moisture ratio calculations in this paper.

Due to the close connection between the hull and the flesh of Cyperus esculentus, it is impossible to carry out moisture detection according to the GB/T21719-2008 standard. To improve the efficiency of Cyperus esculentus moisture detection, Cyperus esculentus need to be sliced or chopped. According to the pre-test of moisture detection and the analysis of variance of the results, the detection time was set as 10 min and the detection state was set as slice.

In this study, the Cyperus esculentus was calculated as wet materials, and the moisture ratio can be described as follows [38]:

$$K=\frac{M_{\mathrm{t}}-\overline{M}}{M_0-\overline{M}}$$

(1)

where K is the moisture ratio of Cyperus esculentus; M_t is the moisture content at moment t of the drying process; and $\overline{M}$ is the average moisture content of Cyperus esculentus after drying. M₀ is the initial moisture content of Cyperus esculentus.

2.4.2. Single-Factor Experiment Evaluation Index

The drying uniformity, germination rate and net drying time were selected as indexes to evaluate the effect of the drying method, tempering temperature, moisture content of Cyperus esculentus at the beginning of tempering and tempering duration on the rate of water evaporation during drying.

The drying uniformity can be described as follows [39]:

$$G=\frac{1}{\sigma }$$

(2)

where G is the uniformity of drying and $\sigma$ is the standard deviation of the water content of Cyperus esculentus sampled using the five-point method per unit area

The dried Cyperus esculentus were soaked in warm water at 30 °C for 36 h to stimulate its vigor. After soaking, the seeds were evenly placed on the germination bed and placed in a 30 °C incubator and periodically replenished with water. The germination rate can be described as follows:

$$Q=\frac{q_1}{q_0}$$

(3)

where Q is germination rate, %; q₀ is the total number of Cyperus esculentus; and q₁ is number of sprouted Cyperus esculentus.

Net drying time is the total time of the hot air drying process of Cyperus esculentus excluding the time of the tempering process.

2.4.3. Multi-Factor Experiment Evaluation Index

The tempering temperature, the moisture content of Cyperus esculentus at the beginning of tempering and the tempering duration were selected as the experimental factors, and the growth rate of drying uniformity, initial germination rate and drying efficiency were selected as evaluation indexes.

The growth rate of drying uniformity can be described as follows:

$$P=(1-\frac{G_1}{G_0})\times 100\%$$

(4)

where P is the growth rate of drying uniformity, %; $G_0$ is the drying uniformity for the hot air drying technology, and the detection method to be determined by experiment; and $G_1$ is the drying uniformity for the tempering drying technology, and the value to be determined from experiment.

The growth rate of the initial germination rate can be described as follows:

$$Q=(1-\frac{Q_1}{Q_0})\times 100\%$$

(5)

where $Q$ is the growth rate of the initial germination rate,%; $Q_0$ is the initial germination rate of the hot air drying technology, %, and the detection method needs to be determined according to the experiment; and $Q_1$ is the initial germination rate of the tempering drying technology, %, and the value needs to be determined from experiment.

The growth rate of drying efficiency can be described as follows:

$$H=(1-\frac{h_1}{h_0})\times 100\%$$

(6)

where $H$ is the growth rate of the drying efficiency, %; $h_0$ is the net drying time of hot air drying, h; and $h_1$ is the net drying time of tempering drying, h.

3. Results and Discussion

3.1. Drying Characteristics Experiment

Through the experiment on drying characteristics, the water evaporation of large beans, medium beans, small beans and beans as seeds were obtained. The data were sorted and fitted, and the results were shown in Figure 3.

The drying process of Cyperus esculentus was divided into the constant rate drying stage, the first deceleration drying stage and the second deceleration drying stage. At the constant rate drying stage, the water evaporation rate was 2.00%/h for 3 h. Water evaporated rapidly at this stage, mainly from the surface of the Cyperus esculentus. The first sudden drop point of drying speed appeared at the fourth hour. This stage lasted 5 h and the evaporation rate of water was 1.56%/h. The easily lost water inside the Cyperus esculentus was evaporated and the surface of the Cyperus esculentus was slightly wrinkled. The second sudden drop point of drying speed appeared at the tenth hour. This stage lasted 10 h and the evaporation rate of water was 0.94 %/h. The water inside the Cyperus esculentus, which was not easily lost, was evaporated in this stage and evaporated slowly. The volume of the Cyperus esculentus decreased and the surface shrinkage increased. The water inside the Cyperus esculentus was concentrated and the temperature of the bean approached the drying temperature.

