The influence of shortterm microwave irradiation on the process of seed extraction from Scots pine cones was investigated in this study. Cones from the Brzesko Forest District (near Kraków, southern Poland) were collected in 2015. The size and changes in the mass of cones during the process of seed extraction, as well as the relations between these parameters, were analyzed. Changes in water content and drying rate of cones were modeled using suitable mathematical equations. The thermal balance of the whole process of seeds extraction was calculated, and the heat necessary to extract the seeds was determined. Cones were initially exposed to 2.45 GHz microwaves with a power of 800 W for either 5 or 15 seconds. Subsequently, cones were placed in a circulating air oven and seeds extracted under convective drying at a constant temperature of 50 ± 0.1 °C. We found that seeds from cones subjected to 15seconds microwave radiation were dramatically damaged, while no significant difference were found in germination and quality of seeds between cones subjected to the 5seconds microwave pretreatment and control cones, both yielding firstclass quality seeds (mean germination capacity > 90%). The results of this study could help developing microwavedryer control algorithms to support the automated process of cone seed extraction in largecapacity extraction plants.
The literature on seed extraction from conifer cones reports processes conducted under laboratory and/or industrial conditions, both in extraction chambers and cabinets. All stages of the process of production of forest reproductive material, from cones collection to nursery cultivation and selection, may have an effect on its genetic diversity  mainly by directional selection  that should be carefully considered (
The effects of microwave irradiation on
The objective of the paper was to elucidate changes in the moisture content of pine cones briefly exposed to microwave irradiation prior to conventional seed extraction, taking into account the effects of cone size, initial moisture content, and drying temperature. Additional goals were to develop a mathematical model describing the drying process and assess the viability of the obtained seeds. The eventual increase of the effectiveness of the process of seed extraction from cones will support research efforts aimed at increasing the production of pine cones (
Scots pine (
Prior to seed extraction, the length (
The pine cones were divided into three lots, each containing 40 specimens (a total of 120). The control lot (“lot C”) was subjected to conventional seed extraction, while the other two lots were initially exposed to 2.45 GHz microwaves with a power of 800 W in a Sharp R200^{®} laboratory microwave oven (Vestel Poland Sp. z o.o., Warsaw, Poland). The exposure time was either 5 seconds (“lot 5 s”) or 15 seconds (“lot 15 s”).
Temperature distribution within the microwave oven was determined in order to appropriately position the cones. A container measuring 12 × 8 cm divided into 24 cells (6 × 4) was placed in the oven, with 10 mL of demineralized water at 21 °C in each cell. The container with water was exposed to the maximum microwave power for 60 and 120 s, and then photographed with a VIGOcam V50^{®} thermographic camera (VIGO System S.A., Ozarów Mazowiecki, Poland) from a distance of 50 cm. The adopted emissivity coefficient for water was 0.97. Images were acquired at a resolution of 384 × 288 pixels and processed using Therm v.2.29.3 software (VIGO System). Temperature was recorded with an accuracy of 0.1 °C. The obtained temperature distribution is shown in
After determining temperature distribution in the oven, cones were individually irradiated. Each cone was placed horizontally in the middle of a rotating glass plate. Before and 10 min after microwave treatment each cone was weighed on a WPS210 S^{®} laboratory balance (Radwag, Radom, Poland) to record their initial (
During seed extraction, cones were placed on the grill inside the oven. The decrease in weight was recorded every 60 ± 1 min over 10 h.
The study sought to describe the seed extraction process by mathematical equations, including the effects of initial microwave irradiation. Two equations were developed: one for the time interval of 00.16(6) h and another one for 0.16(6)10 h. The former equation represents a linear function corresponding to the period required for the microwave treatment (5 s or 15 s) of individual cones comprising a given experimental lot, so that the entire lot could be then simultaneously placed in the convective (circulating air) oven. The latter equation is an exponential function corresponding to the time of seed extraction in the convective oven.
Given that the moisture content of the studied pine cones was much lower than that of so far researched plants or plant products, changes in cone moisture content (
where
The coefficient
where
The drying rate (loss of cone moisture over time) was calculated as the derivative of moisture content duration
The pine cones were weighed using a WPS210S^{®} laboratory scale (Radwag, Radom, Poland) with an accuracy of 0.001 g. The dry weight of cones was determined after seed extraction by drying at 105 ± 1 °C until constant weight. After drying, the number of scales on the cone and the number of obtained seeds were counted. The seeds were cleaned and dewinged. The overall weight of seeds and wings from a given cone was measured with an accuracy of 0.001 g.
The heat of extraction
where
The amount of water that evaporated from the cone (
where
In calculating seed extraction heat, the heat needed to heat up the oven and air was not taken into account. Thus,
