Influence of the Duration of Microwave Irradiation of Scots Pine

Article Inﬂuence of the Duration of Microwave Irradiation of Scots Pine (Pinus sylvestris L.) Cones on the Quality of Harvested Seeds

Monika Aniszewska 1 , Arkadiusz Gendek 1 , Ewa Tulska 1, Paulina P˛eska 1 and Tadeusz Moskalik 2,*

1 Department of Biosystems Engineering, Institute of Mechanical Engineering, Warsaw University of Life Sciences—SGGW, Nowoursynowska 164, 02-787 Warsaw, Poland; [email protected] (M.A.); [email protected] (A.G.); [email protected] (E.T.); [email protected] (P.P.) 2 Department of Forest Utilization, Institute of Forest Sciences, Warsaw University of Life Sciences—SGGW, Nowoursynowska 159/34, 02-776 Warsaw, Poland * Correspondence: [email protected]; Tel.: +48-22-59-381-37

Received: 30 July 2019; Accepted: 2 December 2019; Published: 4 December 2019

Abstract: To improve the process of seed extraction, new solutions have been investigated in an attempt to develop guidelines for the construction of small seed extraction equipment. One of the solutions proposed in this field is the use of electromagnetic radiation in the first stage of hulling cones, reducing their initial moisture content, which will result in quicker scale opening. It is proposed that cones should be irradiated for a relatively short period in the first stage. This operation will allow a quicker loss of moisture from the cones that are still closed, which will result in a more intensive opening of cone scales and will also positively affect the exposure of seeds for the next phase of hulling. The aim of the study was to evaluate the effect of microwave irradiation of pine cones on the quality of the seeds obtained. Cones were exposed to microwaves produced by an 800 W generator. The research was performed in several modes, in which the variable parameters were the duration of microwave irradiation, arrangement of cones with the apex pointed towards either the inner or outer part of the turntable, and the number of cones. The temperature distribution on the surface of and inside the cones was determined using the THERM v2 (Vigo System SA, O˙zarów Mazowiecki, Poland) thermal image processing software. We also assessed the energy (vitality) and germinability (quality class) of seeds that were not exposed and those after microwave treatment. The results of the research allowed us to state that, with the assumed parameters of the process, it is possible to obtain second quality class seeds after exposure to microwaves for 5 s. This result was comparable to the quality of seeds obtained without the use of microwaves. When the irradiation time was increased above 5 s, the vitality of seeds decreased and their quality was not satisfactory.

Keywords: scots pine; cone; seeds; electromagnetic radiation; seed quality class; thermal imagery

1. Introduction Signiﬁcant progress has been made in recent years in the ﬁeld of seed science, which has contributed to the modernisation of existing technical infrastructure facilities or construction of new facilities used for seed collection, quality assessment, and storage. Due to the high processing capacity that these facilities require, the need for regular deliveries of large amounts of cones is high in order to reduce extraction costs. However, this is often not easy to achieve as the availability of cone crops depends on seed years [1]. The most widespread forest tree in Central and Eastern Europe is Scots pine (Pinus sylvestris L.), hence the seed material of this species is typically used for studies.

Forests 2019, 10, 1108; doi:10.3390/f10121108 www.mdpi.com/journal/forests Forests 2019, 10, 1108 2 of 15

