3.1. Physical Characteristics of Scots Pine Cones
Statistics of the physical parameters of the cones used in the study are presented in
Table 2. The physical characteristics of the study material were found to be within the typical range for Europe, where the length of Scots pine cones ranges from 19 to 70 mm, their thickness ranges from 12 to 35 mm [
33,
34], and their weight ranges from 5 to 18.4 g [
1,
35]. The mean moisture content was 34%. The individual parameters had a normal distribution, which was confirmed with the Shapiro–Wilk test.
The analysis demonstrated a positive correlation between the thickness (
D, mm) and length (
H, mm) of cones in the study batch. This was described with linear Equation (1), where the correlation coefficient exceeded the critical value r = 0.1430 (the latter generally depends on the sample size and equation degree) [
36]. The equation indicates that an increase of 1 mm in cone length corresponds to an increase of around 0.23 mm in cone thickness:
In the first and second primary measurement modes (W1 and W2), the initial weight of the cones did not differ in a statistically significant way for various times of exposure to microwave irradiation (p > 0.21). In the third mode (W3), no significant differences in weight were found between the samples exposed for 5 and 10 s, whereas the weight of the cones exposed for 15 s was the smallest and differed statistically (p < 0.01) from the weight of the cones for other exposure times; the difference in the average weight between these batches was 0.2–0.5 g.
3.2. Loss of Weight under the Influence of Microwaves
The loss of weight in the studied cones as a result of microwave irradiation in the primary measurement modes, without accounting for cone arrangement, is shown in
Table 3.
In the mode W1, the loss of weight following microwave irradiation averaged 0.014 g (or 0.15% of the initial weight) after 5 s, 0.021 g (or 0.27%) after 7 s, and 0.103 g (or 1.10%) after 10 s. The linear dependence between loss of weight in the cones and time of exposure to microwaves was described with Equation (2) and is shown in
Figure 3.
where
is a loss of weight (g),
is a model/variant (
Table 1),
x is time of exposure to microwaves (s).
The trend line indicates that an increase in the time of cone heating from 5 to 10 s increased the loss of weight by 0.089 g.
Statistical analysis showed no significant difference in the loss of weight in cones between exposure times of 5 and 7 s (p = 0.95), and these data can be considered as a uniform group. However, a higher loss of weight was observed for the 10-second exposure and was statistically significantly different from the results obtained for other exposure periods (p < 0.01).
The loss of weight in the cones in the second mode (W2), that is, the simultaneous heating of three cones placed on the turntable for 5, 10, or 15 s, is presented in
Table 3. Loss of weight averaged 0.006 g (or 0.07% of the initial weight) after 5 s of heating, 0.037 g (or 0.50%) after 10 seconds, and 0.097 g (or 1.34%) after 15 s.
The relationship between loss of weight and time of exposure in Scots pine cones to microwaves is shown in
Figure 4. For mode W2, it was described with Equation (3), without differentiating between the cone arrangements submodes (
.
It can be seen that the longer heating of cones contributed to increased loss of mass in a similar way to single cones exposed to irradiation in the mode W1.
In the mode W2, ANOVA coupled with a post hoc test indicated a significant effect of the time of exposure of a cone to microwaves on its loss of weight (p < 0.01). The greater the time of exposure, the higher the weight loss of the cones.
Finally, the loss of weight in the third study mode (W3), that is, five cones placed on the turntable and exposed to microwaves for 5, 10, or 15 s, is presented in
Table 3 and
Figure 5. Loss of weight averaged 0.005 g (0.08% of initial weight) after heating for 5 s. Upon exposure to microwaves for 10 and 15 s, loss of weight increased to 0.022 g (0.36% of initial weight) and 0.054 g (1.05% of initial weight), respectively.
The change in weight in the cones in mode W3 was described with Equation (4), which aggregates both cone arrangement submodes:
Similar to the W2 mode, a statistically significant difference occurred between the mean loss of weight in cones for various exposure times (p < 0.01). Loss of weight increased along with an increase in the time of exposure to microwaves.
