Experimental Study on the Characteristics of Camellia oleifera Fruit Shell Explosion by Hot Air Drying

Abstract

The shell explosion by hot air drying is a critical step in the processing of Camellia oleifera fruit (COF), which directly affects the degree of the shell explosion, and the separation effect of Camellia oleifera seed and Camellia oleifera shell after the shell explosion of COF. To reveal the characteristics of the COF shell explosion, a hot air drying device was designed based on mass conservation and drying principles. The physical characteristics of COF and the evolution of drying parameters were thoroughly analyzed with a combination method of drying analysis and experimental. Moreover, under the conditions of air temperature 50–70 °C, relative humidity 20–50%, and air velocity 1.3–1.9 m/s, the internal relationship between COF shell explosion formation through hot air drying and the hot air drying medium was systematically investigated by response surface methodology, and a prediction model for the shell explosion rate of COF by hot air drying was constructed using statistical methods. Results demonstrated that decreasing the relative humidity and increasing the temperature and air velocity of the drying medium could reduce the dehydration time of COF. The moisture content of Camellia oleifera shell was found to be 177.45% d.b. (dry basis) at the initial cracking stage of COF. Furthermore, at temperatures ranging from 50 to 70 °C D_eff values of COF were estimated to be within the range of 0.915 × 10⁻⁹ to 1.782 × 10⁻⁹ m²/s. Similarly, at relative humidity levels of 20 to 50%, D_eff values ranged from 1.226 × 10⁻⁹ to 1.501 × 10⁻⁹ m²/s. At an air velocity of 1.3 to 1.9 m/s, D_eff values ranged from 0.956 × 10⁻⁹ to 1.501 × 10⁻⁹ m²/s. The measured values of the shell explosion rate were in close agreement with that calculated using the fitted model, with a correlation coefficient of 0.997 and a root mean square error of 0.9743. This study will provide a theoretical basis for optimizing the shell explosion process and improving shell explosion rate of COF by hot air drying.

1. Introduction

Camellia oleifera, commonly known as tea tree, is an evergreen tree belonging to the Theaceae family. Alongside olive, oil palm, and coconut, it ranks among the world’s four major woody oil plants [1,2]. With a cultivation history of 2000 years in China [3], Camellia oleifera fruit (COF) plays a vital role as an important oil crop. The oil extracted from COF is abundant in unsaturated fatty acids [4], vitamin E, tocopherol, squalene, sterols, polyphenols, and other bioactive substances [5]. These components exhibit significant inhibitory and preventive effects against diseases such as coronary heart disease, hypertension, and more [6]. As a result, it finds extensive usage across industries, including the industrial, medicinal, healthcare, and beauty sectors [7]. Additionally, COF offers numerous by-products with high added value [8], thereby presenting vast market potential. The fruit of the Camellia oleifera tree consists of shells and seeds. While the shells contain lignin, pentosan, and saponin, devoid of any oil content, they hinder the processing of camellia oil. Consequently, the removal of shells becomes a pivotal step in obtaining Camellia oleifera seeds for the production of premium camellia oil [9]. Currently, three main methods are employed for removing shells from COF: natural-drying-induced shell explosion, mechanically forced shell removal, and hot-air-drying-induced shell explosion. Several studies using shell explosion by hot air drying have indicated that the shell explosion rate of COF could reach more than 90% and that the comprehensive quality of the camellia oil produced was better, with the highest acid value and micronutrient content [10,11,12]. In practical engineering applications, it has been found that the method of shell explosion by hot air drying can ensure the integrity of Camellia oleifera shell and effectively avoid damage to Camellia oleifera seeds caused by mechanical forced stripping, which provides favorable conditions for subsequent separation and storage of Camellia oleifera seeds. Therefore, in the process of removing Camellia oleifera shells, shell explosion by hot air drying technology has great advantages and broad prospects for development.

