Causes of color changes in wood during drying - Springer Link

For. Stud. China, 2010, 12(4): 167–175 DOI 10.1007/s11632-010-0404-8

REVIEW

Causes of color changes in wood during drying Sadoth SANDOVAL-TORRES1,2*, Wahbi JOMAA1, Francoise MARC1, J-R PUIGGALI1 1 2

Université Bordeaux 1, Laboratoire TREFLE site ENSAM, Esplanade Arts et Métiers CEDEX, Talence 33405, France Instituto Politécnico Nacional, CIIDIR Oaxaca Engineering, Hornos 1003, Col. Noche Buena. Santa Cruz Xoxocotlán, Oaxaca 71230, Mexico

© Beijing Forestry University and Springer-Verlag Berlin Heidelberg 2010 Abstract The forest industry operates in a dynamic and global market where change and competition are the rule rather than the exception. The color of wood is one of the most attractive features for the modern wood industry. Even when wood is chosen for its structural qualities, attractive and decorative colors are usually an important factor. In many applications, particularly in furniture, decorative products, decorative veneers and flooring, accurate matching of the color of different samples is required. Wood attributes and properties are important because they have a direct bearing on market opportunities and consumer acceptance for many types of manufactured wood products. The aim of this review is to identify causes of wood discoloration and advances in drying technology to overcome this problem. Wood discoloration is a complex phenomenon, mainly affected by heat, light, physiological and biochemical reactions, as well as from attack by microorganisms.

Key words wood discoloration, wood chemistry, drying

1 Introduction Color is one of the quality attributes that influences perceptions by customers of wood-products. In wood processing operations, wood is mechanically converted by sawing, planning and milling, as well as by drying. In the European markets for high-value tropical hardwoods, the decorative characteristics (appearance) and color consistency of wood-products are important, since wood-based products face severe competition from newer materials (FAO, 2000). Usually it is the color setting that is important in furniture, kitchen fittings, doors, ceilings and wall panels. Wood discoloration problems have been known to wood producers and customers worldwide for many years and have caused large economic losses to the wood industry. Conners et al. (1987), Brunner et al. (1990) and Silvén and Kauppinen (1996), demonstrated the utility of color information to detect defects in wood. Color is an attribute of visual perception, which is determined by the spectral makeup of light reflected from surfaces. Finnish pineis mainly used for surface overlays and home interior decoration because of its warm color and pattern that have an effect on the impression of people and on its valuation (Grekin, 2007). According to Wiedenbeck et al. (2003), changing market conditions of veneers and consumer preferences are key factors in the determination of acceptability of veneergrade logs. Then, color can establish the commercial *

Author for correspondence. E-mail: [email protected]

value and suitability for a given product. Wood discoloration is a very complex phenomenon which can vary greatly with genetic, environmental or treatment conditions (Hon and Shiraishi, 1991). According to Punches and Black (2002), wood may be undesirable (depending on species) if it leads to color variation, since many species are used in flooring, largely for their appearance/color.

2 Chemistry of wood color Wood is a cellular lignocellulosic material consisting of cellulose embedded in a hemicellulose/lignin matrix. The composition of wood varies at all levels from species to species, among cell types and within the cell wall itself. The chemical composition of softwoods (conifers) differs from that of hardwoods (arboreal angiosperms) in the structure and content of lignin and hemicelluloses (Table 1). An unsaturated chemical bond can transfer easily to an excited state with a minimum amount of energy. An atomic group having one π electron, such as an unsaturated bond, is called a chromophore. An atomic group having isolated electron pairs such as –OH, COOH, or –OR is called an auxochrome. Auxochromes assist the action of chromophores by intensifying the coloration or enabling the absorption of light having a longer wavelength (Hon and Shiraishi, 1991). On one hand

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Forestry Studies in China, Vol.12, No.4, 2010

Table 1 Chemical composition of normal and reaction wood in typical softwood and hardwood species Component Radiata pine (Pinus radiata) Birch (Betula papyrifera) Normal wood