Loss of weight, accounting for the cone arrangement on the turntable under a microwave generator, is presented in
Table 4. The loss of weight in the pine cones, depending on the time of their exposure and their arrangement under a microwave generator in submodes W2a and W2b, is shown in
Figure 4. The trend line for the cone arrangement W2b is higher than the trend line for the arrangement W2a, which indicates a higher loss of weight for cones with apexes pointing towards the turntable edge. ANOVA showed a relationship between loss of weight and the cone apex arrangement centrewise or outwards (
p < 0.01). A post hoc test further demonstrated that cone arrangement did not affect loss of weight for exposure times of 5 s (
p = 0.90) or 10 s (
p = 0.39), while it did affect the loss of weight (
p = 0.01) in the case of 15-second exposure.
Loss of weight, depending on the duration of microwave irradiation for the cone arrangement submodes W2a (
) and W2b (
), was described with linear Equations (5) and (6), respectively:
A similar dependence of the loss of weight in cones was observed in the submodes W3a and W3b (
Figure 5). As in the second study mode, a slightly higher weight loss, in both absolute and relative terms, occurred in cones with apexes pointing outwards. Cones with apexes pointing towards the turntable centre lost slightly less moisture. ANOVA coupled with a post hoc test demonstrated that the duration of microwave irradiation resulted in a significant loss of weight (
p < 0.01) for both cone arrangements (W3a and W3b), whereas cone arrangement had no effect on the loss of weight for individual exposure times (
p > 0.17).
Loss of weight, depending on the duration of microwave irradiation for the cone arrangement submodes W3a (
) and W3b (
), was described with the linear Equations (7) and (8), respectively:
The correlation between increased loss of weight and higher time of exposure to microwaves, which was revealed in all cases, confirms the results obtained by Çelen [
17], who carried out a similar study on persimmon drying (through also changing the microwave power).
3.3. Change in Cone Temperature under the Influence of Microwaves
The maximum temperatures of Scots pine cones, both intact (whole cones) and cut, with the indication of the relevant portion of the cone as recorded by a thermal imaging camera, are presented in
Table 5.
In the study mode W1 (
Table 5), statistical analysis indicated that the maximum temperatures of whole cones did not differ in a statistically significant way between the microwave exposure times of 7 and 10 s (
p = 0.16). However, in the experimental protocol of 5-second exposure, the maximum temperature was the lowest, and differed significantly from the maximum temperatures after exposure for 7 (
p = 0.01) or 10 s (
p < 0.01).
In mode W2, without accounting for cone arrangement submodes, statistical analysis demonstrated that the maximum temperatures of whole cones did not differ in a statistically significant way between microwave exposure times of 10 and 15 s (p = 0.08). However, in the case of 5-second exposure, the maximum temperature was the lowest and differed significantly from the maximum temperatures after exposure for 10 (p < 0.01) or 15 s (p < 0.01).
As far as the cone arrangement with apexes pointing towards the turntable centre is concerned (W2a,
Table 5), there was no statistically significant difference in the maximum temperature between microwave exposure times of 10 and 15 s (
p = 0.09), while such differences occurred for other pairs of exposure times (
p < 0.01). In the W2b submode, significant differences in the maximum temperature were observed between all exposure times (
p < 0.02).
Furthermore, for exposure times of 5 and 10 s, there were no significant differences in temperatures depending on the arrangement of cones on the turntable under a microwave generator (p > 0.42), while, for 15-second exposure, cone arrangement had a significant effect on the maximum temperature of cones (p = 0.03).
In mode W3, without accounting for cone arrangement submodes, ANOVA coupled with a post hoc test indicated that the maximum temperatures of whole cones did not differ in a statistically significant way between microwave exposure times of 10 and 15 s (p = 0.06), while, in the cases of 5-second exposure, the maximum temperature was the lowest and differed significantly from the maximum temperatures after exposure for 10 (p < 0.01) or 15 s (p < 0.01).
