1 Introduction

Canola (Brassica napus L.) is an oilseed crop grown in temperate regions and is compatible with a wide range of climates. It is an annual member of the nightshade family (Brassicaceae). Its seeds contain up to 55% oil (Petrie et al. 2020) and 24% protein (Alam et al. 2016). Most varieties of rapeseed (canola) are susceptible to pod shattering, especially under hot, dry, and windy conditions (Wang et al. 2007). Seed should be harvested before the seed dries out, when the moisture content is still high, around 25%, and therefore should be harvested before pod breakage when the seed has a moisture content of 10–12% (d.b). However, the optimum moisture content for storage and germination is around 8% (dry basis) (Boakye Boadu and Siaw 2019). Therefore, seeds need to be dried before storage. An appropriate drying method usually takes into account the maintenance of seed vigor. Several drying methods have been studied, such as fluidized bed drying (Das et al. 2020a; Liu et al. 2014), microwave-assisted drying (Gaukel et al. 2017), and convective drying (Hemis et al. 2015). In fluidized bed drying, the seeds are exposed to high air velocity, which provides uniformity of drying air throughout the drying bed. The seeds are also exposed to a uniform air temperature, which prevents the seeds from overheating, thereby preventing seed damage. To further improve the food drying process, hybrid methods have been investigated in previous research. Hybrid drying processes can accelerate drying, improve drying kinetics and optimize energy consumption. The use of HCP in the drying bed not only provides separation between particles with higher moisture content, but also allows accelerated drying through heat transfer by conduction and convection (Moradi et al. 2020). The effect of HCP on drying kinetics has been reported in several cases (Honarvar et al. 2013; Moradi et al. 2020; Tasirin et al. 2014). Moradi et al. (2020) investigated the presence of different ratios of HCP to green pepper cubes (GBPC) in a spouted bed drying process (Moradi et al. 2020). A 50:50 ratio of HCP to GBPC was optimal in terms of drying time. The drying characteristics of oil palm frond fibers mixed with different weights of sand HCP in a fluidized bed dryer were investigated (Yun et al. 2013). The research showed that a 1:2 specimen to HCP ratio resulted in the shortest drying time. The conditions of the drying process affected not only the kinetics, time, and energy consumption, but also the seed germination characteristics (Chungcharoen et al. 2015). The germination rate of quinoa and amaranth seeds was affected by the power density of microwave-assisted drying (Maqueda et al. 2018). An inverse relationship was reported between microwave power density and germination characteristics. Therefore, it is important to ensure that canola seed drying is done properly to maintain seed vigor. To date, no research has evaluated the effects of HCP on canola seed drying kinetics and germination characteristics because the effect of HCP (as a new hybrid drying method) on drying of some agricultural products has been investigated, but the effect of them on the germination ability of canola seeds has not been investigated. Therefore, the present work investigated how the presence of different amounts of HCP (HCP to sample ratios) in a solar fluidized bed dryer can affect the drying kinetics, drying time, and germination characteristics of canola seed.

2 Materials and Methods

2.1 Sample Preparation

Fresh rapeseed of Neptune cultivar was manually collected from a research field of Faculty of Agriculture of Shiraz University in 2020.The initial moisture content of the samples was measured in an electric oven (at a temperature of 103 ± 2 °C for 17 ± 1 h) using the following formula (ISTA 2010).

$$ MC=\frac{{W}_{1}-{W}_{2}}{{W}_{2}} \times 100$$
(1)

where; Mc = moisture content on dry basis (%), W1 = mass of seed before oven drying (g), W2 = mass of seed after oven drying (g).

The fresh seeds had an average initial moisture content of 25 ± 1% (dry basis). For each drying run, 200 g ± 1% of fresh canola seed was placed in the dryer. Seed drying was continued until the moisture content reached approximately 10% (dry basis). They were then held at 5 °C until the start of the germination test.

