Application of radio-frequency heating to utility poles. part 3. The use of RF heating to eradicate decay fungi in pole material. (Solid Wood Products).
STAVROS AVRAMIDIS (*)
An evaluation of radio-frequency (RF) heating to eliminate decay fungi in roundwood sections was conducted. Shavings colonized by the decay fungi (Gleoeophyllum trabeum (47D) and Postia placenta (120F and 31094B)) were placed in Douglas-fir and western red cedar pole sections at various depths from the pole surface. The pole sections were then subjected to RF heating, after which the shavings were recovered and the viability of the fungi determined. All decay fungi were killed after 2 hours of heating above 65[degrees]C. This confirmed the potential of the RF heating process to destroy decay fungi present in poles during processing.
For over 50 years, the main preservative treatments for utility poles have been creosote and pentachlorophenol. Both of these treatments have traditionally been applied at temperatures of between 800 and 105[degrees]C using pressure cycles of 8 hours or more. In addition, Boultonising is also often used to reduce the moisture content (MC) of the poles before preservative treatment. A benefit of these processes is that the extended heating of the poles sterilized them. Sterilization is important, since often poles are left for extended periods to air-season after debarking. For many species, particularly those with a thick sapwood, colonization by fungi is relatively rapid (13,16,18,19,21,22,24). During the past decade, waterborne preservatives such as chromated copper arsenate (CCA) have increasingly been used for protection of utility poles. Since these treatments are carried out at ambient temperatures, the sterilizing effect of the hot oil treatment is lost.
Prevention of the internal decay in utility poles is an important aspect of the increased attention being given to the customers' interests and the protection of the environment (10,14). Where pretreatment infection is not eradicated, inservice pole failures have been recorded (17). To assist in preventing such problems, the pole industry has implemented remedial treatments using either fumigants (15) or diffusible preservatives, such as fused boron rods (2).
Simple heat treatment is effective for killing organisms present in wood (3, 10), and eliminating fungi that colonize poles during seasoning could reduce the need for chemicals during the early pole life. There is a substantial body of data supporting the use of conventional kiln-drying to eliminate fungi, insects, and nematodes inside wood (5,6,11,23). Alternative heating techniques, such as radio-frequency/vacuum (RF/V) heating, have been used to eradicate the pinewood nematode (7,12). The effectiveness of this approach appears to be dependent on temperature and independent of the vacuum (7). Pohleven et al. (20) demonstrated that RF heating of small samples (50 by 25 by 15 mm) with an RF generator with a power of 6.00 kW at 4.75 MHz eliminated decay fungi in about 25 minutes at a temperature of 70[degrees]C.
This paper examines the potential of RF heating to sterilize 2-m-long pole sections, using a short heating period, at a minimum temperature of 65[degrees]C. The heating temperature and period chosen were based on previous research on what is required to eradicate decay fungi in wood (3) and to fix CCA in pole sections using RF heating (9).
MATERIALS AND METHODOLOGY FUNGI
Two common wood-destroying fungi found in North American softwood products, Postia placenta (UBC 31094B and Madison 120F) and Gloeophyllum trabeum (Madison 47D), were selected for the study. Two of the strains were from a well-established source, while the third strain (from decaying wood in the Vancouver area) was identified by the National Identification Service, Agriculture Canada, Ottawa. The fungi were grown on 25 percent malt agar for 2 weeks at 25[degrees]C until they almost covered the plates.
PREPARATION OF WOOD SAMPLES
The design of the experiment was based on placing shavings inoculated with selected decay fungi into pole sections at specified depths from the surface. To position the shavings at the correct depth, holes were pre-drilled to the required depth and after addition of the shavings the hole was sealed with a pre-cut dowel. The length of the dowel confined the shavings to the chosen location.
Approximately 500 g (1000 mL) of pine sapwood shavings were drilled from southern yellow pine (Pinus spp.) billets. In addition, 90 dowels 70 mm long and 5 mm in diameter and 90 dowels 40 mm long and 5 mm in diameter were prepared from 19- by 19- by 200-mm southern yellow pine billets. These shavings and dowels, as well as commercial machined dowels (10 mm in length, 5 mm in diameter) were placed in different beakers that were sealed with aluminum foil and sterilized by autoclaving at 103 kPa and 120[degrees]C for 25 minutes. The beakers were placed on a laminar flow bench and sterilized water was poured over the shavings and dowels to raise their MC to near 50 percent. All beakers were then sealed and stored in a refrigerator until used.
