Vacuum drying of European oakwood: Color, chemistry and anti-oxidant potency of wood. Improving appearance in forest value added products
S Sandoval, W Jomaa, F Marc, J-R Puiggali
In hardwoods used for decorative and appearance purposes, wood colour is one of the most important factors of wood quality; in addition colour is related with durability and biological decay of wood. Wood discolouration during drying is mainly affected by heat, light, physiological reactions, combinations of reactions, biochemical and chemical reactions, and micro-organisms attack. In freshly felled and stored round wood discolorations are initiated predominantly through physiological reactions of living parenchyma cells. Discolouration during kiln-drying decreases the commercial value of hardwoods, since hardwoods are used in the manufacture of furniture and cabinets. On one hand, heat modifies the cell wall components and induces chemical reactions of nutrients and extractives, by other hand the role of oxygen in kiln dryers is very important due to oxidation reaction of phenolic compounds. The formation of coloured substances from a phenolic compound oxidized with air and the formation of dark materials from hydrolysable extractives are considered causes of discolouration. In order to reduce oxidation reaction, vacuum drying process can be used. In addition, it offers reduced drying times and higher end-product quality in comparison with conventional drying operations. Operating at low pressures reduces the boiling temperature point of water and enables an important overpressure inside the material which is advantageous for drying and especially for species that do not support a high temperature level. In this work, experimental results for the vacuum drying of oakwood with conductive heating are presented for different drying conditions. In particular, surface-wood scans, antioxidant capacity of wood, Fourier Transform Infrared Spectra of dust-wood for different vacuum and convective drying conditions. Sample temperatures and pressure in the dryer are logged during drying. The experimental setup (Figure 1) is a vacuum chamber where pressure is regulated between two values (Pmin, Pmax). The chamber is built in glass; one balance is kept inside the chamber in order to log the mass variation of the sample. A thermometer gives the dryer temperature. The heating source is an electrical resistance which temperature is controlled with the help of a PID controller. Experiments are performed on Oakwood disks (7 cm diameter and 2.5 cm height). The conductive heat source is maintained at different temperatures (46°, 61° and 70°C) and pressure in the chamber is controlled at different intervals (60-100, 150-200, and 250-300 mbar). Temperature inside the wood sample is obtained at two different positions. Conventional drying is carried out for comparison in a tunnel dryer. Antioxidant potential in fresh and dry wood samples is determined by using ABTS+• radical cation method. The top surface of vacuum-dried specimens is imaged with an HP scan. Finally it is shown that oak wood which is prone to discolour is degraded by different mechanisms depending on drying method. Our results suggest that oxidation of extractives and thermal degradation of hemicelluloses are the principal mechanisms of degradation, but its importance depends on drying method. Oakwood can be dried under vacuum conditions with an acceptable diminution of discolorations due to low temperature and reduction of oxygen amount with acceptable drying rates.