Changes in the amounts and composition of acetone soluble extractives (triacylglycerols, free fatty acids, resin acids and phenols), soluble carbohydrates and starch in the outer sapwood of Scots pine (Pinus sylvestris L.) were studied during wet storage of logs. Twenty logs were felled in February and in May, and stored under sprinklers for 810 months. Samples were taken every three weeks at the same as the logs were taken for veneer cutting. Samples of the outer sapwood (10 mm from the cambium) were milled and extracted in a mini-soxhlet apparatus. Lipids and resin acids were analysed by GLC-MS and soluble carbohydrates by HPLC. Starch was analysed enzymatically.
During the winter and early spring soluble carbohydrates (sucrose and monosaccharides) and starch were hydrolysed slowly and they disappeared from the outer sapwood after six weeks of storage during the summer (MayJune).
Triacylglycerols were hydrolysed slowly and the amount of both free fatty acids and mono- and diacylglycerols increased simultaneously. The fatty acid composition did not change remarkably during the storage. Interestingly, after seven months of storage the logs felled in May contained an unknown compound (probably hydroxy fatty acid) and linolenic acid disappeared totally. These wood samples showed a clear yellow colour.
The amount of resin acids decreased only slightly during the storage. However, the relative proportion of levopimaric acid decreased rapidly with a simultaneous increase in the level of dehydroabietic acid. The resin acid composition changed significantly after seven months of storage. The amount of total phenols remained at the same level throughout the storage and no pinosylvin was observed in the outer sapwood.
Discolouration of veneer and lumber was observed after the carbohydrates and lipids disappeared from the wood. Acetone soluble extractives showed a clear yellow colour. It seems reasonable to assume that the discolouration is due to the evaporation and concentration of extractives at the lumber surface during the drying process. However, no specific substance could be singled out as solely responsible in conferring discolouration. Logs should be processed before seven weeks of storage during summer to avoid harmful discolouration.
Key words: extractives, Pinus sylvestris, wet storage, wood chemistry, wood quality.
Correspondence: Pekka Saranpää, The Finnish Forest Research Institute, PO Box 18, FIN-01301 Vantaa, Finland
Telefax: +358-0-85705361
E-mail: pekka.saranpaa@metla.fi