Abstract
Knots in trees, i.e. the branch bases inside stems, contain extraordinary high amounts of polyphenols, which are potent natural antioxidants and biocides. Studies of more than 50 tree species have shown that knots, in most of the studied species, contain remarkably higher amounts of polyphenols than the adjacent stemwood, for many species 20-100 times higher. Knots of softwood species typically contain 5-15% (w/w) of polyphenols, with lignans as the dominating group. Pine species contain a high percentage of stilbenes in their knots, while flavonoids are abundant in knots of certain hardwood species. Spruce knots from Northern Finland contain on average about 10% of lignans. The dominating spruce lignan 7-hydroxymatairesinol (HMR) is a strong antioxidant and has moreover been found to inhibit growth of certain tumors. Production of HMR and marketing it as dietary supplement in the USA has recently been started. In a large pulp mill using Norway spruce wood it is possible to sort out knots and extract up to 100 tons of HMR per year.
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