Wetwood, or water pocket, has higher moisture content (MC) and lower permeability than normal wood, which cause problems for lumber drying. The high moisture content of wetwood usually requires relatively long periods for adequate drying; consequently, it causes a high risk for developing checks, splits, crook, bow and twist of lumber in kiln drying. These problems have not been solved by any physical, chemical or mechanical methods yet. Using biological method to pre-dry lumber containing wetwood is a new concept introduced in this project. Wetwood is formed by bacteria growth inside normal wood. Some fungi are able to kill bacteria and to utilize foetid liquid produced by these micro-organisms. Consequently, the permeability of wetwood can be increased and the lumber drying rate can be improved. The present project intends a research on biological method to pre-dry lumber containing wetwood, and to evaluate efficacy and economic benefit of such a biological treatment.
Wetwood of balsam fir, sub-alpine fir and aspen was cultured on nutrient media, and several species of bacteria and yeasts were isolated. The bacteria and yeasts were re-inoculated on normal wood of balsam fir. All inoculated micro organisms caused wetwood formation in 2 weeks. The MC of the inoculated wood blocks increased from 41% to 220-240%, whereas the control samples without inoculation reached only 110%. When control samples were dried to a MC of 13%, the inoculated wood samples still had MCs between 80% and 105%. The selection of biological control agents was conducted on both agar plates and on balsam fir wetwood blocks, and 2 fungal candidates demonstrated promising results. The field test showed that pre-treating balsam fir wetwood lumber with the selected best biocontrol candidates, wood stain was reduced by 94%, warping reduced up to 13%, and checking reduced up to 30% compared with untreated controls. Drying time was reduced by 33% (24 hours) compared with drying fresh lumber.
CT scanner was able to detect wetwood locations inside a piece of lumber, and the wetwood was identified in heartwood, sapwood or both wood tissues. After the bio-treatment, the wetwood contents of boards were significantly reduced.
Economical analysis showed that the biological treatment would cost $4-7/Mfbm depending on treating method used. Reduction of 33% of drying time by the treatment in this study could save energy cost by $6-13/Mfbm depending on kiln drying energy used. The treatment could reduce lumber degrading loss by $8.5-37.4/Mfbm base on this study. The benefit of the treatment is significant, but will be affected by pre-drying operation, kiln type, energy use and drying schedule. The biological treated lumber is resistant to fungal infection during pre-drying period, and the lumber products are clean and free of moulds and stain infection.
We specifically would like to thank Pierre Lemieux, Scierie Leduc, for providing testing wetwood materials. We also appreciate the support and guidance provided by the project’s industry liaison officers: François Saillant, Natural Resources Canada; Léandre Bélanger, Domtar. Their participation was the key to the success of this project.