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Wood-inhabiting fungi from green timbers represent a serious threat to the native forests of any timber importing country; thus, green timber should be pasteurized. Current pasteurization methods include the use of chemicals that will soon be banned; convective thermal treatment methods take time, use large amounts of radio frequency (RF) energy, and potentially cause surface damage to the wood. This preliminary study investigated the time and energy requirements to heat western hemlock (Tsuga heterophylla) to a pasteurization temperature using dielectric heating technology at radio frequencies an alternative method to chemicals and convective heating. Results revealed that certain temperature-time combinations were successful in pasteurizing the boards. RF heating until the wood shell reached 50[degrees]C and maintaining the temperature for 45 minutes, 56[degrees]C for 30 minutes, and 60[degrees]C for 15 minutes successfully pasteurized hemlock. Energy consumption during this process ranged from 45 to 180 kW/[m.sup.3]. Although the results were very positive, more work is needed with larger sample sizes and a wider range of green moisture content (MC) to capture the MC spectrum of western hemlock.
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Companies exporting wood products internationally face new challenges related to phytosanitary control regulations that attempt to prevent pests from being spread within or between countries by the transport of wood products or packaging. These pests include brown- or white-rot fungi and blue-staining fungi which are responsible for product deterioration and devaluation. Because many of these fungi are considered to be a phytosanitary risk and exports of products and packaging that may contain them are either restricted or banned, ensuring that such fungi are no longer viable in exported wood products is a priority. Traditional methods of sterilization, such as fumigation with methyl bromide or conventional heat treatment sterilization, can no longer meet the needs of the wood products industry. Methyl bromide is being phased out under the Montreal Protocol because it is an ozone-depleting chemical (Luken and Grof 2006). Heat treatment is not always economically feasible, especially for larger size materials. It can also introduce internal stresses that can damage the fibers of some wood species. Consequently, alternative phytosanitary treatments need to be evaluated.
Dielectric heating or radio frequency (RF) treatments have been used successfully in plant pathology, food processing, ceramics, paper, wood drying, and textile industries (Jones 1991; Avramidis 1999, 2004). RF acts through ionic conductivity to generate heat throughout the volume of a product, rather than being transferred to the interior from heated surfaces. The main advantage of this process is that the endogenous water in the board is directly heated within the treated board; therefore, in contrast to convective heating, the temperature increases at a similar rate in the core and in the shell of a product (Avramidis 1999). The intensity of dielectric heating is limited by the water amount and the macro- and microstructure of the product; for example, heating should be appropriate to diffusion rates within the product microstructure so that pressure from the water vapor does not rupture cell walls and cause internal checking (Jones 1991).
RF has been used to pasteurize various materials by heating in order to destroy microorganisms present in the material; however, specific information on heating rates for wood from hemlock is not yet available. Temperatures of 70[degrees] to 75[degrees]C and 85[degrees]C were required to kill the fungi Fusarium culmorum and Trichoconis padwickii in seeds (Cwiklinski and Horsten 1999, Janhang et al. 2005); however, high temperatures affected the rice seed viability. RF treatments between 60[degrees] and 70[degrees]C for 2 minutes killed 98 to 100 percent of the fungi in wood blocks of red oak (Quercus spp.), poplar (Populus alba), and southern pine (Pinus spp.) that were colonized with Gloeophyllum trabeum, Ganoderrna lucidum, Irpex lacteus, and Ceratocystisfimbriata (Tubajika et al. 2007). Fang et al. (2001) recovered no viable fungi when larger wood products (e.g., poles) of Douglas-fir (Pseudotsuga menziesii) and western redcedar (Thuja plicata) colonized by Postia placenta and Gloephyllum trabeum, respectively, were RF-treated for 2 hours with a core temperature of 65[degrees]C. The data from the literature indicate that the time required to pasteurize wood depends on treatment temperature and varies with the type of fungi and the concentration of extractives present in the wood (Jones 1973, Newbill and Morrell 1991). In the current work, the time and energy required to bring western hemlock lumber to different temperatures using RF heating were determined. Additionally, how efficiently different time-temperature combinations pasteurized two dimensions of green hemlock timber that had been inoculated with two types of basidiomycete rot fungi were evaluated.
