Solutes in sap obtained from supercritical CO2 dewatering of radiata pine sapwood, and a new role of sap cyclitols in brown stain formation during kiln drying of green wood

Abstract

Abstract Xylem sap obtained from radiata pine sapwood using a supercritical CO2 (scCO2) dewatering process contained a complex mixture of solutes comprising carbohydrates, amino acids, alkanoic and diterpene acids, neutral diterpenoids, β-sitosterol and cyclitols. Sap also contained free phosphoric acid which is known to catalyse the condensation reactions between glutamic and aspartic amino acids and glucose, galactose and fructose to form Maillard products. When heated these Maillard products lead to the formation of melanoidins, which are partly responsible for the formation of kiln brown stain (KBS) when green radiata pine sapwood is kiln dried. The cyclitols, which are major components in sap, have been shown to undergo phosphoric acid-catalysed dehydration reactions under kiln drying conditions to give methoxyl derivatives of hydroquinone, catechol, resorcinol and O-methylphloroglucinol. These reactive phenols are known to exothermally condense with formaldehyde and could also react with other short carbon chain aldehydes generated during kiln drying. The products of these chemical reactions are extended π-orbital aromatic polymers which are darkly-coloured and would be expected to contribute to KBS. This paper describes the chemistry of sap from radiata pine sapwood using scCO2 dewatering and proposes new theory on KBS formation invoking thermal dehydration and retro Diels-Alder chemical reactions of the sap cyclitols.