Knots in trees – A new rich source of lignans

Recent research in our group has revealed that knots, i.e. the branch bases inside tree stems, commonly contain 5–10% (w/w) of lignans. Norway spruce (Picea abies) knots contain as much as 6–24% of lignans, with 7-hydroxymatairesinol (HMR) as the predominant (70–85%) lignan. Some other spruce species also contain HMR as the main lignan, but some spruce species have also other dominating lignans. Most fir (Abies) species contain secoisolariciresinol and lariciresinol as the main lignans. Lignans occur also in knots of pines (Pinus spp.), although in lower amounts than in spruces and firs. Scots pine (Pinus silvestris) knots were found to contain 0.4–3% of lignans with nortrachelogenin as the main lignan. Lignans have been identified also in knots of some hardwoods, although flavonoids are more abundant in hardwoods. Knots are detrimental in the manufacture of pulp and paper and should preferably be removed before pulping. This is possible using a recently developed industrially applicable process called ChipSep. Recent research has also established novel synthetic routes to several lignans, such as matairesinol, secoisolariciresinol, lariciresinol and cyclolariciresinol, starting from hydroxymatairesinol by applying fairly straight-forward chemical transformations. We conclude that wood knots in certain spruce and fir species constitute the richest known source of lignans in nature. The lignans occur in knots in free form and are easily extracted by aqueous ethanol, or even by water. Not only HMR, but also other potentially valuable lignans, could be produced in a scale of hundreds of tons per year by extraction of knots separated from wood chips at pulp and paper mills.