Soybeans that completed the experiment were cooled and dried and their moisture content and germination rate were determined. The moisture content and germination rate met the drying requirements of this paper. Through the drying characteristics curve of Cyperus esculentus, the drop points of drying speed during the drying process of Cyperus esculentus were determined. The results provided a data base for Cyperus esculentus drying.

3.2. Single-Factor Experiment

(1)

The effect of experimental factors on the drying uniformity

Cyperus esculentus dried with single tempering during hot air drying had better drying uniformity compared to vacuum drying and hot air drying. When the tempering temperature was lower than 20 °C, the drying uniformity of Cyperus esculentus was poor, and the diffusion speed of water inside the soybean was slow. The drying uniformity of Cyperus esculentus was better when the moisture content of Cyperus esculentus was 18~26% at the beginning of tempering. The drying uniformity of Cyperus esculentus was poor when tempering at the beginning and end of hot air drying. The results showed that the second sudden drop point of drying speed was the effective point for the tempering drying of Cyperus esculentus. The best uniformity of drying was achieved when the tempering duration was 4 h. The water inside the Cyperus esculentus could not spread to the outside in time when the tempering duration was too short. When the tempering duration was too long, the moisture inside the Cyperus esculentus stopped spreading outwards and its surface absorbed moisture from the air, which led to a reduction in the drying uniformity.

(2)

Effect of test factors on initial germination rate

The germination rate of single tempering during hot air drying was similar to the vacuum drying and the hot air drying, which proved that the tempering process did not have a negative influence on the germination of Cyperus esculentus seeds and could be used to improve the drying efficiency of Cyperus esculentus seeds. The tempering temperature had no negative influence on Cyperus esculentus seed germination rate. Tempering drying at the beginning and end of hot air drying had an inverse effect on the germination rate of Cyperus esculentus seeds. The germination rate of Cyperus esculentus seeds was increased when the moisture content of Cyperus esculentus was 18~22% at the beginning of tempering. The results showed that the germination rate of Cyperus esculentus seeds was higher when tempering at the second sudden drop point of drying speed. The germination rate of Cyperus esculentus seeds was better, when the tempering duration was 2~4 h. When the tempering duration was less than 2 h, the water inside the Cyperus esculentus seeds could not be fully diffused, and the germination rate was similar to that without tempering. The vitality of the Cyperus esculentus seeds was irreversibly affected when the tempering duration exceeded 4 h, making them less likely to germinate.

(3)

Effect of test factors on the drying time

The tempering process had no adverse effects on the drying process. When the tempering temperature was 20~25 °C, the drying efficiency was effectively improved; when the moisture content of Cyperus esculentus at the beginning of tempering was 18~26%, the net drying time was effectively shortened; when the tempering duration was 2~3.5 h, the whole drying time was obviously reduced and the drying efficiency was improved.

3.3. Multi-Factor Experiment

The results of the multi-factor experiment are shown in

Table 4. Experiment plan and results.

No.	Factors			Evaluation Indexes
	X1 (°C)	X2 (%)	X3 (h)	Y1 (%)	Y2 (%)	Y3 (%)
1	−1	−1	−1	17.97	4.00	26.46
2	1	−1	−1	27.31	3.00	21.67
3	−1	1	−1	14.26	7.50	21.67
4	1	1	−1	20.23	4.50	17.42
5	−1	−1	1	20.57	0.01	21.54
6	1	−1	1	26.77	2.50	28.38
7	−1	1	1	18.69	0.50	23.17
8	1	1	1	24.48	3.50	30.58
9	−1.682	0	0	19.31	4.50	24.54
10	1.682	0	0	29.98	2.50	35.92
11	0	−1.682	0	17.00	2.00	17.88
12	0	1.682	0	11.06	4.50	16.67
13	0	0	−1.682	9.71	4.50	15.25
14	0	0	1.682	19.99	1.50	19.33
15	0	0	0	14.33	2.00	22.58
16	0	0	0	14.31	2.50	23.00
17	0	0	0	14.71	1.50	29.75
18	0	0	0	13.69	3.00	25.79
19	0	0	0	11.02	2.00	25.08
20	0	0	0	12.14	2.50	20.92