2.2 Experimental Setup

A fluidized bed dryer apparatus was used to dry rapeseed (Figure supplementary S1). It was composed of an electric fan (1) (3 phases with 3 kW), a solar flat plate collector (2), a electric heater, and a drying chamber (4). The ambient air is blown on the solar collector by the blower, and after passing through the electric heater, it enters the drying chamber. The air velocity was adjusted so that the seeds floated and vibrated in the dryer (4). After trial and error, the drying air velocity was adjusted to 3 m/s at the air outlet (5). The drying chamber was a transparent fiberglass cylinder with a height and diameter of 400 and 190 mm, respectively. A k-type thermocouple with an accuracy of ± 1 °C was installed between the air heater and the drying chamber to adjust the air temperature to the desired level. For this purpose, an Arduino board was used to record the air temperature at the inlet of the drying chamber and then, if the temperature was lower than the set value, the command to turn on the heater was issued. The air heater was automatically turned on when the solar panel could not produce enough heat in the drying chamber. The fresh canola seeds were placed in the drying bin and the drying process was continued to a moisture content of approximately 10 ± 1% (dry basis).

2.3 Design of the Study

A completely randomized factorial design was used to investigate the effects of different experimental conditions on germination characteristics and drying time. The air temperature was applied in three levels of 40, 50 and 60 °C and the mass percentage of HCP (glass or steel beads of 2.7 mm diameter) to canola seeds in four levels (zero, 25%, 50% and 100%). All experiments were conducted in triplicate. In each experiment, approximately 200 g of fresh canola seed was mixed with either 0 g, 50 g, 100 g, or 200 g of steel or glass HCP and placed in the drying chamber.

2.3.1 Germination Test

To investigate the effects of independent parameters on germination of dried seeds, 100 seeds were randomly selected from each dried sample and planted in 4 petri dishes (25 seeds with 5 cc of distilled water in each petri dish). The petri dishes were placed in an incubator at 25 °C and then the number of germinated seeds, root and shoot lengths were measured at 6-hour intervals for 3 days. The percentage of germination was calculated as follows.

Equation (2):

$$ \text{G}\text{P}=\frac{{\text{N}}_{\text{g}}}{{\text{N}}_{\text{t}}}$$
(2)

Where Nt and Ngare the total number of seeds planted and the number of seeds germinated, respectively. Also, germination rate (GR), coefficient of uniformity of germination (CUG), germination energy (GE), root length vigor index (RLVI) and shoot length vigor index (SLVI) were calculated by Eqs. (3, 4, 5, 6 and 7), respectively (Agarwal, 2003):

$$ GR=\sum _{i=1}^{k}\frac{{n}_{i}}{{t}_{i}}$$
(3)
$$ CUG=\frac{\sum _{i=1}^{k}{N}_{i}}{MGT}$$
(4)
$$ GE=\frac{Germination\,percentage\,on\,determined\,hour}{Total\,EquationNumber\,of\,planted\,seeds}$$
(5)
$$ SLVI=\frac{SL\times ER}{100}$$
(6)
$$ RLVI=\frac{RL\times ER}{100}$$
(7)

Where ti refers to the number of hours from the start of the test, ni indicates the total number of cases of seedling emergence at each measurement hour, k is the total number of time intervals, SL is the shoot length (cm), RL is the root length (cm), MGT is the mean germination time, Ni is the number of seeds germinated at the ith time step, and GR is the germination rate. A control sample was also dried in the shade to a moisture content of approximately 10%. The control seeds were also germinated in a similar manner and the results were compared to the fluidized bed dried seeds.

2.4 Statistical Analysis

Analysis of variance was performed using SAS 9.4 software based on Duncan’s test (p ≤ 0.05). Comparison of means was performed using Duncan’s test at the 5% level, and graphs were generated using Microsoft Excel.