Compression wood

Normal wood

Tension wood

Cellulose

40

36

42

50

O-acetylgalactoglucomannan

20

12 3

2

Glucomannan 1,4-Galactan Arabino-4-O-methylglucoronxylan

8

8

11

11 30

22

Lignin

27

32

24

17

Extractives

2

1

1

1

O-acetyl-4-O-methylglucoronxylan

%

Source: McDonald (2001).

the red hue in wood is commonly associated with the extractive content of wood (Yazaki et al., 1994). Chemistry is also affected by light. The yellow tones of wood are primarily governed by the photochemistry of wood, particularly in lignin. Bleaching, impregnation, steaming, varnishing and similar processes also cause color changes. Processing the surface of wood changes, partially or completely, the natural color and pattern of surfaces (Colacoglu, 2006). It is well known that wood is easily discolored by light or heat and is stained with acid, alkali, metal ion or microorganisms. Wood containing stilbenes or leucoanthocyanidins is changed from a light to a dark color by light. In contrast, wood containing quinonoid pigment is changed from a dark to a light color by light. Wood containing phenolic compounds and particularly ellagitannins is stained to a dark color when it reacts with alkali. Wood containing leucoanthocyanidins or condensed tannins is changed to a reddish color by treatment with acids. Stains with metal ions are caused in wood containing gallic acid, tropolones, or other phenolic compounds having a catechol nucleus (Hon and Shiraishi, 1991). Charrier et al. (1992) studied discoloration in European oak wood. They applied 70°C and 150 Mba (vacuum conditions) using superheated steam and found discoloration to be characterized by a decrease in lightness (–6 to –8 units) compared to non-discolored wood. For this hardwood species, Klumpers et al. (1994) affirmed that its heartwood contains vescalagin, castalagin, grandinin, roburin E and their dimers.

3 Role of phenolic compounds Phenolic compounds have been studied and are related to wood discoloration (Haluk et al., 1991; Mayer et al., 2006). The brown color of heartwood is primarily related to the oxidation of phenolic compounds. Some reactions of phenolic substances were reported by Koch and Bauch (2000). They demonstrated that the discoloration of European beech (Fagus sylvatica) is strongly associated with reactions of low molar mass phenols such as catechin, syringic acid, taxifolin and

conidendrin. Csonka and Nemeth (1998) suggested that flavonoid plays an important role in color changes in hardwoods. The reddish-brown discoloration in the heart of white birch (B. papyrifera) is due to oxidation products of phenolic extractives (Siegle, 1967). Furthermore, catechin isomers have been shown to play a role in the discoloration of Ilomba wood (Yazaki et al., 1985) and western hemlock (Tsuga heterophylla) (Hrutfiord et al., 1985). Based on the effect of extractives, Notburga and Jacques (2004) showed a direct relationship between color and decay resistance, since extractives protect wood from biological decay. The effect of phenolics on wood decay resistance was also demonstrated quite recently for pine heartwood by Harju et al. (2003). Relationships between wood color and phenolic content are discussed in Burtin et al. (2000). These authors reported the effect of the accumulation of native phenolic compounds in the transition zone on wood color and the modifications of hybrid walnut under various steaming conditions. They found during steaming that polyphenol compounds in heartwood conferred the dark color to walnut wood; these compounds may migrate to the sapwood region and change the sapwood color from light to dark. Mononen et al. (2004) demonstrated during drying (conventional and vacuum) of birch wood, that (q)-catechin-7-O-b-Dxylopyranoside decreases, most likely with the breakdown of the glycosidic bond at elevated temperatures. Serious cases of an intense black-brown surface discoloration, found in kiln-dried amabilis fir, were attributed to chemical and anatomical alterations (Kreber and Byrne, 1994). Microscopic studies on discolored specimens demonstrated the presence of high concentrations of dark brown extractives in parenchyma cells and bacterial infestation in longitudinal tracheids. Chemical analysis indicated that 3,3’-dimethoxy-4,4’ -dihydroxystilbene (DDS) was responsible for the deep brown color of amabilis fir. Sutcliffe and Miller (1991) analyzed brown stain in a sample of amabilis fir. A conversion of DDS to a high colored compound had developed discoloration but they hypothesized that other polyphenols could have contributed to discoloration.