To determine whether the adopted seed extraction conditions result in viable seeds, their germination energy and capacity were assessed (
The external parameters of the cones were statistically evaluated using the software Statistica ver. 13 (
The mean lengthtodiameter ratio was 2.13, which places the cones in the longandthick category (
A significant linear relationship (R=0.630) was found between cone diameter (
and between initial weight (
Thus, the longer the cone, the larger its diameter and initial weight. The mean initial moisture content of the cones (
The total seed extraction time for Scots pine cones lasted 10 h, which was sufficient for all cones to open and release seeds. Moisture content for the studied lots is given in
In the case of control cones, moisture content
Moisture content
After 10 h of seed extraction, the lowest final moisture content was found for the cones pretreated with microwaves for 15 s (0.050 kg_{H2O} kg_{dw}^{1}, or 5.0%). In turn, the lowest
for τ∈(0.16(6) 10>.
The extraction process following 15 s exposure to microwaves is given by
for τ∈(0.16(6) 10>.
for 5s u_{p} = 0.233 kg H_{2}O kg_{dw}^{1}, and by
for 15s u_{p} = 0.232 kg H_{2}O kg_{dw}^{1}.
and
The obtained thermal balance shows that the amount of heat supplied to the control cones dried in a convective oven ranged from 257.46 J to 642.98 J, with the heat needed to evaporate water being 1669.374428.89 J (
Descriptive statistics for the weight and number of seeds extracted from pine cones are reported in
The results of the seed quality evaluation are presented in
Analysis of variance revealed no statistical differences in the germination energy (
The process of cones drying using microwave irradiation, which is part of the seed extraction process, was carried out in a typical manner. The decrease in humidity was similar to that observed by
The microwave power applied in the present study led to strong seed damage after 15 s, therefore a shorter exposure time is desirable. Further, a 5 s microwave treatment resulted in first class cones. The increased seed extraction heat in cones exposed to microwaves was associated with the fact that their temperature surged in the first extraction step.
In this study, the microwave treatments affected seed extraction, lowered initial moisture content, and modified the drying curve, as proven by the comparison of
The process of seed extraction from Scots pine cones subjected to initial microwave irradiation may be described using a linear equation for moisture content changes induced by microwave heating, and using an exponential equation for those induced by convective heating. In the latter case, the characteristic values are the initial moisture content
The proposed mathematical description of seed extraction process with a preliminary microwave treatment followed by convective drying may be used to determine the course of cone opening and seed release.
The irradiation of cones with microwaves for 15 s decreased their moisture content by an average of 7%. Due to the need to maintain seed viability, convection drying with temperature control should be used in the further drying process. The reduction of moisture content obtained through microwave irradiation reduces the next convection drying stage by about 2 hours at 35 °C or by about 1 hour at 50 °C. On average, the initial reduction of cones moisture content by 0.5% results in a reduction of convection drying time at 35 °C by 8 minutes.
The application of microwave irradiation in the process of seed extraction from Scots pine cones may be beneficial, but requires further study involving cone exposure to lower microwave powers (less than the 800 W setting used in the present experiments) and different microwave frequencies.
The application of microwaves with a power of 800 W before seed extraction from Scots pine cones with a mean moisture content of 23% decreased cone moisture and caused some scales to separate.
Individual pine cones exposed to a microwave power of 800 W for 5 s yielded firstclass seeds. However, our results suggest that further moisture decreases could be attained by extending the treatment duration at a lower power. At the studied microwave power, however, cone exposure must be shorter than 15 s to reduce the risk of seed damage. This finding should be confirmed in further laboratory studies.
The proposed mathematical descriptions of the process of cone drying using microwave radiation can be used (after further validation) in microwave dryer control algorithms.
The process involving a 5 s microwave pretreatment followed by convective drying accelerated seed extraction from pine cones at low relative humidity as compared to convective drying alone. However, since the combined process required a greater energy input, a cost calculation should be performed to estimate its economic feasibility.
This research did not receive any specific grant from funding agencies in the public, commercial, or noprofit sectors.
Temperature distribution in a container filled with water exposed to microwaves in the oven after 1 min. (a): thermograph; (b): 3Dgraph.
Changes in moisture content (a) and drying rates (b) for cones subjected to seed extraction with 5 s and 15 s microwave pretreatment. (a): Moisture content changes described by linear (L) and exponential (E) functions; (b): drying rate (S: seed extraction).
Cone size and weight statistics for the three investigated lots. Two lots were initially exposed to 2.45 GHz microwaves, the exposure time was either 5 s (“lot 5 s”) or 15 s (“lot 15 s”). The control lot (“lot C”) was subjected only to conventional seed extraction. (
Lot  Param  Mean  Min  Max  Range  Variance  SD  CV  SE 

