The Chemical Nature of Kiln Brown Stain in Radiata Pine

Abstract

Summary This paper presents results of a study on the chemical nature of kiln brown stain (KBS) that develops in kiln dried radiata pine (Pinus radiata D. Don) sapwood. KBS is a chocolate brown discolouration which develops approximately 0.5 mm under the timber surface. Stain free radiata pine was achieved during drying trials on “green” radiata pine sapwood which had been treated to extensive cold and hot water extraction. Differences in chemical composition between the sap and hot water extract were observed. Chemical analyses (of the water-soluble extracts) by a combination of chromatographic and spectroscopic techniques, demonstrated that carbohydrates, cyclitols, amino acids, protein, and phenolics were the main compound classes present. The presence of reducing sugars (glucose and fructose) and amino acids (glutamic acid) in the sap and hot water extracts support the theory that Amadori-Maillard type reactions significantly contribute to the formation of colour in KBS. Furthermore, lignin was also detected and is suspected to contribute to KBS formation. The relative contributions of colour formation to KBS intensity from either phenolics or Maillard-Amadori mechanisms is unknown.