3 Results

Freshly harvested canola seed reached a moisture content of approximately 10% (dry basis) after drying in a solar fluidized bed dryer, either with or without a glass/steel HCP. The effect of various operating conditions on drying time and germination characteristics was studied. All independent variables significantly affected all dependent variables, i.e. drying time (DT), germination percentage (GP), germination rate (GR), coefficient of uniformity of germination (CUG), germination energy (GE), shoot length vigor index (SLVI) and root length vigor index (RLVI) (Table 1).

Table 1 Analysis of variance of Temperature, HCP type and mass percentage of HCP on germination and seedling parameters
Full size table

3.1 Influence of Independent Parameters on Drying Time

3.1.1 Effect of Temperature on Drying Time

A comparison of mean drying times was made for different drying conditions (Fig. 1). The drying time was negatively correlated with the inlet air temperature. According to Duncan’s multiple range test, the mean drying times were 98.25, 52.13, and 35.38 min in response to 40, 50, and 60 °C, respectively. Although the trend was decreasing throughout the drying process (from approximately 99 to 35 min), a decrease of 47% occurred in the first temperature jump, followed by a decrease of 32% in the second jump. This can be attributed to the effect of air temperature on moisture transfer from the interior to the surface.

Fig. 1
figure 1

Mean comparison of drying time for different drying conditions

Full size image

3.1.2 Effect of Application of Glass or Steel HCP on Drying Time

The type and amount of HCP significantly reduced seed moisture, especially in the early stages of seed drying. Steel HCP affected drying time more than glass HCP (Fig. 1). This was more pronounced at lower temperature ranges. Our results indicated that the longest drying time occurred in response to an air temperature of 40 °C with 100% glass HCP. However, at 60 °C, the shortest drying time was obtained with 50% steel HCP.

3.1.3 Effect of HCP Percentage on Drying Time

At air temperatures between 40 and 60 °C, the drying time ranged from 50 to 62 min in the presence of different amounts of steel HCP, with the shortest occurring in response to the 50% HCP (Fig. 1). This was slightly higher when steel was replaced by glass in the drying chamber, in which case the values ranged from 57 to 74 min, with the shortest occurring in response to the 50% HCP. In both cases, increasing the amount of HCP in the chamber to 50% of the total sample weight reduced the drying time. However, the highest temperature resulted in an decrease in drying time.

3.1.4 Interaction Effect of Temperature and HCP Percentage on Drying Time

Accordingly, at all experimental temperatures, as discussed in the previous section, a decreasing trend in drying time was observed as the percentage of HCP increased from zero to 50%, beyond which the drying time increased. This result may be due to the fact that as the inert particles increase, a higher drying rate is observed due to increased heat conduction through the inert particles to the seeds. However, at the higher inert particle levels, the effect of heat conduction may be less than that of the increased mass of materials in the drying chamber. In other words, if the ratio of HCPs to seeds is 0, 25%, 50% and 100%, the total mass of the substrate (mixture of seeds and HCPs) will be 200, 250, 300 and 400 g, respectively. Therefore, on the one hand, the increase of inert particles increases the heat transfer to the seeds, but on the other hand, due to the increase of the mass of HCPs, the mass of the whole bed increases. However, one of the most important limitations in seed drying is the rate of movement of moisture from inside the seed to the drying air (moisture diffusion coefficient). Increasing the heat transfer into the seed increases this coefficient, but due to the limited size of the pores and the structure of the drying material, this coefficient cannot exceed a certain value. Probably beyond 50%, further increase of HCPs only causes more heat absorption of drying air by HCPs and shows an inverse effect on drying rate. However, at the different temperatures, the slope of the changes in drying time versus HCP percentage was nonlinear, i.e., the average slopes of the curves for 40, 50, and 60 °C were − 0.12, -0.11, and − 0.02, respectively.