INOCULATION OF SHAVINGS
A solution of 2 percent malt agar (Difco [TM]) was poured into disposable Petri dishes. Once solid, a sterile pre-cut 8-cm-diameter cellophane (Bio-Rad [TM]) disc was placed over the agar. This procedure was adopted to prevent the wood shavings from contacting the agar surface, as moisture would be drawn into the wood from the agar, causing water-logging of the test samples (4). About 20 mL of sterile 50 percent MC shavings were carefully spread on top of the cellophane. Agar cores (diameter 3 mm) were taken from the edge of a growing colony of Postia placenta 120F, and placed on top of the cellophane in contact with the shavings, to inoculate the plates. All plates were sealed with parafilm and incubated at 25[degrees]C. This procedure was repeated with each of the other fungal strains (Postia placenta 31094B and Gloeophyllum trabeum 47D). Microscopic examination of similar material inoculated with P. placenta confirmed the presence of chlamydospores.
PREPARATION OF THE TEST POLE SECTIONS
Six Douglas-fir (Pseudotsuga menziesii (Mirb) Franco.) and six western red cedar (Thuja plicata D. Don.) round-wood sections (2.2 m in length and 240 mm in diameter) were selected for the experiment. Two additional sections of each wood species acted as controls. The controls would receive shavings inoculated with decay fungi but would not be RF heated. The six sections of each species would be inoculated with replicates of all three fungi at all three depths and then RF heated, to provide six replicate sets of samples for each fungus. The sections had been RF pre-seasoned and CCA treated during earlier phases of the research. All of the experimental pole sections had average initial MCs of 35 percent at 50 mm (tested by an electrical resistance moisture meter and confirmed on selected samples by ovendried weight).
Immediately before the sterilization treatment, 18 holes (10mm in diameter) were randomly drilled in each pole section at [+ or -] 25 cm from the mid-point of the length. All holes were at least 0.75 m from the ends of the pole section. Six holes were drilled to each depth (100 mm, 70 mm, and 25 mm) to provide duplicate samples for each fungus at each depth. All of the holes were then vacuumed to remove shavings and sprayed with ethanol to minimize interference by mold fungi before transferring inoculated shavings. Since RF generates heat within the moist wood and is not produced externally to the pole section, it was not necessary to seal the ends of the sections.
Sterilized tweezers were used to transfer the infected shavings into the holes. About 3 mL of shavings were placed in each hole. After the transfer, pre-sterilized dowels with a suitable length were hammered into the holes to seal them and keep the shavings in place at the desired depth from the pole surface. Each roundwood section was inoculated with two replicates, for each of the three fungi, at three different depths. The inoculated shavings were transferred quickly in order to minimize fungal contamination.
The experiments were carried out in a 0.25-[m.sup.3]-capacity laboratory RF/V dryer; a detailed description can be found in a previous publication (1). The sterilizing experiments were done using an RE oscillator that operated at a fixed frequency of 13.56 MHz, and had a maximum output of 10kW at a maximum electrode voltage of 5 kV.
Altogether, there were 12 heat treatments: 6 for the Douglas-fir sections and 6 for the western red cedar sections. Two insulated fiber optic temperature sensors were used to monitor the temperatures inside the wood during each experiment. (Heat loss from the thin glass fiber to the wood was minimal.) The fiber optic sensors were placed at the mid-point of each section; one was located at the core of the section, with the other located in the outer shell. All of the sterilization treatments were carried out for 2 hours, starting from the time when both temperature sensors reached 65[degrees]C (typically about 30 to 40 min.). The variation of temperature with time was recorded automatically using a computerized data-logging system. A control pole section of each wood species was inoculated with the test fungi and placed under ambient conditions for 1 day.
The dowels were removed immediately after RE heating and the shavings were extracted from each location using flame-sterilized tweezers. The recovered shavings were then placed onto malt agar amended with 4 ppm benomyl and 100 ppm tetracycline. Benomyl inhibits the growth of mold, while tetracycline is added to prevent bacterial growth. The procedure was repeated for each pole section and fungal assessment. The shavings were also removed from the control pole sections. The plates were sealed with Parafilm[TM] and incubated at 25[degrees]C for 2 weeks before being examined to confirm the growth of the fungi after 2 and 4 weeks. The growth characteristics of the fungi were compared with those of the original isolates to confirm their identity.