In the past, various attempts were made to minimise the time of seed extraction. These included raising the temperature of extraction [2], lowering pressure in a drying chamber, mechanical cone trimming (e.g., by cutting off the base), sorting cones by size, or soaking cones in water during the stepwise extraction process [3]. Although numerous studies were conducted, no positive effects in terms of significant reduction in extraction time emerged. One of the reasons for this was the high sensitivity of seeds during drying to external factors, such as excessive temperature or inappropriate air humidity [1]. The observation of the cone hulling process and the review of the existing literature on the subject [2,4,5] resulted in the identification of a problem obtaining coniferous tree seeds in a manner that ensures their optimum quality. The heat and mass transfer in biological materials subjected to electromagnetic radiation (EMR) has been well documented. Consequently, microwave drying is widely considered one of the best methods for the intensification of these processes [6–8]. However, there are divergent opinions on the influence of EMR on biological materials, as it depends on numerous factors, including the moisture content, structure and composition of the material, as well as the frequency and length of the electromagnetic waves used [9–11]. It has been well documented that prolonged microwave irradiation of biological samples can cause structural damage to both tissues and cells [12–23]. The available studies have also investigated the effect of intermittent microwave drying on biophysical properties of various plants, such as rice [24], colza [25], or tree seeds and seedlings [26]. In the case of seeds, a rapid rise in temperature under the influence of microwaves results in the reduction or loss of their viability. The effects of microwave irradiation on Scots pine cones were discussed by Aniszewska and Słowiński[27], who determined the maximum exposure time for enabling the extraction of good quality seeds. Aniszewska also investigated an increase in the surface temperature of cones, depending on the microwave irradiation power and duration [1]. Microwave-assisted drying of canola, soybean, and corn was carried out by Hemis et al. [28], who concluded that microwaves could lead to cracks in seeds, reducing their quality. Based on the aforementioned studies, it cannot be categorically stated whether short-time exposure of biological materials, particularly cones, to radiation is harmful or not. Furthermore, new solutions have been recently investigated to improve the seed extraction process. Attempts have been made to draw guidelines for the construction of small extraction devices, which could increase the efficiency of seed collection. Such devices could be used to dry small quantities of cones using enhanced extraction technologies. One solution, closely related to this, is the use of microwaves in the initial phase of the one- or two-stage process of seed extraction from cones in order to reduce their initial moisture content. The decrease in cone moisture reduces the time taken for scales to lean from the axis. It is proposed that cones should be irradiated for a relatively short period in the first stage of the extraction process, which would result in the rapid loss of moisture from the still-closed cones. This would lead to weight loss and initiate the opening of scales, that is, their separation from one another. Such treatment improves the leaning of scales and exposure of seeds in the next phase of the first stage of cone hulling, which is conducted in an extraction chamber or kiln [1]. However, a precondition for the widespread use of microwaves is the investigation of their effects on seed viability. Hence, the ultimate power and duration of microwave irradiation needs to be carefully examined. Our study on potential extraction process improvements aimed to evaluate the effect of the duration of exposure to microwave radiation and cone temperature on the quality of Scots pine (Pinus sylvestris L.) seeds, collected in 2015 and held in cold storage for three years. For this purpose, the physical parameters of cones, such as weight, length, and thickness, were examined. Furthermore, (a) cone exposure to fixed power microwaves in a number of modes, differing in the duration of irradiation and the number of cones; (b) determination of the temperature distribution on the surface of and inside the cones; and (c) assessment of the quality of seeds, based on their germinability, were measured to highlight the effect of the duration of microwave radiation on the quality of harvested seeds. Forests 2019, 10, x FOR PEER REVIEW 3 of 15 Forests 2019, 10, 1108 3 of 15 2. Materials and Methods 2. Materials and Methods 2.1. Study Material and Physical Characteristics of Cones 2.1. Study Material and Physical Characteristics of Cones The material for the study was obtained from the Brzesko Forest District, State Forest Regional DirectorateThe material in Cracow. for the It was study collected was obtained from a frompine thestand Brzesko aged 119 Forest years District, (from Statean identified Forest Regional source: regionDirectorate of origin: in Cracow. 6th natural It was forest collected region, from comme a pinercial stand forest aged growing 119 years in(from a fresh an mixed identified deciduous source: forestregion site of origin: (49°58 6th′ N, natural20°33′ E). forest The region, cones commercialwere harvested forest in growing December in a 2015 fresh and mixed subsequently deciduous forest cold storedsite (49 in◦58 controlled0 N, 20◦33 0ambientE). The conesconditions: were harvested temperature in December of 5 °C and 2015 relative and subsequently humidity of cold 18% stored with ina maximumcontrolled deviation ambient conditions: of ± 2%. temperature of 5 ◦C and relative humidity of 18% with a maximum deviationA total of of2%. 225 cones, randomly selected from a wider batch, were investigated. The length, ± thicknessA total (the of largest 225 cones, diameter), randomly and selectedthe distan fromce between a wider the batch, maximum were investigated. thickness point The and length, the conethickness base (thewere largest measured diameter), using an and MIB the digital distance calliper between (MESSZEUGE, the maximum Spangenberg, thickness pointGermany) and thewith cone an accuracybase were of measured0.01 mm [29]. using The an initial MIB digitalweight calliperand the (MESSZEUGE,final weight after Spangenberg, drying were Germany)determined with using an aaccuracy WPS 210S of 0.01moisture mm [ 29balance]. The (RADWAG, initial weight Radom, and the Poland) final weight with after an accuracy drying were of 0.001 determined g. using a WPS 210S moisture balance (RADWAG, Radom, Poland) with an accuracy of 0.001 g. 2.2. Microwave Irradiation and Temperature Distribution 2.2. Microwave Irradiation and Temperature Distribution An R200WE laboratory microwave generator (SHARP, Osaka, Japan), with a maximum output powerAn of R200WE 800 W and laboratory supplied microwave with an alternating generator current (SHARP, of frequency Osaka, Japan), of 50 withHz and a maximum voltage of output230 V, waspower used of 800in the W study. and supplied The samples with an were alternating placed under current the of generator, frequency at of a 50 distance Hz and of voltage 160 mm, of 230 on V,a glasswas usedturntable in the with study. a diameter The samples of 255 were mm. placed under the generator, at a distance of 160 mm, on a glassThe turntable temperature with a diameterdistribution of 255 was mm. determined using pictures taken with a VIGOcam v50 thermalThe imaging temperature camera distribution (Vigo System was determined S.A., Ożarów using Mazowiecki, pictures taken Poland) with a VIGOcamwith an IR v50 (Infrared) thermal resolutionimaging camera of 384 × (Vigo 288 px System and thermal S.A., O˙zar sensitivityów Mazowiecki, < 70 mK. The Poland) temperature with an IRreading (Infrared) range resolution was −20 to of +120384 °C.288 The px objects and thermal whose sensitivity temperatures< 70 were mK. Theto be temperature measured were reading placed range on wasa black20 mat to + base120 ◦ atC. × − aThe distance objects of whose 500 mm temperatures from the lenses. were The to be camera measured settings were included placed onmeasurement a black mat parameters base at a distance (target distance:of 500 mm 500 from mm, the ambient lenses. temperat The cameraure: 22 settings °C, ambient included humidity: measurement 48%) and parameters emissivity (target in the distance:8–14 μm band500 mm, for the ambient surface temperature: temperature 22of 0◦C, °C, ambient correspon humidity:ding to the 48%) investigated and emissivity material in (wood): the 8–14 0.95.µm Theband THERM for the surfacev.2.29.3 temperature software (Vigo of 0 ◦SystemC, corresponding S.A., Ożarów to the Mazowiecki, investigated Poland) material was (wood): used 0.95.for thermalThe THERM image v.2.29.3 processing. software (Vigo System S.A., O˙zarów Mazowiecki, Poland) was used for thermal imageThermal processing. captions of the samples, before microwave irradiation (reference samples) and followingThermal irradiation captions (upon of the samples,completion before of microwavecone heating), irradiation were (referencetaken using samples) a thermal and followingimaging camera.irradiation (upon completion of cone heating), were taken using a thermal imaging camera. In order to determine the temperature distribution on the external surface, hot cones were photographedIn order to in determine whole (Figure the 1temperaturea). Next, todetermine distribution the on temperature the external inside, surface, cones hot were cones cut withwere photographedan Ulu knife along in whole the (Figure axis (Figure 1a). Next,1b) and to determine photographed the temperature again with ainside, thermal cones imaging were cut camera. with anThe Ulu cutting knife knife along was the mountedaxis (Figure in a1b) screw and pressphotographed holder, while again the with material a thermal being imaging cut was camera. placed onThe a cuttingprofiled knife base. was mounted in a screw press holder, while the material being cut was placed on a profiled base.

(a) (b) (c)

Figure 1. ScotsScots pine pine cone: cone: ( (aa)) in in whole; whole; ( (bb)) cut; cut; ( (cc)) cut cut with with marked marked halves. halves.