Shell explosion by hot air is a process of natural cracking of the Camellia oleifera shell caused by the exchange of heat and moisture between the fruit and the drying medium, and the fruit’s shell explosion characteristics are directly related to the energy consumption, the degree of the shell explosion, and the separation effect of Camellia oleifera seed and Camellia oleifera shell [13]. In order to reveal the characteristics of COF shell explosion by hot air drying, scholars both domestically and abroad have carried out research. Zhao et al. [14] compared three drying methods—namely hot air drying, heat pump drying, and microwave drying—for their impact on the shell explosion of COF. Microwave drying failed to achieve the desired shell explosion effect, while heat pump drying exhibited lower shell explosion efficiency and consumed more power compared to hot air drying. When dried at 65 °C, the shell explosion rate of COF reached 96.97%. Luo et al. [15] employed a blast-drying oven to dry COF at a hot air temperature of 60 °C. The study found that the fruit could be effectively exploded, resulting in Camellia oleifera seeds with a moisture content of less than 10%. Liao et al. [16] found that under airflow drying at 50 °C for 4 h, the cracking ratio of COFs reached 100%. Wang et al. [13] investigated the influence of hot air temperature on the cracking of COF. The findings revealed that increasing the temperature could accelerate the rate of cracking of COF, but too high a temperature would also hinder the cracking of COF.

Compared to natural drying-induced shell explosion and mechanically forced shell removal, shell explosion by hot air drying can reduce Camellia oleifera losses and increase farmers’ economic income, which is highly welcomed by users [13,14]. However, this technology has not yet formed a widely promoted theory and method for COF shell explosion by hot air drying. One of the main reasons is the lack of comprehensive and systematic research on the characteristic of shell explosion by hot air drying. In order to reveal the characteristics of COF shell explosion by hot air drying, this study built a hot air drying device; systematically analyzed the change rule of effective moisture diffusion coefficient of COF; explored the relationship between the drying characteristics, shell explosion rate of COF, drying medium temperature, relative humidity, and air velocity; and established a prediction model for COF shell explosion by hot air drying, thereby providing a theoretical and scientific foundation for analyzing COF shell explosion by hot air drying and designing an efficient shell explosion machine for COF.

2. Materials and Methods

2.1. Raw Materials

To conduct the experiment, COFs of the Changlin 4 variety were utilized, whose transverse and longitudinal diameters range from 35.4 to 45.7 mm and 30.7 to 40.6 mm respectively. These fruits are extensively cultivated in Nanchang, Jiangxi Province, China. Once fully ripe, the fruits were carefully picked. Subsequently, they were sealed in a fresh-keeping bag and stored in a refrigerator at a temperature of 4 °C [17]. The initial moisture content of the COF was determined using a method involving heating at 105 °C [18]. Three measurements were conducted, and their average value yielded an initial moisture content of COF of 138.1% d.b.

2.2. Hot Air Drying System and COF Drying Process

A COF shell explosion device utilizing hot air drying was developed based on the principles of drying technology. The device consists of a hot air system (WJ-DH-50L, Dongguan Xinqi Machinery Equipment Co., Ltd., Dongguan, China), air ducts, a fan (DPT15-44-30, Jiangmen Jinling Ventilating fan Manufacture Co., Ltd., Jiangmen, China), and a drying room (Figure 1). The drying room incorporates essential components such as a temperature and humidity sensor with a high level of accuracy (COS-03, Shandong Renke Control Technology Co., Ltd., Jinan, China), a material tray, and an exhaust port. The diameter and material of the material tray are 150 mm and stainless steel, respectively. To initiate the experiment, the hot air system is activated, and the temperature and humidity of the drying medium are adjusted accordingly. The fan is then switched on, and the desired air velocity is set. Once the temperature and humidity readings displayed by the sensor stabilize at the predetermined values, the upper opening of the drying chamber is opened. Room-temperature COFs are carefully placed into the material tray within the drying room.

2.3. Physical Properties of COF

In this study, a total of 30 fresh COFs were randomly selected for observation of the natural shell explosion phenomenon. The fresh COFs were placed in a natural environment and observed the cracking behavior every 3 h. Each fruit was numbered to facilitate identification during the observation process. Whenever an individual COF displayed the initial signs of cracking, its shell moisture content was measured. This procedure continued until all 30 fruits had undergone complete cracking. These experiments were carried out in duplicate.