. After removing the insignificant factors after variance analysis of the experimental results, the regression models of the three evaluation indexes were as follows:

$$Y_1=13.26+3.31X_1-1.83X_2+2.05X_3+4.66X_1{}^2$$

(7)

$$Y_2=2.25+0.78388X_2-1.28X_3+1.19X_1{X}_3$$

(8)

$$Y_3=24.44+2.53X_1{}^2-2.05X_2{}^2-2.04X_3{}^2+2.91X_1{X}_3$$

(9)

where Y₁ is the growth rate of drying uniformity, %; Y₂ is the growth rate of the initial germination rate, %; Y₃ is the growth rate of drying efficiency, %; X₁ is the tempering temperature, °C; X₂ is the moisture content of Cyperus esculentus at the beginning of tempering, %; and X₃ is the tempering duration, h.

The regression models were analyzed for variance (

Table 5. Variance analysis of regression model.

Evaluation Indexes	Source of Variation	Sum of Squares	Degree of Freedom	Mean Square	F-Value	p-Value
Growth rate of drying uniformity	Model	584.02	9	64.89	12.03	0.0003 **
	Residual	53.92	10	5.39
	Lack of fit	43.19	5	8.64	4.02	0.0764
	Error	10.74	5	2.15
Growth rate of initial germination rate	Model	48.40	9	5.38	9.37	0.0008 **
	Residual	5.74	10	0.5738
	Lack of fit	4.36	5	0.8725	3.17	0.1154
	Error	1.38	5	0.2750
Growth rate of drying efficiency	Model	408.99	9	45.44	4.80	0.0111 *
	Residual	94.69	10	9.47
	Lack of fit	46.36	5	9.27	0.9591	0.5177
	Error	48.33	5	9.67

Note: ** means extremely significant difference (p < 0.01), * means significant difference (p < 0.05).

). The results showed that the regression equation model significance test results of the increase in the growth rate of drying uniformity, the growth rate of initial germination rate, and the growth rate of drying efficiency were extremely significant (p < 0.01), and the results of the lack of fit test were not significant (p > 0.05), indicating that the regression model had a good fit in the test range.

The experimental results were processed and analyzed by Design-Expert software. The response surfaces showed the effect of the experimental factors on each indicator (Figure 4, Figure 5 and Figure 6).

(1)

Effect of experimental factors on the growth rate of drying uniformity

As can be seen from Figure 4, the growth rate of drying uniformity first decreased and then increased with increasing tempering temperature, increased with the increase of tempering duration, and decreased with the increase of moisture content of Cyperus esculentus at the beginning of tempering.

The reason for the above phenomenon was that when the drying temperature was 40 °C and the drying wind speed was fixed, the lack of tempering temperature slowed down the diffusion of water within the Cyperus esculentus seeds and increased the diffusion rate when the temperature increased. The difference in drying degree among Cyperus esculentus seeds decreases gradually. The increase in tempering duration improved the uniformity of water distribution within the Cyperus esculentus seeds and reduced the effect of water gradients, so that water evaporated more evenly within the Cyperus esculentus seeds. The lower the moisture content of Cyperus esculentus at the beginning of tempering, the more pronounced the degree of moisture uniformity within the Cyperus esculentus seeds, and the differences in drying uniformity between seeds was reduced.

(2)

Effect of test factors on the growth rate of initial germination rate

As shown in Figure 5, the interaction between tempering temperature and tempering duration was the most significant. The initial germination rate decreased with increasing tempering temperature when the duration of tempering was short, and increasing with increasing tempering temperature when the duration of tempering was long.

The reason for the above phenomenon was that when the drying temperature was 40 °C and the drying wind speed was fixed, the smaller the difference between the hot air drying temperature and the tempering temperature, the smaller the effect on the germination rate of the Cyperus esculentus seeds. The germination rate of Cyperus esculentus seeds decreased under low temperature for a long tempering duration.

(3)

Effect of test factors on the growth rate of drying efficiency

As can be seen from Figure 6, the drying efficiency decreased and then increased as the tempering temperature increased, and increased and then decreased as the tempering duration and the moisture content of the retardation time increased.