Sadoth SANDOVAL-TORRES et al.: Causes of color changes in wood during drying

4 Causes of wood discoloration 4.1 Heat It is generally accepted that color changes increase as the temperature increases. Hardwoods generally discolor at lower temperature than softwoods, but it is generally observed that heat can directly alter the color by causing hydrolysis and oxidation of wood components. According to White and Dietenberger (2001), darkening of wood due to heat is caused by thermal degradation of hemicelluloses and lignin and can be initiated at temperatures as low as 65°C, depending on pH, moisture content, heating medium, exposure period and species. Hence, hydrolysis of hemicelluloses can occur at temperatures within the range employed to kiln-dry wood. On one hand, researches at high temperature have been carried out in order to improve physical properties. Wikberg and Maunu (2004) reported that chemical modifications in wood structures, occurring at high temperatures, are accompanied by several favorable changes in physical structure: reduced shrinkage and swelling, low equilibrium moisture content, better decay resistance, enhanced weather resistance and a decorative dark color. On the other hand, some studies have concluded that high temperatures degrade wood. Beyer et al. (2006) found heat induces thermal processes which usually lead to non-beneficial effects such as yellowing, discoloration, or unpleasant odor. In earlier studies discoloration of birch had been investigated using colorimetric methods, with results indicating a notable color change occurring during kiln drying, of which temperature appeared to be the most important parameter (Kreber and Haslett, 1997; Luostarinen and Verkasalo, 2000; Kreber et al., 2001; Luostarinen and Luostarinen, 2001; Mononen et al., 2002; Sundqvist, 2002; Mononen et al., 2004). 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. Bekhta and Niemz (2003) affirmed heat treatment mainly resulted in a darkening of wood tissues, improvement of the dimensional stability of wood and reduction of their mechanical properties. In another work, Bourgios et al. (1991) concluded that the decrease in lightness was caused by a decrease in hemicellulose contents, especially pentosan. Sundqvist (2002) exposed white birch (Betula pubescens), Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) to temperatures of 65°C, 80°C

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and 95°C for 0, 1, 3 and 6 d. Each species showed pronounced darkening when the temperature exceeded 80°C. Later, Sundqvist et al. (2006) studied hydrothermal treatment of birch wood and its relation to color. They found considerable amounts of acetic and formic acid released from the wood, related to high treatment temperatures and long treatment times. At both low and high temperatures, vapor emissions can affect color. Schafer and Roffael (2000) reported formaldehyde emanates from wood at temperatures as low as 40°C and higher temperatures increase the formaldehyde emission tremendously; this fact can induce color change reactions in wood. Buerger maple is an important furniture and building material in China. During drying this material tends to become red and fuscous (Gao et al., 2004). The authors suggest that high temperature and humidity accelerate the oxidation reaction of polyphenol, leuco-fancy pigment and tannin, which changes the color to red; the hydroxyl (–OH) is oxidized and the carbonyl (–C=O), carboxyl (–COOH), esters and ketone groups discolor Buerger maple. Kang (2006) dried Japanese pine (Pinus densiflora), Korean pine (Pinus koraiensis) and larch (Larix leptoleptis) using five different methods. He found that the strength of discoloration reactions depend largely on wood temperature. McCurdy and Pang (2006) concluded that low temperature and low relative humidity can diminish the non-uniformity of color in softwoods. In American black cherry wood, heat treatment of heartwood during veneer production intensifies the reddish-brown heartwood colour, probably by promoting the polymerisation of (+)-catechin and other flavonoid monomers (Koch et al, 2003). 4.2 Microorganisms Considerable interest exists to controlling, designing and developing future integrated fungal control programs in wood. Microorganisms can develop discoloration in wood. Fungi attacks diminish wood quality since they modify color. Blue stain is caused by microscopic fungi that commonly infect only sapwood, using sapwood compounds, such as simple sugars and starches. According to Viiri et al. (2001), many bark beetles distribute spores of fungi laterally or internally in their digestive tract while constructing breeding chambers and galleries in the phloem of their host trees. As they grow, fungal hyphae suppress water transportation in the host, causing discoloration of wood and helping the beetles to kill the trees. They cannot grow in heartwood or in most wet woods that do not contain the necessary food substances. Blue stain fungi are prone to cause bluish or grayish discoloration of wood but they do not cause decay. Wood blue stain fungi that belong to the family Ophiostomataceae Nannf. are propagated by xylophagous insects and are usually used as response