3.1.5 Interaction Effect of Temperature and HCP Type on Drying Time

Temperature and HCP type had an interaction effect on drying time (Fig. 2B). As can be seen, the drying time of seeds decreased with increasing temperature. However, a greater decrease in drying time was observed in the experiments conducted with glass HCP. The average slope of decrease in drying time by steel and glass particles was − 2.92 and − 3.26, respectively.

Fig. 2
figure 2

Interaction effect of different parameters on the drying time, (a) percentage of HCP and temperature, (b) temperature and HCP type, (c) HCP percentage and type

Full size image

3.1.6 Interaction Effect of HCP Type and HCP Blending Percentage on Drying Time

Figure 2C shows the interaction effect of HCP type and its blending percentage on drying time. According to this, the slope of variation for steel and glass curves were − 0.48 and − 0.33, respectively, when HCP was used in the range of 0–25%. Similarly, for the 25–50% HCP ranges, the values were − 0.14 and − 0.10, respectively. However, in the 50–100% HCP range, the steel and glass curves had slopes of 0.21 and 0.33, respectively. Therefore, it can be concluded that increasing the HCP blend percentage in the 0–50% range resulted in a shorter drying time.

3.2 Influence of Independent Parameters on Germination Viability

Physiological parameters of seed quality or seed vigor can determine the performance and activity of seed lots with acceptable germination in a wide range of environments. Seed vigor is an important determinant of seed quality. The drying process had a significant effect on canola seed vigor. Drying temperature, HCP type, and mixture percentage were considered to be the most important factors affecting canola seed vigor during drying. The performance of seed or seed vigor included several factors, i.e. GP, GR, SLVI, RLVI and GE in the current research. The results of germination test showed that all the above factors decreased with the increase of temperature (Figs. 3, 4, 5, 6 and 7). Also, our results showed that steel HCP caused lower GP compared to glass HCP. This may be due to the higher conductivity and lower specific heat of steel compared to glass, which allows the seeds to receive more heat in less time. In the experiments conducted with steel HCP at 40 °C, GP decreased as the percentage of HCP in the mixture increased to 25%. However, an increase in GP was observed at higher blend percentages, which may be due to a reduction in the temperature of the seeds affected by an increase in the bulk mass of seeds and HCP. The intensity (temperature rate in time) of heat transfer to the seed plays an important role in the quality of seed germination, and from this point of view, the glass cells with GLSS HCP were more suitable. In Figs. 3, 4, 5, 6 and 7, the control group consisted of seeds dried in the shade. As can be seen in the control experiment, germination was lower than that observed in the 25% glass bead experiment at 40 °C. The uniformity of canola seed germination is an important characteristic for variety evaluation. The coefficient of uniformity of germination (CUG) was significantly affected by the interaction of HCP percentage and temperature (Table 1). Mean comparisons showed that the mean values of CUG decreased as the percentage of HCP increased from 0 to 25% and decreased as it increased from 25 to 100% at 40 °C. However, at 50 and 60 °C, the CUG decreased compared to 40 °C (Fig. 7).The germination characteristics showed that an increase in the mixture percentage of glass HCP caused an increase in the values of GP, GR, CUG, GE, SLVI and RLVI. The highest values of GP (92), GR (3.29), CUG (6.84), GE (3.16), SLVI (3.24) and RLVI (5.01) were obtained when glass HCP particles were used, probably because the glass HCP in the drying bed reduced the temperature of the drying bed and thus increased the germination characteristics.

Fig. 3
figure 3

Mean comparison of germination percentage for different drying conditions

Full size image
Fig. 4
figure 4

Mean comparison of germination rate for different drying conditions

Full size image
Fig. 5
figure 5

(A) Mean comparison of root length vigor index for different drying conditions, (B) Mean comparison of shoot length vigor index of shoot for different drying conditions

Full size image
Fig. 6
figure 6

Mean comparison of germination energy for different drying conditions

Full size image
Fig. 7
figure 7

Mean comparison of coefficient of uniformity of germination for different drying conditions