The reason for the above phenomenon was that when the drying temperature was 40 °C and the drying wind speed was fixed, the lack of tempering temperature slowed down the diffusion of water within the Cyperus esculentus seeds; the diffusion speed and the drying efficiency increased with increasing tempering temperature. Insufficient tempering duration led to an uneven distribution of moisture in the Cyperus esculentus seeds, and too long reduced the drying efficiency of the Cyperus esculentus seeds. The uniformity of water distribution in the Cyperus esculentus seeds decreased when the tempering duration was too short, and the drying efficiency decreased when the tempering duration was too long.

3.4. Model Validation of Tempering Process Parameters

To obtain the optimal combination of the tempering parameters, the model was optimized using the Optimization function of Design Expert software with the objective of maximizing the growth rate of drying uniformity, the growth rate of initial germination rate and the growth rate of drying efficiency. The optimal combination of process parameters within the experimental range was as follows: tempering temperature of 24 °C, moisture content of Cyperus esculentus at the beginning of tempering of 23%, and tempering duration of 3 h. Under this combination, the software predicted the growth rate of drying uniformity was 21.122%, the growth rate of initial germination rate was 4.205%, and the growth rate of drying efficiency was 22.832%.

The validation test was carried out in the Cyperus esculentus planting demonstration base in Nong’an County, Changchun, Jilin Province. The test was repeated three times to eliminate trial randomization error. The results of the validation test are shown in

Table 6. The results of validation tests.

Test Number	Growth Rate of Drying Uniformity (%)	Growth Rate of Initial Germination Rate (%)	Growth Rate of Drying Efficiency (%)
1	20.570	4.211	22.754
2	22.310	4.117	22.313
3	21.477	4.341	23.677
Average	21.452	4.223	22.915
Error	1.564	0.428	0.362

. The error between the test results and the software predictions was within the feasible range, which proved that the test results were consistent with the optimized results, and the design of the process parameters of tempering was reasonable.

4. Conclusions

This paper analyzed the drying characteristics of Cyperus esculentus through experiments and explored the influence of tempering process parameters (the tempering temperature, the moisture content of Cyperus esculentus at the beginning of tempering and the tempering duration) on the drying quality (the drying uniformity, the initial germination rate and the drying efficiency) of Cyperus esculentus seeds through single-factor and multi-factor experiments. The following conclusions were obtained:

(1) Through the drying characteristics experiment, it was determined that the drying process of Cyperus esculentus at a certain temperature was mainly divided into the constant rate drying stage, the first deceleration drying stage and the second deceleration drying stage. According to the moisture loss characteristic curve of Cyperus esculentus, the sudden drop points of drying speed in the drying process of Cyperus esculentus were obtained, which provided basic data for the research and optimization of the Cyperus esculentus tempering process.

(2) The single-factor experiment results showed that tempering drying could improve the drying uniformity, initial germination rate and drying efficiency of beans better than vacuum drying and continuous hot air drying. In addition, the trend of drying quality with the tempering process parameters was analyzed by single-factor experiment and the range of tempering process parameters was determined.

(3) The star point design response surface methodology was used to establish the regression model between experimental factors and evaluation indexes. The optimal process parameter combination of the tempering process during hot air drying of Cyperus esculentus obtained through the regression model is as follows: tempering temperature of 24 °C, moisture content of Cyperus esculentus at the beginning of tempering of 23%, and tempering duration of 3 h. Under this combination, the tempering process increased the drying uniformity, germination rate and drying efficiency of Cyperus esculentus seeds by 21.122%, 4.205% and 22.832%, respectively. The error between the verification test results and the optimization test results were within the feasible range.

In this paper, the tempering process parameters during hot air drying of Cyperus esculentus seeds were optimized. The experimental results showed that the tempering process could effectively improve the drying uniformity and the seed germination rate, shorten the drying time and improve the drying efficiency. This provided a drying method with a theoretical basis for Cyperus esculentus seeds production and drying. This study can provide a reference for the actual production of Cyperus esculentus, and provide a theoretical basis for further research on the drying process of Cyperus esculentus seeds. In the future, we will investigate the effect of the tempering process on the nutritional quality (oil, starch, sugar, protein, dietary fiber) of Cyperus esculentus.