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inductors. The pathogenicity of these fungi in conifers manifests itself by the rapid development of phloem necrosis in the vascular system after fungal inoculation (Shein et al., 2001). Blue stain has no effect on the strength of wood. Generally, discoloration in living trees and logs is caused by microorganisms while discoloration in kiln dried sawn wood is related to wood extractives (Kreber and Byrne, 1994). According to Kim et al. (2007), currently Japanese red pine (P. densiflora) and Korean pine (P. koraiensis) are major wood resources in Korea. Korean pine has been replanted extensively during the past several decades, but pine sapwood is usually highly susceptible to dark discoloration caused by sap staining fungi. Ilomba wood (Pycnanthus angolensis) has been attributed to reactions of phenolic extractives caused by microorganisms or enzymes (Yazaki et al., 1985; Hon and Minemura, 1991).


of freshly cut fruit and occur in living parenchyma cells where they create amorphous globules of colored material. They found that internal darkening of hard maple (Acer saccharum) developed at or above 43°C when the wood moisture content was at or above the fiber saturation point. Some authors suggest enzymatic action in color changes. Browning reactions are generally catalyzed by polyphenoloxidases (Lee and Whitaker, 1995) or peroxidases (Higuchi, 1997) and result in the formation of quinines which subsequently polymerize to varying degrees, leading to brown pigments. Fast drying leads to an increased enrichment towards surfaces; in fact, when this carbohydrate-enriched zone is being heated in the presence of amino acids working as catalysts, a degradation of the carbohydrates takes place at which colored degradation products are formed. 4.5 Oxygen

4.3 Chemical reactions Chen and Workman (1980) and Kubinsky and Ifju (1973) studied the effect of steaming on the color of press-extracted wood fluids, concluding that extractives in wood fluids are largely wood pigments. Aydin and Colakoglu (2005) reported that changes in natural coloration of wood occur as a result of chemical reactions that take place between naturally occurring precursors to stain formation (phenolic extractives) and enzymes present in wood, hence inactivation of enzymes could be one strategy to control color alteration. According to Terziev et al. (1993) and Terziev (1995), yellowing of sapwood is explained by the enrichment of sugars and nitrogenous compounds towards wood surfaces during the initial or capillary phase of drying. 4.4 Biochemical reactions 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. Masson et al. (2000) studied the effect of kiln drying on the chemical composition of oak wood, Quercus petraea, and found part of the xylans and glucomannans degraded during drying. Storage and kiln drying of sawn wood can cause discoloration due to physiological and biochemical reactions (Luostarinen et al., 2000; Mononen et al., 2004). According to Yeo and Smith (2004) wood discoloration reactions are similar to those causing browning