Full size image

3.2.1 Effect of Drying Temperature on Germination Viability

Mean comparisons using Duncan’s test showed that the mean values of GP at 40, 50 and 60 °C were 81, 71.88 and 58, respectively (Fig. 9A). Therefore, an increase in temperature from 40 to 50 °C and from 50 to 60 °C caused a decrease in GP of 11.3% and 19.3%, respectively (Figs. 3 and 9A). Seeds at 40 °C germinated faster than any of the other temperature treatments during the first 48 h and then reached a germination peak (Fig. 9B, C and D). Seed germination at 60 °C was slower compared to the other temperatures (Table 2). The germination percentage of seeds at 40 °C reached 75% after 30 h, while at 50 °C it reached only 55% and 37% after 30 h (Fig. 9A). At temperatures higher than 40 °C, seed germination was lower and the slope decreased (Table 2). Accordingly, an increase in drying temperature caused a greater decrease in GP, probably because the degradation of seed vigor is more likely at higher temperatures. Our results suggest that the high moisture content of canola seeds should be removed at a low drying temperature (40 °C), because drying at high temperature can significantly reduce seed vigor.

Fig. 9
figure 8

(A) Effect of different drying temperature on germination percentage, T40: 40 temperature, T50: 50 temperature, T60: 60 temperature; (B) Effect of 40 temperature + heat carrier glass and steel particles on germination percentage, 40 g: 40 temperature + heat carrier glass particles, 40s: 40 temperature + heat carrier steel Particles; (C) Effect of 50 temperature + heat carrier glass and steel particles on germination percentage, 50 g: 50 temperature + heat carrier glass particles, 50s: 50 temperature + heat carrier steel particles; (D). Effect of 60 temperature + heat carrier glass and steel particles on germination percentage, 60f: 60 temperature + heat carrier glass particles, 60s: 60 temperature + heat carrier steel particles

Full size image
Table 2 Reductions of germination percentage canola seeds with increasing temperature
Full size table

3.2.2 Effect of HCP Type e on Germination Viability

The highest values of germination characteristics were obtained with glass HCP particles, while the lowest values were caused by the application of steel HCP particles.

3.2.3 Effect of HCP Percentage on Germination Viability

The percentage of HCP affected germination viability (Fig. 3). Thus, it can be predicted that for a given level of mixing ratio of HCP to seed, the amount of heat absorbed by the HCP is greater than the heat transferred to the drying seed. However, a negative effect on drying time was observed when HCP had a mixing percentage of 100%. Accordingly, as the percentage of HCP increased, the values of GP, GR, CUG, SLVI, RLVI, GE were effectively reduced. For example, the mean value of germination percentage became 71.33%, 71.16%, 69.5%, and 69.16% in response to HCP percentages of zero, 25%, 50%, and 100%, respectively.

Notably, HCPs have a lower heat capacity and a higher affinity for heat conductivity compared to biological materials such as canola seed. Thus, HCPs rapidly absorb convective heat from the hot air and transfer the absorbed heat to the seeds by conduction. The surfaces of inert materials such as sand, steel, or glass HCP quickly absorb and store heat, causing the seed mass to dry faster.