Forests 2019, 10, 1108 4 of 15 Forests 2019, 10, x FOR PEER REVIEW 4 of 15

Finally,Finally, to to identify identify the the cross-section cross-section spot withspot thewith highest the highest temperature, temperature, two surfaces two weresurfaces marked: were onemarked: on the one apex on side the andapex the side other and onthe the other base on side the ofbase the side cone of (Figure the cone1c). (Figure 1c). TheThe durationduration of the the entire entire study study procedure, procedure, from from turning turning on the on microwave the microwave generator generator to taking to takingpictures pictures with witha thermal a thermal imaging imaging camera, camera, was was very very short, short, just just 15–30 15–30 s, s, depending onon thethe measurementmeasurement mode. mode. FollowingFollowing microwave microwave irradiation, irradiation, cones cones were were stored stored for 21 for days 21 untildays spontaneousuntil spontaneous opening, opening, when seedswhen could seeds be could collected. be collected. 2.3. Measurement Modes 2.3. Measurement Modes Three primary measurement modes (W1, W2, and W3) were applied for various combinations of Three primary measurement modes (W1, W2, and W3) were applied for various combinations cone arrangement and irradiation time, resulting in a total of 15 combinations (Table1). of cone arrangement and irradiation time, resulting in a total of 15 combinations (Table 1). Table 1. Measurement modes. Table 1. Measurement modes. Number of Cones Under Duration of Microwave Mode Variant Cone Arrangement Number of Cones Under the Duration of Microwave Mode Variant Cone Arrangement the Generator (pcs.) Irradiation (s) Generator (pcs.) Irradiation (s) W1W1 - - - - 1 1 5, 5, 7, 7, 1010 W2W2 W2a W2a Apexes Apexes towards towards the centre the centre 3 3 5, 5, 10, 10, 1515 W2bW2b Apexes Apexes towards towards the edge the edge 3 3 5, 5, 10, 10,15 15 W3W3 W3a W3a Apexes Apexes towards towards the centre the centre 5 5 5, 5, 10, 10, 1515 W3bW3b Apexes Apexes towards towards the edge the edge 5 5 5, 5, 10, 10,15 15

InIn mode mode 1 1 (W1), (W1), a a single single cone cone was was placed placed at at the the centre centre of of the the turntable turntable (Figure (Figure2a). 2a). In In mode mode 2 2 (W2),(W2), three three cones cones were were placed placed on on the the turntable turntable with with apexes apexes pointing pointing either either towards towards the the centre centre (W2a, (W2a, FigureFigure2a) 2a) or or towards towards the the edge edge (W2b). (W2b). In In mode mode 3 3 (W3), (W3), ﬁve five cones cones were were placed placed on on the the turntable turntable with with apexesapexes pointing pointing either either centrewise centrewise (W3a) (W3a) or or outwards outwards (W3b, (W3b, Figure Figure2c). 2c).

(a) (b) (c)

FigureFigure 2. 2.Cone Cone arrangement arrangement on theon turntable:the turntable: (a) single (a) single cone (W1); cone (b(W1);) three (b cones) three with cones apexes with pointing apexes towardspointing the towards centre (W2a);the centre (c) ﬁve(W2a); cones (c) withfive cones apexes with pointing apexes towards pointing the towards edge (W3b). the edge (W3b).

ThreeThree di differentﬀerent times times of of cone cone exposure exposure to to microwave microwave irradiation irradiation werewere appliedapplied inin eacheach mode.mode. However,However, in in mode mode W1, W1, a a sample sample self-ignited self-ignited after after 15-second 15-second exposure exposure to to microwaves. microwaves. Therefore,Therefore, aa di differentﬀerent time time sequence sequence was was applied applied in in this this mode, mode, which which resulted resulted in in a a reduction reduction of of the the maximum maximum durationduration of of irradiation irradiation to to 10 10 s. s. TheThe experimental experimental procedure procedure was repeatedwas repeated 25 times. 25 Consequently,times. Consequently, measurements measurements were completed were forcompleted 25 cones for in mode 25 cones W1, in 75 mode cones W1, in mode 75 cones W2, in and mode 125 W2, cones and in 125 mode cones W3. in mode W3.

2.4. Seed Quality 2.4. Seed Quality After cone opening, seeds were manually extracted and dewinged. Then, the quality of seeds was After cone opening, seeds were manually extracted and dewinged. Then, the quality of seeds assessed by determining their germination energy and capacity in accordance with the standards [4] was assessed by determining their germination energy and capacity in accordance with the standards [4] PN-76/9211-02 [30] and PN-R-65700 [31]. A Jacobsen germinator (LABORSET, Łódź,

Forests 2019, 10, 1108 5 of 15

PN-76/9211-02 [30] and PN-R-65700 [31]. A Jacobsen germinator (LABORSET, Łód´z,Poland), featuring an electronic lighting and water temperature control system, was used for this purpose [32].

2.5. Statistical Analysis Statistical analysis of the physical characteristics of the studied cones was performed using Statistica 13 software (Dell Inc., Round Rock, USA). The distribution of each parameter was tested for normality using the Shapiro–Wilk test. The differences in mean values of the parameters were determined using ANOVA coupled with Tukey’s HSD (honest significant difference test) post hoc test for unequal sample sizes. The analysis was performed at the significance level α = 0.05.

3. Results

3.1. Physical Characteristics of Scots Pine Cones Statistics of the physical parameters of the cones used in the study are presented in Table2. 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.

Table 2. Statistics of the physical parameters of the Scots pine cones used in the study.

Distance Between the Statistics Length (mm) Thickness (mm) Maximum Thickness Point Initial Weight (g) and the Cone Base (mm) Mean ( SD) 43.92 4.2 19.87 1.64 11.12 1.6 6.789 1.68 ± ± ± ± ± Minimum 35.2 16.2 7.0 4.224 Maximum 57.6 25.7 18.7 12.973 Range 22.4 9.5 11.7 8.749 Coeﬃcient of 9.50 8.26 13.9 24.7 variation Standard deviation 0.28 0.11 0.10 0.11

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:

D = 0.2348H + 0.95586; r = 0.5983. (1)

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 Table3. Forests 2019, 10, 1108 6 of 15