2.4. Parameters of Shell Explosion by Hot Air Drying

2.4.1. Setting of Shell Explosion Rate Standard

After a specific duration of hot air drying, the COFs undergo surface cracks, leading to the exposure of Camellia oleifera seeds. These seeds can easily detach from the Camellia oleifera shell due to the action of shock. The fruit is considered to have exploded if all the Camellia oleifera seeds have fallen off, and this event is recorded as “1”. For fruits in which not all the seeds have completely fallen off, the explosion count is calculated using the following equations [14]:

$${\mathrm{m}}_{\mathrm{i}}={\displaystyle \frac{{\mathrm{z}}_{\mathrm{i}}}{{\mathrm{x}}_{\mathrm{i}}}}$$

(1)

$$\mathrm{c}={\sum }_{\mathrm{n}=1}^{\mathrm{y}}{\mathrm{m}}_{\mathrm{i}}$$

(2)

$$\mathrm{Y}={\displaystyle \frac{\mathrm{c}}{\mathrm{y}}}\times 100\mathrm{\%}$$

(3)

where ${\mathrm{m}}_{\mathrm{i}}$ is the exploding number of the $\mathrm{i}$-th COF; ${\mathrm{x}}_{\mathrm{i}}$ is the total number of seeds contained in the i-th COF; z_i is the number of seeds shed from the i-th COF; y is the total number of COF samples; c is the exploding number in the sample; and $\mathrm{Y}$ is the shell explosion rate of COF.

2.4.2. Setting of Shell Explosion Rate Standard

The dry basis moisture content of COF was calculated from Equation (4) [3,18]:

$${\mathrm{M}}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{m}}_{\mathrm{t}}-{\mathrm{m}}_{\mathrm{d}}}{{\mathrm{m}}_{\mathrm{d}}}}\times 100\mathrm{\%}$$

(4)

where ${\mathrm{M}}_{\mathrm{t}}$ is the moisture content (d.b.) of COF (g/g); ${\mathrm{m}}_{\mathrm{t}}$ is the weight of COF at time $\mathrm{t}$, (g); and ${\mathrm{m}}_{\mathrm{d}}$ is the weight of COF dry matter (g).

2.4.3. Calculation of Drying Rate

The drying rate (DR) of COF was calculated from Equation (5) [19]:

$$\mathrm{D}\mathrm{R}={\displaystyle \frac{{\mathrm{M}}_{{\mathrm{t}}_1}-{\mathrm{M}}_{{\mathrm{t}}_2}}{{\mathrm{t}}_1-{\mathrm{t}}_2}}$$

(5)

where ${\mathrm{t}}_1$ and ${\mathrm{t}}_2$ represent the time (h) and ${\mathrm{M}}_{\mathrm{t}1}$ and ${\mathrm{M}}_{\mathrm{t}2}$ represent the dry basis moisture content (g/g).

2.4.4. Calculation of Moisture Ratio

The moisture ratio (MR) of COF was determined according to Equation (6) [20]:

$$\mathrm{M}\mathrm{R}={\displaystyle \frac{{\mathrm{M}}_{\mathrm{t}}-{\mathrm{M}}_{\mathrm{e}}}{{\mathrm{M}}_{\mathrm{o}}-{\mathrm{M}}_{\mathrm{e}}}}$$

(6)

where ${\mathrm{M}}_{\mathrm{e}}$ is the equilibrium moisture content (d.b.) of COF (g/g) ${\mathrm{M}}_{\mathrm{o}}$ is the initial moisture content (d.b.) of fresh COF (g/g). Considering that the equilibrium moisture content of COF is small, the moisture ratio here can be simplified as followed [21,22]:

$$\mathrm{M}\mathrm{R}={\displaystyle \frac{{\mathrm{M}}_{\mathrm{t}}}{{\mathrm{M}}_{\mathrm{o}}}}$$

(7)

2.4.5. Calculation of Moisture Effective Diffusivity ${\mathrm{D}}_{\mathrm{e}\mathrm{f}\mathrm{f}}$

The moisture effective diffusivity reflects the moisture migration of COF during the drying process, and its value represents the evaporation intensity of moisture on the surface of COF. Assuming that the volumetric shrinkage of the material during drying is neglected, it can be obtained from Fick’s second law that the moisture effective diffusivity is constant under certain drying conditions. The moisture effective diffusivity of COF satisfies the following equation [23]:

$$\mathrm{M}\mathrm{R}={\displaystyle \frac{6}{{\mathsf{\pi }}^2}}{\sum }_{\mathrm{n}=1}^{\mathrm{\infty }}\mathrm{e}\mathrm{x}\mathrm{p}(-{\displaystyle \frac{{\mathrm{n}}^2{\mathsf{\pi }}^2{\mathrm{D}}_{\mathrm{e}\mathrm{f}\mathrm{f}}}{{\mathrm{r}}^2}}\mathrm{t})$$