3.2.4 Interaction Effect of Temperature and Percentage of HCP on Germination Viability

The interaction of temperature and percentage of HCP affected the GP (Fig. 8A). Accordingly, lower temperatures (40 and 50 °C) had a weaker effect of HCP on GP. In fact, the average slopes of GP versus the temperatures of 40 and 50 °C were evaluated as -0.02 and − 0.037, respectively. However, at a higher temperature (60 °C), HCP reduced GP more, so that the average slope was − 0.1. Therefore, the use of HCP at high temperatures reduced GP and other germination traits. Similarly, GR, SLVI and RLVI, CUG and GE were significantly affected by the interaction of HCP percentage and temperature (Table 1). Comparison of the means showed that GR, SLVI, RLVI, CUG and GE increased in response to an increase in the percentage of HCP from 0 to 25% and then decreased with an increase in HCP from 25 to 100% at 40 °C. However, the effect of 50 °C was an exception and did not follow this trend. A similar result has been reported in the available literature (Hartmann-Filho et al., 2016). Comparison of the means showed that CUG usually increased with an increase in HCP percentage from 0 to 25%, but decreased with a further increase in HCP from 25 to 100% at 40 °C. However, at 50 and 60 °C, CUG increased with an increase in HCP from 0 to 100% (Fig. 7). Since the use of HCP at high temperatures adversely affects drying time (Fig. 2A) and GP (Fig. 8A), it is suggested that HCP be used at lower temperatures. Air temperatures of 50 and 60 °C negatively affected the physiological parameters of seed quality. Therefore, it is recommended that the drying temperature should not exceed 40 °C to avoid any damage to the quality of rapeseed.

Fig. 8
figure 9

Interaction effect of different parameters on GP, (a) Interaction effect of temperature and HCP percentage on GP of seeds, (b) Interaction effect of temperature and HCP type on GP of seeds, (c) Interaction effect of HCP type and percentage on GP of canola seeds

Full size image

3.2.5 Interaction Effect of Temperature and HCP Type on Germination Viability

The results of the germination tests showed that as the drying temperature increased, the GP decreased. Accordingly, the average slopes of the variations of GP versus temperature with respect to the two different types of glass and steel HCP were found to be -1.53 and − 0.78, respectively (Fig. 5B). Comparison of the mean values showed that the GR, SLVI, RLVI, GE and CUG decreased in value when the steel HCP was used (Figs. 4, 5, 6 and 7). Therefore, as the temperature increased, the effects of the steel and glass HCPs on the GR became increasingly similar. Thus, drying the canola seeds in a shorter time resulted in a higher GR of the dried seeds. It is recommended that glass HCP be used at a drying temperature of 50 °C.

3.2.6 Interaction Effect of Type and Percentage of HCP on Germination Characteristics of Canola Seeds

The interaction of type and percentage of HCP affected germination GP, GR, SLVI and RLVI (Figs. 3, 4 and 5). Accordingly, higher values of GP, GR, SLVI and RLVI were obtained when glass beads were used, and the amount of GP, GR, SLVI and RLVI decreased with the increase of steel HCP percentage, while the opposite trend was observed for glass HCP. GP values increased with the increase of glass HCP percentage, while this value decreased with the increase of steel HCP percentage (Fig. 3C).

However, an increase in the values of the germination traits was observed at the mixture percentage of 100%, which could be due to a reduction in the temperature of the seeds, as influenced by an increase in the bulk mass of the seeds as a function of the percentage of HCP.

