Summary
Black-colored heartwood of sugi (Cryptomeria japonica D. Don) logs with bark attached were smoked,
heated, and smoke-heated separately to improve the heartwood color. After each treatment, changes of
heartwood color, amounts of extracts (hot water, 1 % sodium hydroxide, and ethanol-toluene), metal ions
(sodium, potassium, calcium and magnesium), and pH were examined. In addition, changes of heartwood
color by UV irradiation (wavelength at 254 nm and 365 nm) also were studied. Heating and smoke-heating
of logs prevented blackening of the heartwood, leading to a yellow-white heartwood color,
whereas smoking did not largely change the heartwood color. Almost no differences in the amounts of
extracts and metal ions were found in the control and treated woods. Thermal and smoke treatments
decreased the pH from the original 8.1 to 6.0 and 7.4, respectively. The results obtained suggest that the
changes of pH by both heating and smoking relate to the heartwood color changes in black-colored sugi.
In the heated and smoke-heated woods, redness and yellowness were increased by the subsequent UV
irradiation at 365 nm, whereas brightness was decreased. Apparently, the heartwood color of black-colored
sugi was changed from yellow-white to red by the UV irradiation, the red color being the normal
heartwood color of sugi. However, UV irradiation did not cause significant changes in the heartwood
color in the control and smoked woods. These results suggest that UV irradiation of the thermally treated
wood showing the resulting yellow-white color recovered the redness as found in normal red-colored
heartwood, which seems to be caused by chemical changes of pigments under a weak acidic condition
in the black-colored heartwood of sugi.
Steaming-caused chemical changes of sugi (Cryptomeria japonica) wood monitored by NIR spectroscopy