(8)

where $\mathrm{n}$ is the number of experimental samples and $\mathrm{r}$ is the volume equivalent radius of COF (m). When the drying time is long enough, take $\mathrm{n}$ = 1 and perform logarithmic transformation on both sides of the above Equation (8) to obtain the following equation [24]:

$$\mathrm{l}\mathrm{n}\mathrm{M}\mathrm{R}=\mathrm{l}\mathrm{n}({\displaystyle \frac{6}{{\mathsf{\pi }}^2}})-({\displaystyle \frac{{\mathsf{\pi }}^2{\mathrm{D}}_{\mathrm{e}\mathrm{f}\mathrm{f}}}{{\mathrm{r}}^2}})\mathrm{t}$$

(9)

It can be seen from Equation (9) that the natural logarithm of the moisture ratio in the hot air drying process of COF has a linear relationship with the drying time, and the slope k satisfies the following equation:

$${\mathrm{D}}_{\mathrm{e}\mathrm{f}\mathrm{f}}=-{\displaystyle \frac{{\mathrm{r}}^2}{{\mathsf{\pi }}^2}}\mathrm{k}$$

(10)

Considering COF as a sphere, the volume-equivalent radius satisfies the following equation [25]:

$${\displaystyle \frac{4}{3}}\mathsf{\pi }{\mathrm{a}}^2\mathrm{b}={\displaystyle \frac{4}{3}}\mathsf{\pi }{\mathrm{r}}^3$$

(11)

where a is the transverse radius (m), b is the longitudinal radius (m). Randomly select 60 COFs, measure their transverse and longitudinal radius, and obtain the average values a = 0.0201 m and b = 0.0177 m.

2.5. Experiment Method

To comprehensively study the impact of hot air temperature, relative humidity, and air velocity on the characteristics of COF shell explosion during hot air drying, an orthogonal experiment was conducted. The aim was to observe the influence of these factors on the shell explosion rate. Prior to the experiment, the hot air system and fan were started, and the temperature, relative humidity, and air velocity of the drying medium were adjusted until they reached a stable state. Nine COFs with uniform size, shape, no cracking, and no mildew were selected from the same fresh-keeping bag. These fruits were placed in a natural environment and allowed to return to room temperature. Out of the nine fruits, three were randomly chosen, peeled, mixed, and smashed to measure their initial moisture content. The remaining six fruits were measured for initial temperature and weight, ensuring a total weight of 200 ± 5 g. Subsequently, the fruits were placed on the material tray within the drying room for hot air drying. The initial temperature, humidity, and air velocity of the drying medium were recorded during the experiment. At regular intervals, the top of the drying chamber was opened, and the surface temperature of the COFs was measured using an infrared thermometer. The fruits were quickly removed and weighed, and the exploding shell data were recorded. They were then returned to the drying chamber for further drying. The experiment was concluded when the shell explosion rate of the COFs reached 90%. The drying rate and moisture ratio of the fruits were calculated under different conditions. All the experiments were carried out in the triplicate and the average moisture content was used to draw the drying curves. Double standard deviation was to draw error bars [26]. The shell explosion rate observed 7 h before the drying test was used as the evaluation index. The experimental results were analyzed using the response surface method [27] to discuss the comprehensive influence of the three factors (temperature, relative humidity and air velocity) on the shell explosion effect during hot air drying. A Box–Behnken design [28] was used for 17 experiments carried out in orthogonal experiments, and the shell explosion rate was response value. Finally, a regression equation for COF shell explosion by hot air drying was established. The experimental factors are coded as

Table 1. Experimental codes.

Codes	Factors
	Temperature (A)/°C	Relative Humidity (B)/%	Air Velocity (C)/m/s
−1	50	20	1.3
0	60	35	1.6
1	70	50	1.9

[29].

