Improvement of heartwood color of black-colored sugi (Cryptomeria japonica D. Don) by UV irradiation after smoke heating

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.