Microwave-treatment of frozen wood packaging material

As part of the wood packaging material (WPM) regulation in international trade, the dielectric
heating (DH) is soon to be included in ISPM15 (IPPC 2009) as an approved phytosanitary
treatment. It has been considered that when using dielectric radiation (i.e. microwaves or radiofrequencies),
reaching 60°C or more throughout the entire profile of the wood during at least 60s
(i.e. 60°C/60s) ensures the eradication of any noxious organism present in the wood. Regarding
the treatment itself, the main requirements mentioned in the draft of ISPM15 Annex 1 (IPPC
2011) concern the way to achieve “uniformity of heating” as well as the treatment duration,
which must not exceed 30min. Concerning wood characteristics, the only restriction to the DH
concerns the thickness of the wood pieces, which must not exceed 20cm; no other restriction
exists regarding wood moisture content, density or initial temperature. As reported in the draft of
ISPM15 Annex 1, it is generally considered that “when using microwaves as a heating source,
the coldest part of the wood is the surface”. This statement implies that achieving 60°C (or
more) during 60s at the surface of the wood should guarantee compliance with IPPC
requirements (i.e. 60°C throughout the profile of the wood).
However, since ice and liquid water exhibit very different properties towards microwaves,
initially frozen wood (9cmX9cm and 17cmX17cm in cross-section) was irradiated in a 28.8 kW
microwave oven (2.45GHz) in order to assess whether achieving 60°C/60s at the surface of less
than 20cm-thick planks ensures higher core temperatures. The temperature pattern observed after
the treatment was compared with the one observed after the treatment of initially thawed wood
pieces. It was observed that, in some conditions, initially frozen pieces exhibit inside temperature
(much) lower than 60°C, despite complying with 60°C/60s all over the surface of the wood.
These results show that when wood is heated with microwaves, its coldest part may not be the
surface. Our results also strongly suggest that the impact of wood properties on post-treatment
temperature pattern should be further investigated in order to better identify the limits of the DH
(at least on frozen wood).