2.6. Statistical Analysis

In this paper, the experimental data were calculated as the average of three repeated experiments. Statistically significant differences were analyzed by variance with a significant level of p < 0.05. The correlation coefficient and root mean square error will be used to analyze the reliability of the regression model of COF shell explosion by hot air drying [30]. The calculation equations of correlation coefficient [31] and root mean square [32] are as follows:

$${\mathrm{R}}^2=1-{\displaystyle \frac{{\sum }_{\mathrm{i}=1}^{\mathrm{N}}{({\mathrm{X}}_{\mathrm{e}\mathrm{x}\mathrm{p},\mathrm{i}}-{\mathrm{X}}_{\mathrm{p}\mathrm{r}\mathrm{e},\mathrm{i}})}^2}{{\sum }_{\mathrm{i}=1}^{\mathrm{N}}{({\mathrm{E}}_{\mathrm{e}\mathrm{x}\mathrm{p},\mathrm{i}}-{\mathrm{E}}_{\mathrm{p}\mathrm{r}\mathrm{e},\mathrm{i}})}^2}}$$

(12)

$$\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{E}=\sqrt{{\displaystyle \frac{{\sum }_{\mathrm{i}=1}^{\mathrm{N}}{({\mathrm{X}}_{\mathrm{e}\mathrm{x}\mathrm{p},\mathrm{i}}-{\mathrm{X}}_{\mathrm{p}\mathrm{r}\mathrm{e},\mathrm{i}})}^2}{\mathrm{N}}}}$$

(13)

where ${\mathrm{X}}_{\mathrm{e}\mathrm{x}\mathrm{p},\mathrm{i}}$ is the $\mathrm{i}$th test value; ${\mathrm{E}}_{\mathrm{e}\mathrm{x}\mathrm{p},\mathrm{i}}$ is the average value of the tested value; ${\mathrm{X}}_{\mathrm{p}\mathrm{r}\mathrm{e},\mathrm{i}}$ is the $\mathrm{i}$th predicted value; and $\mathrm{N}$ is the number of tests.

3. Results and Discussion

3.1. Analysis of Physical Characteristics of COF

The moisture content of shell plays a crucial role in influencing the cracking behavior of COF. A lower moisture content contributes to easier cracking of the fruit [33]. Analysis from Figure 2 reveals that the initial cracking of Camellia oleifera shell occurs within a maximum moisture content range of 219–228% (d.b.), while the minimum range is 115–128% (d.b.). Notably, the highest occurrence of initial cracks in Camellia oleifera shell is observed when the moisture content ranges from 160–180% (d.b.), accounting for 18.89% of the total.

Table 2. Moisture content of Camellia oleifera shell at initial cracking.

No.	1	2	3	4	5	6
Moisture content (% d.b.)	142.5	157.0	137.3	123.8	207.0	227.2
No.	7	8	9	10	11	12
Moisture content (% d.b.)	122.8	141.7	181.3	204.2	203.0	209.0
No.	13	14	15	16	17	18
Moisture content (% d.b.)	153.3	136.5	182.0	190.1	116.2	172.3
No.	19	20	21	22	23	24
Moisture content (% d.b.)	217.9	179.1	166.1	181.8	178.3	173.5
No.	25	26	27	28	29	30
Moisture content (% d.b.)	162.5	190.4	167.2	204.3	152.0	193.1

further demonstrates that the average moisture content of Camellia oleifera shell at the point of initial cracking is measured to be 177.45% (d.b.), which is consistent with the conclusions of the research teams from China Agricultural University and Hunan Agricultural University [16].

3.2. Analysis of Drying Rate of COF

Based on the initial moisture content and weight changes during drying, the drying rate and moisture ratio of COF were analyzed in relation to time. From Figure 3A, it is evident that, under constant relative humidity and air velocity conditions, the characteristics of COF shell explosion by hot air drying are significantly influenced by the hot air temperature. Higher temperatures result in a shorter drying time to reach a certain moisture ratio. This can be attributed to the increased temperature difference between the COF and the drying medium, leading to greater kinetic energy of moisture molecules and faster rates of heat and mass transfer [34]. At hot air temperatures of 60 °C and 70 °C, the hot air drying rate of COF initially increases and then decreases, following a relatively consistent pattern. In contrast, at a temperature of 50 °C, the hot air drying rate exhibits an upward trend, which differs from the other two temperature conditions. This discrepancy may arise due to uneven distribution of the hot air temperature field within the drying room during the explosion process. Consequently, the COFs experience non-uniform heating, resulting in varying moisture evaporation rates and different cracking times for each fruit. Additionally, after the explosion of COFs, the effective moisture evaporation area increases, leading to an accelerated drying rate. As a result, the drying rate of COFs at 50 °C shows a gradually increasing trend. The experimental findings demonstrate that the hot air drying characteristics of COFs are closely related to the temperature. Higher temperatures result in a shorter drying time to achieve a specific moisture ratio.