Table 3. Loss of weight in the primary measurement modes (mean SD). ± Loss of Weight (g) Exposure Time 5 s 7 s 10 s 15 s Forests 2019, 10, x FOR PEER REVIEW 6 of 15 W1 0.014 0.008 0.021 0.018 0.103 0.049 - ± ± ± W2 0.006 0.003 - 0.037 0.018 0.097 0.037 Table 3. Loss of weight± in the primary measurement modes± (mean ± SD). ± W3 0.005 0.003 - 0.022 0.007 0.054 0.022 ± ± ± Loss of Weight (g) Exposure Time 5 s 7 s 10 s 15 s In the mode W1, theW1 loss of weight 0.014 ± following0.008 0.021 microwave ± 0.018 0.103 irradiation ± 0.049 averaged - 0.014 g (or 0.15% of the initial weight) afterW2 5 s, 0.0210.006 g (or ± 0.27%)0.003 after - 7 s, and 0.037 0.103 ± 0.018 g (or 0.097 1.10%) ± 0.037 after 10 s. The linear dependence between lossW3 of weight 0.005 in ± the 0.003 cones and - time of 0.022 exposure ± 0.007 to 0.054 microwaves ± 0.022 was described withIn Equation the mode (2) W1, and the is shown loss of in weight Figure following3. 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 y = 0.0183x 0.0849; r = 0.7732. (2) dependence between loss of weightW1 in the cones− and time of exposure to microwaves was described with Equation (2) and is shown in Figure 3. where yWi is a loss of weight (g), wi is a model/variant (Table1), x is time of exposure to microwaves (s).

0.20

0.15

0.10

Loss of weight, g 0.05

0.00 4567891011 Time of exposure to microwaves, s

Figure 3. Dependence between loss of weight and time of exposure to microwaves in the first Figuremeasurement 3. Dependence mode (W1). between loss of weight and time of exposure to microwaves in the first measurement mode (W1). 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. 𝑦 = 0.0183𝑥 − 0.0849; r = 0.7732 Statistical analysis showed no significant difference in the loss of weight. in cones between exposure(2) wheretimes of𝑦 5 and is 7a sloss (p = 0.95),of weight and these(g), 𝑤 data is cana model/variant be considered as(Table a uniform 1), x group.is time However, of exposure a higher to microwavesloss of weight (s). was observed for the 10-second exposure and was statistically significantly different fromThe the trend results line obtained indicates for that other an exposure increase periodsin the ti (mep < of0.01). cone heating from 5 to 10 s increased the loss ofThe weight loss of by weight 0.089 g. in the cones in the second mode (W2), that is, the simultaneous heating of three conesStatistical placed on analysis the turntable showed for 5,no 10, significant or 15 s, is presenteddifference inin Table the 3loss. Loss of ofweight weight in averaged cones between 0.006 g exposure(or 0.07% times of the of initial 5 and weight) 7 s (p after = 0.95), 5 s of and heating, these 0.037 data gcan (or be 0.50%) considered after 10 as seconds, a uniform and 0.097group. g However,(or 1.34%) aftera higher 15 s. loss of weight was observed for the 10-second exposure and was statistically significantlyThe relationship differentbetween from the loss results of weight obtained and for time other of exposure exposure in periods Scots pine (p < cones 0.01). to microwaves is shownThe in loss Figure of 4weight. For mode in the W2, cones it was in describedthe second with mode Equation (W2), that (3), withoutis, the simultaneous di fferentiating heating between of threethe cone cones arrangements placed on the submodes turntable ( yforW2 )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 yW = 0.0087x 0.0417; r = 0.8414. (3) (or 1.34%) after 15 s. 2 − 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 (𝑦).

Forests 2019, 10, x FOR PEER REVIEW 7 of 15

0.20 ForestsForests 20192019,, 1010,, x 1108 FOR PEER REVIEW 7 7of of 15 15 0.16 0.20 0.12 W2b W2 0.16 0.08

Loss of weight, g 0.12 W2b 0.04 W2 W2a 0.08

Loss of weight, g 0.00 4 6 8 10 12 14 16 0.04 Time of exposure to microwaves, sW2a

0.00 Figure 4. Dependence4 between 6 loss of weight 8 and time 10 of exposure 12 to microwaves 14 in the 16 second measurement mode (W2, W2a, W2b). Time of exposure to microwaves, s

Figure 4. Dependence between loss of weight and time of exposure to microwaves in the second 𝑦 = 0.0087𝑥 − 0.0417; r = 0.8414. (3) Figuremeasurement 4. Dependence mode (W2, between W2a, W2b).loss of weight and time of exposure to microwaves in the second Itmeasurement can be seen mode that the(W2, longer W2a, W2b). heating of cones contributed to increased loss of mass in a similar 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. 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 In the mode W2, ANOVA coupled𝑦 = with 0.0087𝑥 a post − hoc 0.0417; test indicated r = 0.8414 a. significant effect of the time(3) of of exposure of a cone to microwaves on its loss of weight (p < 0.01). The greater the time of exposure, exposure of a cone to microwaves on its loss of weight (p < 0.01). The greater the time of exposure, the higherIt can thebe seenweight that loss the of longer the cones. heating of cones contributed to increased loss of mass in a similar the higher the weight loss of the cones. way Finally,to single the cones loss exposed of weight to irradiation in the third in thestudy mode mode W1. (W3), that is, five cones placed on the Finally, the loss of weight in the third study mode (W3), that is, five cones placed on the turntable turntableIn the and mode exposed W2, ANOVA to microwaves coupled for with 5, 10, a postor 15 hoc s, is te presentedst indicated in aTable significant 3 and Figureeffect of 5. the Loss time of and exposed to microwaves for 5, 10, or 15 s, is presented in Table3 and Figure5. Loss of weight weightof exposure averaged of a cone0.005 to g microwaves(0.08% of initial on itsweight) loss of after weight heating (p < 0.01). for 5 s.The Upon greater exposure the time to microwavesof exposure, averaged 0.005 g (0.08% of initial weight) after heating for 5 s. Upon exposure to microwaves for forthe 10higher and the15 s,weight loss of loss weight of the increased cones. to 0.022 g (0.36% of initial weight) and 0.054 g (1.05% of 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 initialFinally, weight), the respectively. loss of weight in the third study mode (W3), that is, five cones placed on the turntableweight), respectively.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, 0.10loss of weight increased to 0.022 g (0.36% of initial weight) and 0.054 g (1.05% of initial weight), respectively.

