Thermal Modification of Color in Red Alder Veneer. Part II. Effects of Season, Log Storage Time, and Location of Wood in Stems

The value of red alder lumber is diminished by discoloration caused by the enzyme-mediated polymerization of the diarylheptanoid xyloside, Oregonin that results in the formation of red-colored chromophores in freshly felled wood. This discoloration can be reduced by pre-steaming wood prior to kiln drying of lumber or veneer slicing, but in practice, there is still variation in the color of heat-treated wood, particularly in veneer sliced from heat-treated cants processed at different times of the year. There is seasonal variation in the concentration of Oregonin that is involved in the discoloration of red alder wood and it is hypothesized here that heat-treated red alder wood will be redder and darker when the wood is obtained from logs harvested during spring when the concentration of Oregonin is known to be higher than in other seasons. The aim of this research was to test this hypothesis, and also examine the effects of log storage time and location of wood in stems on the color of heat-treated red alder wood. The color of red alder wood subjected to an isothermal heat treatment at 70°C was strongly influenced by the season in which parent trees were harvested and the length of time that logs were stored prior to heat treatment of wood. In particular, wood harvested in spring and stored for 2 wk prior to heat treatment was significantly darker than similarly treated wood obtained from logs harvested in other seasons, and redder than wood harvested in summer and winter. If the storage time of logs harvested in spring and summer was extended to 4 wk, however, the heat-treated wood became lighter and less red. Heat-treated wood from the inner part of the logs was redder and darker than heat-treated wood from the outer part of the logs except occasionally, when the outer sapwood was obtained from logs harvested in spring or summer. Careful control of log storage time, heating temperature, and duration of heat treatment could be used to minimize seasonal variation in the color of veneer sliced from heated red alder cants