Figure 3B demonstrates that, under constant drying medium temperature and air velocity conditions, a relative humidity of 20% results in the shortest time to reach a specific moisture ratio. Conversely, there is minimal difference in the change of moisture ratio when the relative humidity is between 35% and 50%. This behavior can be attributed to the fact that lower relative humidity corresponds to a drier drying medium with increased moisture absorption capacity, resulting in faster material drying [35]. The experimental results indicate a clear relationship between relative humidity and the hot air drying characteristics of COF. Specifically, lower relative humidity leads to a shorter drying time to achieve a specific moisture ratio.

Figure 3C reveals that, under constant temperature and relative humidity of the drying medium, the air velocity of the drying medium impacts the drying rate and moisture ratio of COF. Notably, the longest time to reach a specific moisture ratio is observed at the air velocity of 1.3 m/s, while there is minimal difference in the moisture ratio when the air velocity is 1.6 m/s or 1.9 m/s. This can be attributed to the fact that increasing the air velocity enhances the transfer of moisture between the surface of COF and the drying medium, thereby accelerating the drying process. In conclusion, the hot air drying characteristics of COF are influenced by the air velocity. Higher air velocity results in shorter drying times to achieve a specific moisture ratio and faster drying rates.

3.3. Analysis of Effective Moisture Diffusion Characteristics of COF

Equation (10) reveals a linear relationship between time $\mathrm{\tau }$ and $\ln \mathrm{M}\mathrm{R}$ during the hot air drying process of COF. Figure 4 illustrates the changes in $\ln \mathrm{M}\mathrm{R}$ with drying time under various drying conditions. The slope k of this linear relationship was obtained through linear fitting, enabling the calculation of the moisture effective diffusivity [36] of COF under different hot air drying conditions by combining Equation (9). The computed results are presented in

Table 3. Effective moisture diffusion coefficient of COF.

No.	Equation of Linear Regression	R2	Deff/10−9 m2/s
1	ln MR = 0.0883 − 2.418 × 10−5 t	0.9601	$0.905$ ± 0.033 e
2	ln MR = 0.1229 − 4.154 × 10−5 t	0.9620	$1.555\pm 0.048$ b
3	ln MR = 0.1035 − 4.851 × 10−5 t	0.9693	$1.816\pm 0.031$ a
4	ln MR = 0.0923 − 3.260 × 10−5 t	0.9450	$1.220\pm 0.017$ d
5	ln MR = 0.9105 − 3.232 × 10−5 t	0.9762	$1.210\pm 0.007$ d
6	ln MR = 0.1031 − 3.143 × 10−5 t	0.9318	$1.176\pm 0.074$ d
7	ln MR = 0.1385 − 3.962 × 10−5 t	0.9653	$1.483\pm 0.010$ c

Note: The different lowercase letter in the same row indicates that the means are significantly different at p < 0.05 according to the Duncan test.

.

3.4. Response Surface Experiment Design and Results

A three-factor and three-level experiment was conducted, as outlined in Table 1, which consisted of a total of 17 experimental groups. The objective of this study was to investigate the impact of temperature, relative humidity, and air velocity on the shell explosion rate of COF during hot air drying. With a fixed drying time of 7 h for each group, a regression equation model for the shell explosion rate (Y) was developed. The experimental design and corresponding results are presented in

Table 4. Test plan and results.

No.	Factors			Index
	Temperature/°C	Relative Humidity/%	Air Velocity/m/s	Shell Explosion Rate/%
1	−1	−1	0	11.11
2	1	−1	0	69.31
3	−1	1	0	6.67
4	1	1	0	66.67
5	−1	0	−1	2.08
6	1	0	−1	59.72
7	−1	0	1	16.67
8	1	0	1	75.28
9	0	−1	−1	40.58
10	0	1	−1	28.59
11	0	−1	1	54.09
12	0	1	1	36.37
13	0	0	0	33.33
14	0	0	0	31.11
15	0	0	0	38.41
16	0	0	0	42.38
17	0	0	0	34.47

.

3.5. Establishment of Model for COF Shell Explosion Rate by Hot Air Drying

The experimental data presented in Table 4 were analyzed and fitted, and the results of variance analysis for the shell explosion rate model of COF are displayed in

Table 5. Variance analysis of experimental results.