Materials and methods
Two RF heating laboratory apparatus were used: an RFV-dryer operating at 6.8 MHz and an RF-oven operating at 40.7 MHz with corresponding wavelengths of 44.2 and 7.4 m, respectively (Fig. 1). Because the wood boards used in this study were much smaller (maximum length of 2 m) than the RF wavelengths, the RF energy was distributed uniformly throughout the wood and, therefore, frequency differences did not affect heating rates. In both cases, the upper electrode was connected to the generator (hot) while the lower one was grounded. Matching networks were used to ensure efficient transfer of energy into the wood. Transmission sections with forward and reverse reflectometers were used to monitor the RF energy transferred to the wood samples. Ultrahigh molecular weight polyethylene plates, 25 mm in thickness, were placed between the wood and the metal components of the heaters to prevent direct conduction of heat from the wood to the heater and for better field distribution and stabilization. Internal wood temperatures were measured using four fiber-optic temperature probes with a quick response time placed in the shell (2 to 3 mm from surface) and core (geometric center) locations at the middle and end (50 mm from the butt) of each board, respectively. Temperature data were transferred to a computer through a data acquisition system. The maximum wood load in the RFV-dryer is about 0.23 [m.sup.3] and 0.3 [m.sup.3] in the RF-oven.
[FIGURE 1 OMITTED]
Freshly cut green wood boards of western hemlock (Tsuga heterophylla) were obtained from a local sawmill. Of the 180 total boards (40 by 90 and 90 by 90 mm in cross section), half were slotted for the determination of heating rates and half were inoculated with two species of fungi: Fomitopsis pinicola (brown rot) and Phellinus pini (white rot) from the Breuil laboratory culture collection at the University of British Columbia, Faculty of Forestry. Two strains from each fungal species were used to inoculate the boards.
During phase 1, RF heating tests were conducted on single boards at 60 kW/[m.sup.3] for the 90 by 90s and 70 kW/[m.sup.3] for the 40 by 90s power densities. For the heating experiments, the following terminology is used:
"core"--the sensor is located inside the board 45 mm and 20 mm deep for the 90 by 90s and the 40 by 90s, respectively,
"shell"--the sensor is located close to the board surface at depths of 2 to 3 mm,
"middle"--defines the geometric center of the board, and "end" is located 50 mm from the board butt.
Experiments focused on determining the amount of time necessary for the most disadvantageously placed sensor (usually the shell-end sensor) to reach and maintain for 30 minutes a temperature of 56[degrees]C. The boards slotted for the heating rates were divided into four different categories according to heartwood/sapwood proportion that was visually evaluated and microscopically verified on some specimens: 100 percent heartwood with or without pith and over or under 50 percent sapwood. Other tests at the higher power densities of 120 kW/[m.sup.3] (90 by 90s) and 140 kW/[m.sup.3] (40 by 90s) were also conducted. In half of the cases, however, the required conditions could not be achieved without significant overheating of the board core and development of internal checking (honeycomb).
In phase 2, the boards were inoculated in a clean environment. For each board, 10 equidistant holes were drilled along the length and inoculated with one of the two fungal species. Then the holes were covered with clean dried wood dowels in order to prevent further contamination (Fig. 2). After inoculation, the boards were covered with plastic sheets that allowed air circulation and were incubated for 1 month in a warm environment of at least 20[degrees]C. Upon completion of incubation, each board was cut in half; one half was heated with a power density of 60 kW/[m.sup.3] and the other half with 90 kW/[m.sup.3]. Seven different combinations of time-temperature heating treatments were tested with three replicates having high, medium, and low moisture content (MC) (Table 1). Before RF heating, a control wood section was cut from each half to check for the presence of fungi. Following RF heating, four wood sections (cookies) per meter of approximately 25 to 30 mm in thickness (Fig. 3) were cut from the board areas that had been inoculated with one of the two fungal species. The top 10 to 20 mm of wood was removed to reduce airborne contamination of the samples, after which three samples from the upper left (UL), upper middle (UM), and upper right (UR) were plated onto 1 percent oxoid malt extract agar (OMEA) with 0.001 percent benomyl, an antibiotic that reduces growth of molds. Afterward, a second section from the cookie was removed and a fourth sample was taken from the middle (MI) area where decay fungi had been inoculated. The inoculated plates were left to incubate at 22[degrees]C for 14 days before the presence of fungi was evaluated.
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