0.08 0.10 W3 0.06 W3b 0.08

0.04 W3 Loss of weight, g 0.06 W3b

0.02 W3a 0.04 Loss of weight, g 0.00 0.02 4 6 8 10 12W3a 14 16 Time of exposure to microwaves, s

Figure 5. Dependence0.00 between loss of weight and time of exposure to microwaves in the third measurement mode4 (W3, W3a, 6 W3b). 8 10 12 14 16 Time of exposure to microwaves, s

Forests 2019, 10, 1108 8 of 15

The change in weight in the cones in mode W3 was described with Equation (4), which aggregates both cone arrangement submodes:

yW = 0.0047x 0.0206; r = 0.8361. (4) 3 − 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 Table4. 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 Figure4. 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.

Table 4. Loss of weight (g) following irradiation, by cone arrangement, in the second (W2a and W2b) and third (W3a and W3b) study modes.

Loss of Weight (g) Exposure Time 5 s 10 s 15 s W2a 0.006 0.003 0.034 0.015 0.078 0.025 ± ± ± W2b 0.006 0.002 0.040 0.021 0.127 0.034 ± ± ± W3a 0.005 0.002 0.022 0.006 0.050 0.025 ± ± ± W3b 0.006 0.003 0.021 0.008 0.060 0.017 ± ± ±

Loss of weight, depending on the duration of microwave irradiation for the cone arrangement submodes W2a (yW2a) and W2b (yW2b), was described with linear Equations (5) and (6), respectively:

yW a = 0.0011x 0.0559; r = 0.8749, (5) 2 − y = 0.0007x 0.0314; r = 0.8714. (6) W2b − A similar dependence of the loss of weight in cones was observed in the submodes W3a and W3b (Figure5). 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 (yW3a) and W3b (yW3b), was described with the linear Equations (7) and (8), respectively:

yW a = 0.0050x 0.0224; r = 0.8434, (7) 3 − y = 0.0045x 0.0193; r = 0.8117. (8) W3b − 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). Forests 2019, 10, 1108 9 of 15

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 Table5.

Table 5. Maximum temperatures of intact and cut cones by measurement modes.

Time of Exposure to Maximum Temperature Maximum Temperature Half No. Microwaves (s) of the Whole Cone (◦C) of the Cone Half (◦C) W1 5 62.52 10.24 1 53.25 15.24 ± ± 2 43.10 7.04 ± 7 88.29 14.49 1 60.58 14.37 ± ± 2 72.74 10 103.43 13.22 1 75.58 4.69 ± ± 2 77.67 W2a 5 63.07 11.76 1 49.15 6.36 ± ± 2 48.04 10 84.69 8.59 1 68.35 8.93 ± ± 2 63.71 9.03 ± 15 99.23 0.16 1 - ± 2 78.76 0.45 ± W2b 5 48.58 7.78 1 42.43 5.01 ± ± 2 - 10 79.35 10.87 1 66.5 4.48 ± ± 2 56.65 0.99 ± 15 95.19 1.45 1 75.85 1.42 ± ± 2 - W3a 5 52.53 6.29 1 43.73 4.42 ± ± 2 50.00 10 78.35 9.15 1 60.58 1.55 ± ± 2 - 15 85.14 3.59 1 66.56 3.63 ± ± 2 67.21 W3b 5 42.65 7.04 1 37.70 4.23 ± ± 2 - 10 70.26 6.02 1 60.47 1.81 ± ± 2 60.19 15 84.26 3.75 1 66.17 4.21 ± ± 2 -

In the study mode W1 (Table5), 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, Table5), there was no statistically significant di fference in the maximum temperature between Forests 2019, 10, 1108 10 of 15 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). Forests 2019, 10, x FOR PEER REVIEW 10 of 15 Furthermore, for exposure times of 5 and 10 s, there were no significant differences in temperatures dependingFurthermore, on the arrangement for exposure of conestimes on of the 5 turntableand 10 s, under there a microwavewere no significant generator (differencesp > 0.42), while, in fortemperatures 15-second exposure,depending cone on the arrangement arrangement had of cones a significant on the turntable effect on under the maximum a microwave temperature generator of cones(p > 0.42), (p = 0.03).while, for 15-second exposure, cone arrangement had a significant effect on the maximum temperatureIn mode W3,of cones without (p = 0.03). accounting for cone arrangement submodes, ANOVA coupled with a post hoc testIn mode indicated W3, without that the accounting maximum for temperatures cone arrangement of whole submodes, cones ANOVA did not coupled differ inwith a statisticallya post hoc significanttest indicated way that between the maximum microwave temperatures exposure times of whole of 10 and cones 15 sdid (p =not0.06), differ while, in a in statistically the cases of 5-secondsignificant exposure, way between the microwave maximum exposure temperature times wasof 10 theand lowest 15 s (p = and 0.06), di while,ffered in significantly the cases of 5-second from the maximumexposure, temperaturesthe maximum after temperature exposure was for 10the ( plowest< 0.01) and or 15differed s (p < significantly0.01). from the maximum temperaturesAnalysing after the dataexposure for thefor arrangement10 (p < 0.01) or of 15 cones s (p < 0.01). with apexes pointing towards the turntable centreAnalysing (W3a), there the was data no for statistically the arrangement significant of cones difference with apexes in the maximumpointing towards temperature the turntable between microwavecentre (W3a), exposure there was times no of statistically 10 and 15 ssignifican (p = 0.11),t difference while such in dithefferences maximum occurred temperature for other between pairs of exposuremicrowave times exposure (p < 0.01). times A of similar 10 and tendency15 s (p = 0.11), was while observed such fordifference the W3bs occurred submode, forin other which pairs cones of wereexposure arranged times with (p < apexes 0.01). A pointing similar towardstendency the was edge. observed Mean for cone the temperatures W3b submode, following in which exposure cones forwere 10 orarranged 15 s can with be consideredapexes pointing as a uniformtowards datathe edge. set ( pMean= 0.15), cone whereas temperatures differences following in mean exposure values occurredfor 10 or for 15 others can be pairs considered of exposure as a timesuniform (p < data0.01). set (p = 0.15), whereas differences in mean values occurredStatistical for other analysis pairs confirmedof exposure the times positive (p < 0.01). correlation between the temperature of cones and their arrangementStatistical analysis with apexes confirmed pointing the positive either outward correlation or centrewise between the for temperature the 5-second of exposure cones and time their arrangement with apexes pointing either outward or centrewise for the 5-second exposure (p = 0.03), while, in the case of microwave exposure of 10 or 15 s, cone arrangement had no significant time (p = 0.03), while, in the case of microwave exposure of 10 or 15 s, cone arrangement had no effect on temperature (p > 0.84). significant effect on temperature (p > 0.84). The presented thermal images of the studied materials (Figure6) indicate non-uniform temperature The presented thermal images of the studied materials (Figure 6) indicate non-uniform distribution for both the whole cone (Figure6a,c) and the cone cut in half with a guillotine cutter temperature distribution for both the whole cone (Figure 6a,c) and the cone cut in half with (Figure6b,d) in the mode W1. As far as the arrangement of the study objects is concerned, warmer a guillotine cutter (Figure 6b,d) in the mode W1. As far as the arrangement of the study objects colours were seen at the cone base, which implies that the temperature of the base was higher compared is concerned, warmer colours were seen at the cone base, which implies that the temperature of to that of the apex. the base was higher compared to that of the apex.