Analysing the data for the arrangement of cones with apexes pointing towards the turntable centre (W3a), there was no statistically significant difference in the maximum temperature between microwave exposure times of 10 and 15 s (p = 0.11), while such differences occurred for other pairs of exposure times (p < 0.01). A similar tendency was observed for the W3b submode, in which cones were arranged with apexes pointing towards the edge. Mean cone temperatures following exposure for 10 or 15 s can be considered as a uniform data set (p = 0.15), whereas differences in mean values occurred for other pairs of exposure times (p < 0.01).
Statistical analysis confirmed the positive correlation between the temperature of cones and their arrangement with apexes pointing either outward or centrewise for the 5-second exposure time (p = 0.03), while, in the case of microwave exposure of 10 or 15 s, cone arrangement had no significant effect on temperature (p > 0.84).
The presented thermal images of the studied materials (
Figure 6) indicate non-uniform temperature distribution for both the whole cone (
Figure 6a,c) and the cone cut in half with a guillotine cutter (
Figure 6b,d) in the mode W1. As far as the arrangement of the study objects is concerned, warmer colours were seen at the cone base, which implies that the temperature of the base was higher compared to that of the apex.
In mode W2, upon microwave irradiation for 5 s, the maximum temperature of whole cones and their halves was higher for the cone arrangement with apexes pointing centrewise (W2a). When cones were exposed to microwaves for 10 or 15 s, the maximum temperatures were comparable, regardless of the cone orientation with respect to the turntable. For 15-second exposure, the temperature approached 100 °C for whole cones and 80 °C for cone halves. These figures are comparable to those obtained by Çelen [
17] during the microwave drying of vegetables (87–155 °C). Sample images of both intact and cut cones, made with a thermal imaging camera, are shown in
Figure 7.
Based on the obtained results, it was concluded that the arrangement of cones under a microwave generator with apexes pointing towards the centre of the turntable results in a faster heating of the cone bases, while the opposite arrangement results in faster heating of the cone apexes.
In mode W3, in the case of 5-second exposure, temperatures of both whole cones and cone halves were significantly higher for the cone arrangement with apexes pointing centrewise (W3a), whereas, in the case of microwave irradiation for 10 or 15 s, no significant differences in cone temperatures were observed for both arrangements (W3a, W3b). In the majority of cases, the highest temperature occurred in the apex portion of both whole cones and cones cut in half.
Considering the number and distribution of seeds in cones [
37], it is more favourable for seed viability to place a greater number of cones under a microwave generator with apexes pointing towards the edge of the turntable, as there are less seeds at the apex than in the middle or at the base of cones.
3.4. Analysis of the Germination Energy and Capacity of Scots Pine Seeds
The results concerning the germination energy and capacity as well as quality of seeds for the three primary study modes are presented in
Table 6.
In modes W1 and W2, seeds exposed to microwave irradiation fell short of the lowest quality class as their germination energy was below the threshold of 50% and their germination capacity was below 70% for all three exposure times. Only the seeds irradiated for 5 s in the study mode W3 reached quality class II. Their germination energy was 74% and their germination capacity was 81%, thus falling within the range of 70% to 84% for germination energy and 81% to 90% for the germination capacity specified in the relevant standard. Increasing the duration of microwave irradiation to 10 or 15 s led to the reduced viability of seeds, which were classified as substandard.
In the control sample (not exposed to microwave radiation), seeds reached a germination energy of 72% and germination capacity of 83%, so they could be classified as quality class II.
For microwave exposure times of 5 and 10 s, in all study cases, there was a clear relationship between seed viability (germination energy and capacity) and the number (total weight) of cones placed together under a microwave generator. An increase in the number of cones placed under a microwave generator resulted in a higher germination energy and capacity of seeds.