High-Frequency Energy–Assisted Vacuum Drying of Fresh Eucalyptus globulus Christian Hansmann, 1,2 Robert Stingl, 1 Oscar Gonz alez Prieto, 3 Carlos Baso Lopez, 3 and Helmuth Resch 1 1 Institute of Wood Science and Technology, Department of Material Sciences and Process Engineering, BOKU, University of Natural Resources and Applied Life Sciences, Vienna, Austria 2 WOOD K PLUS, Competence Centre for Wood Composites and Wood Chemistry, Linz, Austria 3 Depto. De Ingenieria de los Recursos Naturales y Medio Ambiente, E.U.I. Te ´ cnica Forestal-Campus de Ponteveddra, Universidad de Vigo, Vigo (Pontevedra), Spain Worldwide, eucalyptus tree plantations have been established in appropriate climates because of fast growth and wood qualities suit- able mainly for pulp. A potential exists of converting eucalyptus trees into lumber that may be of higher value than pulp. Conven- tional drying of lumber of Eucalyptus globulus is often difficult because of the occurrence of drying stresses, leading to collapse and checking. The special method of vacuum drying while heating the wood with high-frequency energy (75–77 mbar, 46–51  C) was used to obtain short drying times (5–13 days from green state to 10% final moisture content) and low crack amount. Keywords Cell collapse; Cracking; Drying quality; Drying rate; Moisture content; Rapid drying; Wood INTRODUCTION Worldwide, eucalyptus tree plantations have been established in suitable climates because of fast growth and excellent wood qualities, mainly for use in pulp and paper. In Portugal and Spain, Eucalyptus globulus Labill. has been the preferred species. It is also known as southern blue gum or under the commercial name Tasmanian oak. Eucalyptus globulus Labill. is the most extensively planted eucalyptus in the world. [1] On the Iberian peninsula the species covers an area of about 1 million hectares. [2] Beside the common utilization of Eucalyptus globulus as windbreak trees or as a major source of fuel wood in many countries of the world, Eucalyptus globulus is much used for pulpwood, particularly so because its bark, acceptable in most pulping processes, adds greatly to the yield. Other uses include the extraction of essential oils from the leaves, [3] honey production from the flowers, plantings for erosion control, and roadside plantings to provide a noise and headlight buffer. [1] Additionally, a potential exists of converting trees of most eucalyptus species into lumber that may be of higher value than pulp. Eucalyptus globulus features a light- colored wood, its mechanical properties are excellent, and its appearance resembles some other hardwood species, in particular oak. [1,4,5] However, utilization as solid wood is subjected to several problems during processing. Sawing of logs is difficult and the quality of lumber is poor because of growth stress problems. [1,6,7] When lumber is being dried, drying stresses develop and may cause drying defects such as collapse, case hardening, honeycomb, checking, and cupping. This is especially true for Eucalyptus globulus, which in fact is known to be very susceptible to these drying defects. [1,6,8–10] Collapse is even possible with air drying, especially in the summer showing that technical drying (i.e., drying with controlled climate conditions) is necessary in order to obtain sufficient quality. [11] However, more effective drying methods for Eucalyptus globulus have to be developed for the increasing demand. [12] With conventional kiln drying, better drying quality could be achieved compared to uncontrolled air drying, especially when a steaming treat- ment is applied. [5,6] This steam treatment, which can be applied in different stages of the drying process, allows for a higher rate of drying and a considerable reduction of shrinkage and collapse. [5,13,14] However, drying time is still very long as a very mild drying scheme has to be applied. [6,10,15,16] Also, high-temperature drying was tried in order to achieve higher drying rates. While drying from 20 to 25% at temperatures above 100  C was reported to be successful, [6] other studies concluded that high-temperature drying was not suitable for this species, either from the Correspondence: Christian Hansmann, Institute of Wood Science and Technology, Department of Material Sciences and Process Engineering, BOKU, University of Natural Resources and Applied Life Sciences, Peter-Jordan-Straße 82, 1190 Vienna, Austria; E-mail: hansmann@boku.ac.at Drying Technology, 26: 611–616, 2008 Copyright # 2008 Taylor & Francis Group, LLC ISSN: 0737-3937 print/1532-2300 online DOI: 10.1080/07373930801946759 611