(a) (b)

FigureFigure 6. 6.Thermal Thermal imagesimages ofof conecone heatingheating in the study mode mode W1: W1: (a (a) )whole whole cone, cone, 7 7s;s; (b ()b cone) cone halves, halves, 77 s; s; (c ()c) whole whole cone, cone, 10 10 s; s; and and ((dd)) conecone halves,halves, 1010 s.s.

Forests 2019, 10, 1108 11 of 15

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 wereForests exposed 2019, 10, tox FOR microwaves PEER REVIEW for 10 or 15 s, the maximum temperatures were comparable, regardless11 of 15 of the cone orientation with respect to the turntable. For 15-second exposure, the temperature approached temperature approached 100 °C for whole cones and 80 °C for cone halves. These figures are 100 ◦C for whole cones and 80 ◦C for cone halves. These ﬁgures are comparable to those obtained by comparable to those obtained by Çelen [17] during the microwave drying of vegetables (87–155 °C). Çelen [17] during the microwave drying of vegetables (87–155 ◦C). Sample images of both intact and Sample images of both intact and cut cones, made with a thermal imaging camera, are shown in cut cones, made with a thermal imaging camera, are shown in Figure7. Figure 7.

(a) (b)

(e) (f)

FigureFigure 7. 7.Thermal Thermal imagesimages ofof conecone heating:heating: ( a)) whole cone cone with with the the apex apex pointing pointing outwards, outwards, heated heated forfor 5 5 s; s; ( b(b)) cone cone halves halves withwith thethe apexapex pointingpointing outwards, heated heated for for 5 5 s; s; (c (c) )whole whole cone cone with with the the apex apex pointingpointing centrewise, centrewise, heated for for 10 10 s; s; (d (d) )cone cone halves halves with with the the apex apex pointi pointingng centrewise, centrewise, heated heated for 10 for s; ( 10e) s; (ewhole) whole cone cone with with the the apex apex pointing pointing outwards, outwards, heated heated for for 15 15s; s;and and (f) ( fcone) cone halves halves with with the the apex apex pointingpointing outwards, outwards, heated heated forfor 1515 s.s.

BasedBased on on the the obtained obtained results, results, it was it concludedwas concluded that the that arrangement the arrangement of cones of under cones a microwaveunder a generatormicrowave with generator apexes pointingwith apexes towards pointing the towards centre of the the centre turntable of the results turntable in a faster results heating in a faster of the coneheating bases, of whilethe cone the oppositebases, while arrangement the opposite results arrangement in faster heating results of in the faster cone heating apexes. of the cone apexes.In mode W3, in the case of 5-second exposure, temperatures of both whole cones and cone halves were significantlyIn mode W3, higher in the for case the of cone 5-second arrangement exposure with, temperatures apexes pointing of both centrewise whole cones (W3a), and whereas, cone inhalves the case were of significantly microwave irradiationhigher for the for cone 10 or arra 15 s,ngement no significant with apexes differences pointing in centrewise cone temperatures (W3a), werewhereas, observed in the for case both of arrangements microwave irradiation (W3a, W3b). for In 10 the or majority15 s, no ofsignificant cases, the differences highest temperature in cone occurredtemperatures in the were apex observed portionof for both both whole arrangements cones and (W3a, cones W3b). cut in In half. 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.

Forests 2019, 10, 1108 12 of 15

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 Table6.

Table 6. Quality class of Scots pine seeds for the three measurement modes.

Microwave Germination Germination Seed Quality Mode Exposure Time (s) Energy (%) Capacity (%) Class W1 5 38 42 substandard 7 33 35 substandard 10 10 11 substandard W2 5 62 65 substandard 10 19 20 substandard 15 0 0 substandard W3 5 74 81 II 10 39 45 substandard 15 2 3 substandard

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.

4. Discussion The data presented here indicate that exposure to microwaves for more than 5 s caused seed damage. In seed extraction plants, pine cone hulling temperatures do not exceed 50 ◦C[2,5,38]. Subsequently, extracted seeds, depending on the species, are dried at temperatures of up to 45 ◦C, as a higher temperature results in reduced seed viability or even damage. In the current study on microwave irradiation, the highest temperatures were reached in cones placed individually under a microwave generator, while temperatures were lower when three or ﬁve cones were placed together on the turntable. Following 5-second exposure to microwaves, temperatures recorded on the cone surface exceeded 60 ◦C, implying that the temperatures inside the cone were over 40 ◦C. In the case of 15-second exposure, a temperature close to 100 ◦C was recorded on the cone surface, which corresponded to approximately 80 ◦C inside. The temperatures observed upon short exposure times were close to the surface temperatures of coﬀee beans (approx. 67–72 ◦C) reached by Dong et al. [39] by using a microwave power of 0.5–1.0 kW. Forests 2019, 10, 1108 13 of 15

The moisture content levels to which seeds can be safely dried are significantly influenced not only by ambient temperature but also by air humidity [40], which should not exceed 28% on average. According to studies by Zał˛eski and Ani´sko[5] and Ani´skoet al. [41], the viability of seeds in drying is reduced not only by high temperatures, but also by intensive tissue dehydration. Furthermore, the seeds of various tree species react differently to drying [8,40]. The minimum admissible moisture content for Scots pine seeds is 3.5%, and the same applies to black alder and common birch; the maximum drying temperature for Scots pine, as well as black alder and common birch seeds, is 45 ◦C[5]. Although the use of microwaves in drying biological materials has been the subject of numerous studies in recent years [42,43], the problem of uneven heating remains a barrier to industrial application. Hence, it is obvious that devices using EMR should be adapted to the drying process instead of adapting the process and materials to standard microwave systems. In order to mitigate the adverse effects of high temperatures, the optimum model for microwave power control during drying needs to be developed. Microwave power should be gradually reduced depending on the weight and number of cones in hulling, which, combined with the monitoring of the moisture content of the material, should result in the extraction of seeds with the desired properties after a relatively short time of drying. The relationship between seed quality and microwave power and between the weight of the material and exposure time need to be investigated by determining radiation flux (W/g) and unit irradiation (J/g).