4 Discussion

Air temperature affects the transfer of moisture from the interior to the surface and shortens the drying time. This is in good agreement with previous research on a similar topic (Pornpraipech et al. 2017; Puttalingappa et al. 2022). Macedo et al. (2020) found similar results after drying banana slices between 40 and 80 °C, as their moisture curves approached each other as the temperature increased to 80 °C (Macedo et al. 2020). Apparently, as the temperature increased, problems such as case hardening and internal resistance became more likely, slowing the mass transfer rate to a 15% reduction in value (Lewis and Trabelsi 2022). Due to their higher thermal conductivity compared to glass beads, steel beads heat the canola more quickly, accelerating drying. Thermal conductivity refers to the intrinsic ability of a material to transfer or conduct heat. Metals such as silver, copper, and gold have much higher thermal conductivities than ordinary glass, which typically has much lower thermal conductivities. The specific heat capacity of a substance is defined as the amount of heat energy required to raise the temperature of a unit mass of the substance by one degree Celsius or one Kelvin.The specific heat capacity is an important property in thermodynamics and is used in various applications, such as the design of heating and cooling systems. Metals generally have low specific heat capacities, resulting in rapid temperature changes when heat is applied. In contrast, glasses have higher specific heat capacities, meaning that it takes more energy to raise the temperature of a given mass of glass compared to the same mass of metal. The higher conductivity and lower heat capacity of steel compared to glass further improved the heat transfer process, as indicated by a lower requirement for steel HCP. Notably, the lower the temperature, the more important the effect of HCP. The results are consistent with a previous research case (Macedo et al. 2020). The presence or absence of HCP, their type and amounts (0 to 100% of sample weight) not only removed seed moisture, especially at the beginning of the drying process, but also improved the overall heat transfer through conductive heating and convection.When HCP was applied, heat transfer occurred as conduction and convection affected the drying time. When the amount of HCP was increased to 100% (i.e., a total of 400 g seeds per unit volume of HCP), but the air flow rate and temperature were kept constant, the drying time increased slightly, probably because it took longer for the entire bed to heat up before evaporation started and continued. Moradi et al. (2020) obtained similar results when drying green pepper cubes in a spouted bed in the presence of aluminum HCP (Moradi et al. 2020). However, the effect of HCP in reducing drying time became less significant as the drying temperature increased, probably because an increase in air temperature caused an increase in the temperature gradient between the inner and outer layers of the seeds, thus transferring sufficient thermal energy to the dried products to reduce the need for HCP. This confirms the previous results of a study on spouted bed drying of green pepper slices by aluminum HCP, in which the effect of HCP was reduced by increasing the temperature (Moradi et al. 2020). The interaction of temperature and HCP type had an interaction effect on drying time (Fig. 2B). As can be seen, the drying time of seeds decreased with increasing temperature. However, a greater decrease in drying time was observed in the experiments conducted with glass HCP. The average slope of decrease in drying time by steel and glass particles were − 2.92 and − 3.26, respectively, which is consistent with a previous research on the effect of glass and steel HCP on drying kinetics of green peas (Hatamipour and Mowla 2006). The interaction of HCP type and its mixture percentage had an effect on drying time. The curve of steel HCP was more downward than the curve of glass HCP, indicating a stronger effect of steel beads on drying time in a fluidized bed of seeds compared to the effect of glass beads. Nonlinear trends due to complex interactions between material properties, moisture content, and drying conditions such as shrinkage, stress-strain relationships, and changes in physical properties during drying can contribute to nonlinear behavior. Further research and analysis in the specific context of drying time vs. HCP would be required to fully understand and characterize these nonlinear trends. Physiological parameters of seed quality or seed vigor can determine the performance and activity of seed lots with acceptable germination in a variety of environments. The drying process had a significant effect on canola seed vigor. Drying temperature, HCP type and mixing percentage were considered to be the main factors influencing canola seed vigor by drying. In another research, the effect of drying temperature on germination of Pinus bungeana seeds was investigated using a temperature range of 15 to 30 °C. A lower germination rate was observed at higher temperatures (Guo et al. 2020). This may be due to the higher conductivity and lower specific heat of steel compared to glass, which allows the seeds to receive more heat in less time. Increasing the temperature in the seed drying stage resulted in seed deterioration with lower germination percentage and less uniformity (Nair et al. 2011). The results of