Project	Source of Variation	Sum of Squares	Sum of Squares.	Mean Square	F	p
shell explosion rate Y	model	6841.93/6807.89	9/5	760.21/1361.58	44.68/97.81	<0.0001 **/<0.0001 **
	A	5989.10/5989.10	1/1	5989.10/5989.10	352.03/430.22	<0.0001 **/<0.0001 **
	B	342.57/342.57	1/1	342.57/342.57	20.14/24.61	0.0028 **/0.0004 **
	C	330.76/330.76	1/1	330.76/330.76	19.44/23.76	0.0031 **/0.0005 **
	AB	75.34/75.34	1/1	75.34/75.34	4.43/5.41	0.0734/0.0401 *
	AC	0.2352	1	0.2352	0.0138	0.9097
	BC	8.21	1	8.21	0.4825	0.5097
	A2	25.48	1	25.48	1.50	0.2606
	B2	65.03/70.11	1/1	65.03/70.11	3.82/5.04	0.0915/0.0464 *
	C2	0.0059	1	0.0059	0.0003	0.9856
	residual.	119.09/153.13	7/11	17.0/13.92
	Lack of fit	39.21/73.26	3/7	13.07/10.47	0.6546/0.5241	0.6209/0.7860
	sum	6961.02/6961.02	16/16
	R2	0.9829/0.9780
	R2Adj	0.9609/0.9680

Note: The numbers after “/” are the results of variance analysis after removing insignificant factors; ** means extremely significant (p < 0.01), * means significant (0.01 ≤ p < 0.05).

. The overall model was found to be highly significant (p < 0.01). Specifically, the linear main effects of temperature, relative humidity, and air velocity were observed to be extremely significant (p < 0.01) for the shell explosion rate indicator (Y), while the remaining factors were not significant for the shell explosion rate. To further refine the model, interaction terms and quadratic terms with large P values were removed [37], followed by another round of variance analysis. The updated results are presented in Table 5. The model square determination coefficient (R²) and the adjusted square determination coefficient (R²_adj) were 0.9780 and 0.9680, respectively, indicating that the regression equations were well fitted and the models were reliable. The F-values of A, B, and C were 430.22, 24.61, and 23.76 respectively, indicating that the order of the influence of the three factors on shell explosion rate from large to small is temperature, relative humidity, and air velocity, and the effects of air velocity and relative humidity are not considerably different [38]. At this stage, the interaction term AB and the quadratic term B² became significant, providing insights into the relationship between the levels of each factor. The air temperature is the dominant factor affecting the moisture diffusion in materials; high temperature can provide sufficient internal energy for the evaporation of moisture inside the material, causing the moisture temperature to rise and quickly migrate to the outer surface, thereby enabling the material to quickly dehydrate [39]. In shell explosion by hot air drying, the shell explosion rate of COF is inversely proportional to its moisture content [13]. Therefore, the air temperature has the greatest impact on COF shell explosion by hot air drying, and the research team from Hunan Agricultural University has also confirmed this conclusion [16]. Relative humidity is a parameter that characterizes the ability of a drying medium to accept moisture, and air velocity is a parameter that affects the evaporation rate of moisture on the external surface of material. They are both important drying parameters that control the evaporation process of moisture on the surface of material, and directly affect the drying rate of the material [40,41].Hence, relative humidity and air velocity have a significant effect on COF shell explosion by hot air drying.

$$\mathrm{Y}=-63.230+1.723\mathrm{A}-3.438\mathrm{B}+21.433\mathrm{C}+0.029\mathrm{A}\mathrm{B}+0.018{\mathrm{B}}^2$$

(14)

The modified lack of fit item p = 0.7860 was not significant (p > 0.1), indicating that there are no other factors that affect the test indicators, and the analysis of the experimental results is reasonable [42].

