The Chemical Nature of Kiln Brown Stain in Radiata Pine

Abstract

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. KeywordsAmino acids Kiln brown stain Kiln drying Maillard reaction Radiata pine Reducing sugars Sap Brought to you by |

Causes of color changes in wood during drying

“…During drying, for some species (mainly softwoods), hydrophilic and lipophilic extractives migrate towards the surface and become distributed on the board surface causing a brown stain. Zavarin (1984), Lavisci et al (1991), Terziev (1995, Wiberg (1996), Schmidt and Kreber (1998), Terziev and Boutelje (1998), Terziev and Nilsson (1999), McDonald et al (2000) suggested the formation of kiln brown stain in radiata pine. P. radiata is associated with elevated contents of reduced sugars and amino acids.…”

“…Biochemical reactions are mentioned by Kreber et al (1998) and McDonald et al (2000), suggesting that the formation of kiln brown stain in radiata pine, P. radiata, is associated with elevated contents of reducing sugars and amino acids, which form MaillardAmadori type compounds at elevated temperatures. Decomposition of polyoses yields elevated amounts of soluble carbohydrates.…”

Effect of Treatment Duration and Clamping on the Properties of Heat-Treated Okan Wood

“…Their study proposed that E* occurred during heat treatment results from chemical changes in lignin due to the darkening of the lignin which was associated with the generation of chromophoric groups, mainly the increase in carbonyl groups. McDonald et al (2000) stated that when wood was exposed to high temperatures, aldehydes and phenols were formed, which led to the formation of colored compounds as a result of the chemical reactions that occurred.…”

Production and characterization of bio-oil and biochar from the pyrolysis of residual bacterial biomass from a polyhydroxyalkanoate production process

“…Amino acid composition (free and bound) was performed according to the method described by McDonald et al [29]. Briefly, protein in the RBB (20 mg) was hydrolyzed in hydrochloric acid into amino acids, and then free/hydrolyzed amino acids were sequentially derivatized using acetylchloride/isobutanol (1:4 v/v) mixture followed by heptafluorobutylanhydride in CH 2 Cl 2 .…”