the present study are consistent with previous findings on drying of green pea seeds (Ghalavand et al. 2011). An increase in the proportion of glass HCP in the mixture resulted in an increase in germination characteristics, which are particularly important for field management practices and final yield assessment (Khalequzzaman et al. 2023). Accordingly, an increase in drying temperature resulted in a greater decrease in GP, probably because the degradation of seed vigor is more likely at higher temperatures. A similar result was reported by Abbasi et al. (2012), indicating that drying soybean seeds at 45 and 55 °C directly affected the germination percentage and vigor of soybean seeds (Abbasi 2012). Our results suggest that the high moisture content of canola seeds should be removed at a low drying temperature (40 °C), because drying at high temperature can significantly reduce seed vigor. Kumar et al. (2010) reported similar results for canola seed drying (Kumar et al. 2010). The use of high temperatures in seed drying has been reported to cause oxidation of fatty acids and accumulation of toxic substances such as lipoxygenase (LOX) and malondialdehyde (MDA) in the seed, ultimately reducing seed vigor (Andreou and Feussner 2009). Meanwhile, increasing temperature causes lipid peroxide accumulation by LOX, which in turn damages the cell membrane and reduces seed vigor. Accordingly, temperatures of 50 and 60 °C negatively affected the physiological parameters of seed quality. Therefore, it is recommended that drying temperature should not exceed 40 °C to avoid damage to canola seed quality. Accordingly, higher values of germination characteristics were obtained in the experiments with glass beads. However, when steel HCP was used, an increase in HCP percentage caused a decrease in germination values (Fig. 3), possibly because the steel beads created a higher thermal conductivity on the seed surface. Thus, lower GP values were caused by steel HCP compared to glass HCP. The highest values of germination characteristics were obtained with glass HCP particles, while the lowest values were caused by the application of steel HCP particles. Accordingly, higher values of germination characteristics were obtained in the experiments with glass beads. However, when steel HCP was used, an increase in the percentage of HCP caused a decrease in germination values (Fig. 3), possibly because steel beads created a higher thermal conductivity on the seed surface. This is because high temperature in seed drying can cause seed aging by accelerating seed deterioration, reducing seed viability and vigor, and damaging seed properties such as membrane permeability and enzyme destruction. The effect of high temperature on seed drying is influenced by factors such as seed moisture content, drying time, and drying speed (Huang et al. 2021; Siddique and Wright 2003). There is an interaction between temperature and percentage of HCP: the germination characteristics increased the percentage of HCP from 0 to 25% and then decreased, a similar result was reported in the available literature (Hartmann Filho et al. 2016; Ghalavand et al. 2011; Das et al. 2020a, b, 2021). On the other hand, an increase in temperature led to the accumulation of lipid peroxides by LOX, which in turn damaged cell membranes and reduced seed vigor (Ahmed et al. 2016; Sharma et al. 2012). Air temperatures of 50 and 60 °C negatively affected the physiological parameters of seed quality. Therefore, it is recommended that drying temperature should not exceed 40 °C to avoid damage to canola seed quality.There is an interaction between temperature and HCP type.The effect of HCP type on germination characteristics is not similar because they are not the same in terms of heat transfer (Hartmann Filho et al. 2016).Glass and steel HCP caused less damage to seeds even at 50 °C. Kumar et al. (2010) reported similar results for canola seed drying (Kumar et al. 2010). Steel beads had a higher affinity for thermal conductivity on seed surfaces, resulting in a lower GP value compared to the use of glass HCP. This may be due to the higher conductivity rate and lower specific heat of steel compared to glass HCP, which allowed the seeds to receive more heat in less time. In experiments with steel HCP at 40 °C (best temperature), even germination traits decreased in value as the mixture ratio of HCP to seed increased.

5 Conclusion

A solar fluidized bed dryer containing glass and steel HCP was used to study the drying of fresh canola seed for storage and germination. Temperatures ranged from 40 to 60 °C, either with or without the presence of HCP in the drying chamber. A minimum drying time of 30 min was achieved when the inlet air was heated to 60 °C in the presence of 50% steel HCP per sample weight. The inlet air temperature affected germination, seed viability and seedling vigor. The experimental results indicated that an optimum temperature of 40 °C resulted in optimum germination characteristics of canola seeds in the presence of 25% glass HCP per sample weight. The use of glass HCP improved the drying process by providing adequate temperature distribution without overheating the seeds. Germination rate (GR), shoot length vigor index (SLVI), root length vigor index (RLVI), and germination energy (GE) were significantly affected by the amount of HCP and the temperature of the dryer. The germination parameters were highest when glass HCP was used.