5. Conclusions Loss of weight increased along with the time of the cones’ exposure to microwaves. Depending on the study mode, weight loss ranged from 0.005 g (W3) to 0.103 g (W1). Loss of weight was also related to the number of cones placed under a microwave generator. The greater the number of cones, the lower the weight loss and the higher the quality of the extracted seeds. Analysis of the maximum temperature of Scots pine cones, both intact and cut, revealed that the portions further away from the turntable centre became hotter. Furthermore, cone bases reached higher temperatures with cone apexes pointing centrewise, rather than outwards. Taking into consideration all the experimental conditions, cone exposure to microwaves for more than 5 s signiﬁcantly reduced the viability of seeds. For 15-second microwave irradiation, a rise in temperature resulted in damage to seeds and loss of germinability. The seeds extracted from Scots pine cones irradiated for 5 s in the mode W3 reached the quality class II, which was the highest among all studied cone batches and the same as that of their control counterparts.

Author Contributions: M.A., A.G., E.T., and P.P. conceived and designed the experiments; M.A., A.G., E.T., and P.P. performed the experiments; M.A., A.G., E.T., and T.M. analysed the data and wrote the paper. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Aniszewska, M. The change in weight and surface temperature of a pine cone (Pinus sylvestris L.) as a result of microwave irradiation. For. Res. Pap. 2016, 77, 56–67. [CrossRef] 2. Antosiewicz, Z.; Zał˛eski,A. Technika i technologia wyłuszczania nasion sosny, ´swierkai modrzewia w Polsce. Las Pol. 1987, 23, 7–11. (In Polish) 3. Aniszewska, M. Changes in humidity and temperature inside the pine cones (Pinus sylvestris L.) in two stages seed extraction. For. Res. Pap. 2013, 74, 205–214. [CrossRef] 4. Zał˛eski,A. Nasiennictwo Le´snychDrzew i Krzewów Iglastych [Seeds of Forest Trees and Coniferous Shrubs]; Oﬁcyna Edytorska Wydawnictwo Swiat:´ Warszawa, Poland, 1995; ISBN 978-83-85597-27-8. (In Polish) 5. Zał˛eski,A.; Ani´sko,E. Suszenie nasion wybranych gatunków drzew. Not. Nauk. Inst. Badaw. Le´snictwa 2003, 1, 1–4. (In Polish) Forests 2019, 10, 1108 14 of 15

6. Li, Z.Y.; Wang, R.F.; Kudra, T. Uniformity Issue in Microwave Drying. Dry. Technol. 2011, 29, 652–660. [CrossRef] 7. Alpert, P.; Oliver, M.J. Drying without dying. In Desiccation and Survival in Plants: Drying without Dying; Black, M., Pritchard, H.W., Eds.; CABI: Wallingford, UK, 2002; pp. 3–43. ISBN 978-0-85199-534-2. 8. Kermode, A.R.; Finch-Savage, B.E. Desiccation sensitivity in orthodox and recalcitrant seeds in relation to development. In Desiccation and Survival in Plants: Drying without Dying; Black, M., Pritchard, H.W., Eds.; CABI: Wallingford, UK, 2002; pp. 149–184. ISBN 978-0-85199-534-2. 9. Khraisheh, M.A.M.; McMinn, W.A.M.; Magee, T.R.A. Quality and structural changes in starchy foods during microwave and convective drying. Food Res. Int. 2004, 37, 497–503. [CrossRef] 10. Nowacka, M.; Sled´z,M.;´ Wiktor, A.; Witrowa-Rajchert, D. Physical and Chemical Properties of Microwave Dried Food Products. Zywno´sćNauka˙ Technol. Jako´sć 2012, 6, 5–20. [CrossRef] 11. Rz ˛aca,M.; Witrowa-Rajchert, D. Influence of convective-microwave drying parameters on radicalscavenging activity of dried apples. Zywno´sćNauka˙ Technol. Jako´sć 2007, 14, 222–230. 12. Jakubowski, T. Effect of seed potatoes microwave stimulation on the growth and yields of potato (Solanum tuberosum L.). Acta Agrophys. 2010, 16, 295–313. 13. Pietruszewski, S.; Kania, K. Microwave effect on the germination kinetics of seeds of white lupine and consumptive pea. Acta Agrophys. 2011, 18, 121–129. 14. Słowiński,K. The influence of microwave radiation emitted to non-disinfected nursery soil on the survivability and chosen biometric characteristics of Scots pine (Pinus sylvestris L.). Zesz. Nauk. Uniw. Rol. Kołł ˛ataja Krakowie 2013, 517, 122. 15. Friesen, A.P.; Conner, R.L.; Robinson, D.E.; Barton, W.R.; Gillard, C.L. Effect of microwave radiation on dry bean seed infected with Xanthomonas axonopodis pv. phaseoli with and without the use of chemical seed treatment. Crop Prot. 2014, 65, 77–85. [CrossRef] 16. Sahin, H. Effects of Microwaves on the Germination of Weed Seeds. J. Biosyst. Eng. 2014, 39, 304–309. [CrossRef] 17. Çelen, S. Effect of Microwave Drying on the Drying Characteristics, Color, Microstructure, and Thermal Properties of Trabzon Persimmon. Foods 2019, 8, 84. [CrossRef] 18. Ryszard, J.; Dolińska,R.; Błaszczak, W. Microscope analysis of two generations of wheat grain crops grown from microwave heated seeds. Acta Agrophys. 2007, 10, 727–737. 19. Tkalec, M.; Malarić,K.; Pavlica, M.; Pevalek-Kozlina, B.; Vidaković-Cifrek, Ž. Effects of radiofrequency electromagnetic fields on seed germination and root meristematic cells of Allium cepa L. Mutat. Res. Genet. Toxicol. Environ. Mutagen. 2009, 672, 76–81. [CrossRef] 20. Sharma, S.; Parihar, L. Effect of mobile phone radiation on nodule formation in the leguminous plants. Curr. World Environ. 2014, 9, 145–155. [CrossRef] 21. Stefi, A.L.; Margaritis, L.H.; Christodoulakis, N.S. The aftermath of long-term exposure to non-ionizing radiation on laboratory cultivated pine plants (Pinus halepensis M.). Flora 2017, 234, 173–186. [CrossRef] 22. Stefi, A.L.; Margaritis, L.H.; Christodoulakis, N.S. The effect of the non-ionizing radiation on exposed, laboratory cultivated maize (Zea mays L.) plants. Flora 2017, 233, 22–30. [CrossRef] 23. Vian, A.; Davies, E.; Gendraud, M.; Bonnet, P. Plant responses to high frequency electromagnetic fields. BioMed Res. Int. 2016, 2016, 1830262. [CrossRef] 24. Xu, F.; Chen, Z.; Huang, M.; Li, C.; Zhou, W. Effect of intermittent microwave drying on biophysical characteristics of rice. J. Food Process Eng. 2017, 40, e12590. [CrossRef] 25. Isaev, A.V.; Bastron, A.V.; Yakhontova, V.S. The research of unevenness degree influence of heating of colza seeds in emf uhf on their energy of germination and viability. Bull. KrasGAU 2016, 4, 131–137. 26. Mishra, R.; Dash, A.; Pattanaik, S.; Nayak, P.; Mohanty, R. Seed Germination and Seedling Survival Percentage of Shorea robusta Gaertn.f. in Buffer Areas of Similipal Biosphere Reserve, Odisha, India. J. Ecosyst. Ecograph. 2015, 4, 153. 27. Aniszewska, M.; Słowiński,K. Effects of microwave irradiation by means of a horn antenna in the process of seed extraction on Scots pine (Pinus sylvestris L.) cone moisture content and seed germination energy and capacity. Eur. J. For. Res. 2016, 135, 633–642. [CrossRef] 28. Hemis, M.; Choudhary, R.; Gariépy, Y.; Raghavan, V.G.S. Experiments and modelling of the microwave assisted convective drying of canola seeds. Biosyst. Eng. 2015, 139, 121–127. [CrossRef] Forests 2019, 10, 1108 15 of 15