5 s 

45.84  35.70  52.50  16.80  25.64  5.06  11.05  0.90 

21.55  17.00  24.60  7.60  3.56  1.89  8.75  0.33  
2.13  1.69  2.55  0.86  0.04  0.19  8.98  0.03  

7.96  4.79  11.32  6.53  3.36  1.83  23.01  0.32  
15 s 

45.83  37.80  56.80  19.00  18.66  4.32  9.43  0.76 

21.28  18.40  25.00  6.60  3.29  1.81  8.52  0.32  
2.16  1.89  2.62  0.72  0.03  0.16  7.55  0.03  

7.97  5.30  13.15  7.85  3.83  1.96  24.55  0.35  
C 

45.24  37.60  56.10  18.50  19.92  4.46  9.87  0.80 

21.67  17.80  26.10  8.30  4.12  2.03  9.36  0.36  
2.09  1.73  2.50  0.77  0.03  0.18  8.61  0.03  

8.01  5.10  12.68  7.58  3.90  1.98  24.11  0.35 
Parameters of the mathematical equation describing seed extraction from Scots pine cones. (
Lot  Param  Mean  Min  Max  Range  SD  CV  SE 

5 s 

0.232  0.205  0.269  0.064  0.015  6.425  0.003 

0.227  0.199  0.263  0.064  0.015  6.575  0.003  

0.057  0.029  0.087  0.058  0.009  16.285  0.002  
b  0.31  0.14  0.44  0.30  0.07  21.57  0.01  
15 s 

0.227  0.189  0.264  0.075  0.015  6.688  0.003 

0.157  0.120  0.206  0.086  0.018  11.731  0.003  

0.050  0.020  0.076  0.056  0.011  21.868  0.002  
b  0.39  0.29  0.47  0.18  0.04  11.14  0.01  
C  0.233  0.205  0.267  0.062  0.013  5.587  0.002  

0.058  0.046  0.077  0.031  0.008  13.740  0.001  
b  0.32  0.21  0.40  0.24  0.07  21.97  0.01 
Results (pvalues) of the Duncan test for differences in cone moisture content after microwave irradiation (
Lot  Param  5 s  15 s  C 