3.6. Analysis of the Influence of Experimental Factors on Response Values

Multivariate regression fitting analysis was conducted on the experimental results presented in Table 4, resulting in the generation of response surface plots and contour maps to examine the influence of factors such as hot air temperature, relative humidity, and air velocity on the shell explosion rate. As shown in Figure 5, the lower projection of the response surface is a contour line, and the contour lines are elliptical, indicating that the two factors have a significant interaction. The steepness of the slope of the response surface of each factor reflects the impact on the shell explosion rate (Y)—the steeper the degree, the greater the impact [43]. By analyzing Figure 5, it is evident that within the selected range for each factor, the interactions between air velocity and relative humidity, as well as temperature and air velocity, are not significant. Conversely, the interaction between temperature and relative humidity shows significance. As depicted in Figure 5A, when temperature and relative humidity are held constant, the explosion rate increases with higher air velocities. This can be attributed to the accelerated moisture exchange rate between the COF and the drying medium, resulting in increased drying efficiency and shell explosion. As shown in Figure 5B, when temperature and air velocity are held constant, the shell explosion rate increases with higher relative humidity. Relative humidity serves as a crucial parameter that characterizes the moisture-absorbing capacity of air. A lower relative humidity signifies stronger moisture accommodation ability, while a higher relative humidity indicates weaker capacity. When relative humidity decreases, the drying medium exhibits enhanced capacity to absorb evaporated moisture from the surface of COF, thereby accelerating its drying rate and subsequently increasing the explosion rate. Examining Figure 5C, it is observed that when relative humidity and air velocity are constant, the explosion rate increases with higher temperatures. Increased temperature in the drying medium allows the COF to absorb more heat energy, providing the necessary internal energy for vigorous movement and rapid diffusion of moisture. Simultaneously, higher temperatures lead to an increase in the moisture effective diffusivity, facilitating efficient heat and moisture transfer between the COF and the drying medium. This enhances the dehydration rate.

3.7. Analysis of the Influence of Experimental Factors on Response Values

To validate the reliability of the shell explosion rate model for COF during hot air drying, random selection of different drying medium conditions was performed for experimental verification. The results of these experiments are presented in

Table 6. Test results.

No.	Drying Condition.			Predictive Value of Shell Explosion Rate/%	Tested Value of Shell Explosion Rate%	R2	RMSE
	Temperature/°C	Relative Humidity/%	Air Velocity/m/s
1	50	20	1.9	26.22	27.56	0.9993	0.9743
2	50	20	1.3	10.49	10.33
3	50	50	1.9	10.27	9.67
4	60	20	1.6	45.32	46.79
5	60	35	1.9	45.60	47.10
6	70	35	1.6	66.14	65.33
7	70	20	1.9	80.94	81.17

. The data analysis reveals that the correlation coefficient (R²) between the predicted values and the tested values is 0.9993, while the root mean square error (RMSE) is 0.9743. These findings indicate that the shell explosion rate model developed for COF through hot air drying exhibits high accuracy and reliability.

4. Conclusions

From the results of this study, the following conclusions could be drawn: the drying rate and moisture ratio of COFs were related to the temperature, relative humidity, and air velocity of the drying medium; reducing relative humidity and increasing the temperature and air velocity of hot air could shorten the dehydration time of COF; and the lower the moisture content, the easier the COF was to crack and the more easily Camellia oleifera seeds fell off. Meanwhile, the moisture content of the Camellia oleifera shell at the initial cracking stage of COF was determined. The effective moisture diffusion coefficient of COF varies within the range of 0.905 × 10⁻⁹ m²/s to 1.816 × 10⁻⁹ m²/s when the drying temperature is 50–70 °C. Similarly, at a relative humidity range of 20–50%, the moisture diffusion coefficient ranges from 1.210 × 10⁻⁹ m²/s to 1.555 × 10⁻⁹ m²/s. For air velocities between 1.3 m/s and 1.9 m/s, the moisture diffusion coefficient falls between 1.176 × 10⁻⁹ m²/s and 1.555 × 10⁻⁹ m²/s. The response surface analysis revealed that the interaction between air velocity and relative humidity—as well as air temperature and air velocity—had no significant effect on the shell explosion rate. However, relative humidity squared and the interaction between air temperature and relative humidity showed a significant effect on the shell explosion rate. The air temperature, relative humidity, and air velocity all had a significant influence on the shell explosion rate, and their degrees of influence showed a decreasing trend. Meanwhile, high temperature, high air velocity, and low relative humidity were helpful in increasing the shell explosion rate of COFs. A model for the COF shell explosion rate by hot air drying was developed. Statistical analysis showed that the model had good agreement and reliability. The correlation coefficient and root mean square error between the measured shell explosion rate and the predicted value of the model were 0.9993 and 0.9743, respectively. Thus, the proposed model may be reliably used to predict the shell explosion rate of COFs by hot air drying.