29. Jasińska,A.K. Zmienno´sćgenetyczna Pinus sylvestris L. na terenie ostoi flor trzeciorz˛edowychw południowej Europie i południowo-zachodniej Azji. In Zastosowanie Metod Statystycznych w Badaniach Naukowych IV; Jakubowski, J., W ˛atroba, J., Eds.; Wydawnictwo StatSoft Polska: Kraków, Poland, 2012; pp. 449–458. (In Polish) 30. BN-76/9211-02—Le´snictwo; Wydawnictwa Normalizacyjne: Warsaw, Poland, 1976. (In Polish) 31. N-R-65700—Materiał siewny. Nasiona drzew i krzewów le´snychi zadrzewieniowych; Polish Committee for Standardization: Warsaw, Poland, 1998. (In Polish) 32. Banach, J.; Skrzyszewska, K.; Sobaszek, D. Assessment comparison of viability of Scots pine seeds executed by X-ray and germination methods. Acta Agrar. Silvestria Ser. Silvestris 2015, 53, 3–13. 33. Staszkiewicz, L. Staszkiewicz Investigation on Pinus sylvestris L. from South-estern Europe and from Caucasus and its relation to the pine from other territories of Europe based on morphological variability of cones. Flor. Geobot 1968, 14, 259–315. 34. Białobok, S.; Boratyński,A.; Bugała, W. Biologia Sosny Zwyczajnej [Biology of Scots Pine]; Polska Akademia Nauk Instytut Dendrologii: Poznań-Kórnik, Poland, 1993; ISBN 978-83-85599-21-0. (In Polish) 35. Aniszewska, M. The impact of selected morphological features of Scotch Pine cones on the progress of hulling process. In˙z.Rol. 2008, 1, 9–15. 36. Bruchwald, A. Statystyka Matematyczna Dla Le´sników; Wydawnictwo SGGW: Warszawa, Poland, 1989; ISBN 978-83-00-02187-1. (In Polish) 37. Aniszewska, M. Connection between shape of pine (Pinus sylvestris) cones and weight, colour and number of seeds, extracted from them. Electron. J. Pol. Agric. Univ. For. 2006, 9, 3. 38. Tyszkiewicz, S. Wyłuszczanie Nasion Le´snych; PaństwoweWydawnictwo Rolnicze i Le´sne:Warszawa, Poland, 1951. (In Polish) 39. Dong, W.; Cheng, K.; Hu, R.; Chu, Z.; Zhao, J.; Long, Y. Effect of Microwave Vacuum Drying on the Drying Characteristics, Color, Microstructure, and Antioxidant Activity of Green Coffee Beans. Molecules 2018, 23, 1146. [CrossRef] 40. Witowska, O.; Ani´sko,E.; Zał˛eski,A. The effect of drying seeds of European larch, Douglas-fir and mountain pine foreseen for a long-term storage on their viability. For. Res. Pap. 2009, 70, 253–269. [CrossRef] 41. Ani´sko,E.; Witowska, O.; Zał˛eski, A. Effect of drying conditions on viability of common birch, black alder, Scots pine and Norway spruce seeds. For. Res. Pap. 2006, 2, 91–113. 42. Sham, P.W.Y.; Scaman, C.H.; Durance, T.D. Texture of Vacuum Microwave Dehydrated Apple Chips as Affected by Calcium Pretreatment, Vacuum Level, and Apple Variety. J. Food Sci. 2001, 66, 1341–1347. [CrossRef] 43. Funebo, T.; Ahrné, L.; Kidman, S.; Langton, M.; Skjöldebrand, C. Microwave heat treatment of apple before air dehydration—Effects on physical properties and microstructure. J. Food Eng. 2000, 46, 173–182. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).