5 s 

  0.0001*  0.1114^{ns} 

  0.0043*  0.4021^{ns}  

  0.0001*  0.6098^{ns}  
15 s 

0.0001*    0.0001* 

0.0043*    0.0300*  

0.0001*    0.0001*  
C 

0.1114^{ns}  0.0001*   

0.4021^{ns}  0.0300*    

0.6098^{ns}  0.0001*   
Seed extraction heat (expressed in J) for control cones (C) and cones exposed to microwave irradiation for 5 s (“5”) and 15 s (“15”). (
Variable  Mean  Min  Max  Range  Variance  SD  CV  SE 

Q_{1} C  418.09  257.46  642.98  385.52  9992.8  99.96  23.91  17.95 
Q_{2} C  2780.10  1669.37  4428.89  2759.52  458593.5  677.20  24.36  121.63 
Q C  3198.20  1926.83  5070.73  3143.90  600941.1  775.20  24.24  139.23 
Q_{1} 5  457.71  271.97  640.25  368.28  11311.0  106.35  23.24  18.80 
Q_{2} 5 M  84.76  23.55  155.40  131.85  1121.0  33.49  39.50  5.92 
Q 5 M  542.47  295.51  753.62  458.11  15386.0  124.04  22.87  21.93 
Q_{1} 5 PM  407.86  243.64  571.02  327.38  9033.0  95.04  23.30  16.80 
Q_{2} 5 PM  2566.11  1624.63  3830.84  2206.20  311131.0  557.79  21.74  98.60 
Q 5 PM  2973.97  1868.27  4382.83  2514.56  416670.0  645.50  21.71  114.11 
Q 5 MPM  3516.44  2163.79  5068.08  2904.29  580192.0  761.70  21.66  134.65 
Q_{1} 15 M  1127.92  740.51  1849.79  1109.28  80562.0  283.83  25.16  50.18 
Q_{2} 15 M  1052.57  541.54  1980.17  1438.62  88940.0  298.23  28.33  52.72 
Q 15 M  2180.49  1420.09  3829.96  2409.87  299574.0  547.33  25.10  96.76 
Q_{1} 15 PM  355.15  232.22  570.85  338.64  8376.0  91.52  25.77  16.18 
Q_{2} 15 PM  1641.72  894.73  2670.05  1775.32  196848.0  443.68  27.02  78.43 
Q 15 PM  1996.87  1151.80  3193.10  2041.31  281204.0  530.29  26.56  93.74 
Q 15 MPM  4177.36  3007.05  6750.64  3743.59  1024785.0  1012.32  24.23  178.95 
Seed weight (SW, g) and number of seeds per cone (N, pcs.) obtained from the three studied lots of pine cones. (SD): standard deviation; (CV): coefficient of variation; (SE): standard error of the mean.
Lot  Param  Mean  Min  Max  Range  Variation  SD  CV  SE 

5 s  SW  0.172  0.067  0.320  0.253  0.005  0.072  41.731  0.013 
N  23  10  44  34  77  9  38  1.6  
15 s  SW  0.156  0.050  0.342  0.292  0.004  0.064  41.271  0.011 
N  20  4  37  30  54  7  36  1.3  
C  SW  0.201  0.075  0.421  0.346  0.007  0.085  38.581  0.015 
N  27  10  48  38  73  9  31  1.5 
Assessment of germination quality for seeds from the three studied lots. (Ge_{14d}): germination energy at 14 days (%); (Gc_{28d}): germination capacity at 28 days (%); (Mean Gc): Mean germination capacity (%).
Lot  Sampleno.  Ge_{14d}(%)  Gc_{28d}(%)  Quality class  Mean Gc (%)  Mean quality class 

5 s  1  86  90  II  93  I 
2  92  97  I  
3  86  90  II  
4  92  96  I  
15 s  1  0  0    0   
2  0  0    
3  0  0    
4  0  0    
Control  1  91  93  I  94  I 
2  96  99  I  
3  90  91  I  
4  91  92  I 