RESULTS AND DISCUSSION
TEMPERATURE FIGURE OF THE RP HEAT TREATMENT
Temperature versus time plots did not differ significantly between heat treatments, and representative graphs for each species are shown in Figure 1. The temperature at the end of heating exceeded the requisite 65[degrees]C. The initial rates of temperature increase were slower in the Douglas-fir pole sections than in western red cedar, probably due to the differences in their density. It took about 50 minutes for both sensors inside the Douglas-fir roundwood to exceed 65[degrees]C, but less than 40 minutes for both of the temperature sensors to exceed 65[degrees]C for the western red cedar pole sections. The wood temperature at the inner monitoring position in Douglas-fir pole sections increased more rapidly than that nearer to the wood surface during the initial heating, due to the surface cooling effect (8). However, this difference was less obvious for the western red cedar pole sections.
RECOVERY OF THE FUNGI
Since all of the experimental sections were removed after exposure to a temperature of 65[degrees]C for 2 hours, a time-based or temperature-based mortality curve for the elimination of the selected decay fungi could not be obtained. However, after 2 and 4 weeks of incubation, no decay fungi were recovered from any shavings placed in pole sections subjected to RF heating (Tables 1 through 3).
An important confirmation of the effectiveness of the RF heating process was the recovery of the fungi from the shavings placed in the control pole sections. This was confirmed in all cases. For two fungal strains, vigorous growth of the initial isolate was observed during incubation of the recovered shavings. In one series of experiments involving the UBC isolate of Postia placenta, the growth of the isolate was slowed by the presence of molds. No fungi (mold or decay) grew from the shavings recovered from the RF-heat-treated sections. An RF heat treatment used to fix CCA in roundwood would also successfully eradicate decay fungi throughout the pole cross section, in both the outer sapwood and the much drier heartwood. RF can achieve this effect rapidly, because heat is generated throughout the cross section, thereby allowing more rapid temperature rises than would occur with conventional oil or steam heating.
RF heating for 2 hours after the internal temperature of the pole reached 65[degrees]C completely eliminated test decay fungi (Postia placenta and Gloeophyllum trabeum) in Douglas-fir and western red cedar roundwood, at all depths tested. Thus, the use of either conventional RF/V conditioning of poles or RE fixation of preservatives will also sterilize the roundwood.
(*.) Forest Products Society Member.
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TABLE 1. Recovery of Postia placenta 120F from shavings placed in RF-heat-treated Douglas-fir and western red cedar pole sections, as well as from control sections stored at ambient temperatures. Distance from Fungal survival Wood species the wood surface Non-heated (controls) (a) (mm) (%) Douglas-fir 100 100 75 100 25 100 Western rcd cedar 100 100 75 100 25 100 Fungal survival Wood species Heated (b) (%) Douglas-fir 0 0 0 Western rcd cedar 0 0 0 (a)Values represent the percentage of fungal recovery from six replicates from two non-heated control sections after 4 weeks. (b)Values represent the percentage of fungal recovery from 12 replicates from 6 heated sections after 4 weeks. TABLE 2. Recovery of Postia placenta (UBC 31094B) from shavings placed in RF-heat-treated Douglas-fir and western red cedar pole sections, as well as from control sections stored at ambient temperatures. Distance from Fungal survival Wood species the wood surface Non-heated (controls) (a) (mm) (%) Douglas-fir 100 50 75 50 25 50 Western red cedar 100 50 75 50 25 50 Fungal survival Wood species Heated (b) (%) Douglas-fir 0 0 0 Western red cedar 0 0 0 (a)Values represent the percentage of fungal recovery from six replicates from two non-heated control sections after 4 weeks. Fungal growth was retarded in some controls due to mold contamination. (b)Values represent the percentage of fungal recovery from 12 replicates from 6 heated sections after 4 weeks. TABLE 3. Recovery of Gloephyllum trabeum 47D from shavings placed in RF-heat-treated Douglas-fir and western red cedar pole sections, as well as from control sections stored at ambient temperatures. Distance from Fungal survival Wood species the wood surface Non-heated (controls) (a) (mm) (%) Douglas-fir 100 100 75 100 25 100 Western red cedar 100 100 75 100 25 100 Fungal survival Wood species Heated (b) (%) Douglas-fir 0 0 0 Western red cedar 0 0 0 (a)Values represent the percentage of fungal recovery from six replicates from two non-heated control sections after 4 weeks. (b)Values represent the percentage of fungal recovery from 12 replicates from 6 heated sections after 4 weeks.
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|Author:||Fang, Fang; Ruddick, John N.R.; Avramidis, Stavros|
|Publication:||Forest Products Journal|
|Date:||Nov 1, 2001|
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