Discoloration of red alder has been studied by Simpson (1991). In his work, Simpson suggested that a red to orange discoloration occurs in freshly cut red alder wood due to an oxidative reaction between extractives and the atmosphere. In another study, Abe and Matsumura (1994) found that reactions between extractives and atmospheric oxygen under weakly alkaline conditions were responsible for color alteration in Sugi wood (Criptomeria japonica). Oakwood (Quercus spp.), beech (Fagus sylvatica) and rubberwood (Hevea brasiliensis), which are prone to stains, can be dried under vacuum without the occurrence of discoloration due to the absence of oxygen (Charrier et al., 1995; Charrier et al., 2002). The formation of colored substances from a phenolic compound oxidized with air and the formation of dark materials from hydrolysis of hemicelluloses have been considered causes for discoloration. It is well known that vacuum drying reduces drying time and oxidation (Cividini and Travan, 2003; Seyfarth et al., 2003), after which a free-oxygen drying method can alleviate discoloration (Sandoval-Torres et al., 2009). Experimental research in the absence of oxygen was carried out by Leiker and Adamska (2004). They applied vacuum-microwave drying to avoid material degradation, where the wood in most cases was free of checks and always free of color changes, because of the short process duration and the absence of oxygen. 4.6 Light Another cause of discoloration is light. Light can modify wood color by affecting lignin molecules. These days, several authors appear interested in photodiscoloration, since polymers in wood are sensitive to light exposure (Chang and Cheng, 2001; Kzokou and Kamdem, 2006). Pandey (2005a) showed that the

Sadoth SANDOVAL-TORRES et al.: Causes of color changes in wood during drying

effect of light on lignin leads to the formation of free radicals, reacting with oxygen to produce chromophoric carbonyl and carboxyl groups, which are responsible for color changes. Pandey (2005b) found the degradation is triggered by the formation of free radicals by UV irradiation. Photochemical reactions are initiated by the absorption of UV-visible light, mainly by lignin, which leads to the formation of aromatic and other free radicals. These free radicals may then cause degradation of lignin and photo-oxidation of cellulose and hemicelluloses. Kudo and Saito (1980) reported that photo-discoloration of heartwood, which contains extractives, was higher than that of sapwood and that photo-degradation of extractives also results in the generation of carbonyl groups. Some authors discussed photo-stabilization of wood; for example photo-stabilization of wood by copper treatments may retard the formation of carbonyl groups and reduces delignification during weathering (Temiz et al., 2005). In addition to light action, there are many reports that humidity treatment alters the properties of wood. Mitsui and Tolvaj (2005) showed that the color of irradiated wood changed markedly with high relative humidity; therefore little change in color was observed at low relative humidity. Deka et al. (2007) investigated color stability of chemically treated and thermally modified wood, comparing it to non-modified wood during long term artificial UV light irradiation, by means of EPR and DRIFT spectroscopy. Deka et al. (2007) demonstrated that some degree of color stability of treated and modified wood during artificial UV light irradiation resulted from modifications in lignin molecules and monomers of phenolic compounds. Given this condition, control of wood color during drying is a very difficult task, and becomes a complex operation.

5 Color requirement in the wood industry The Composite Panel Association has published the Buyers and Specifi ers Guide for Decorative Surfaces. This guide is primarily published for those who seek information on decorative surfaces (architects, designers and specifiers). The first section of the guide describes each decorative surface treatment, by detailing the characteristics of surface material. The application to decorative overlays (high pressure laminates, thermally fused papers, decorative foils, heat transfer foils and wood veneer) requires high quality appearance of wood-based material. As well, Magnum Wood Flooring has published a Granding Book. In this book, the reader can elucidate that color is a very important specification for wood markets. Other enterprises like WISA and OMNOVA Inc., produce wood-based material for decorative applications. These facts open new opportunities in wood science and technology research. For example, sliced red alder veneer is commonly used as a decorative overlay on composite

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wood panels (particleboard and medium density fiberboard), which are then used in the manufacture of cabinets and furniture (Hibbs et al., 1994). Red alder wood, however, acquires a mottled orange color after felling, which is undesirable when the wood is used for cabinets and furniture (Kozlik, 1962; Simpson, 1991). Thompson et al. (2004) studied the thermal modification of color in red alder (Alnus rubra). They suggested discoloration depends on the strength of reactions that produce orange/red chromophores in wood. 5.1 Current technology Drying methods have been proposed to overcome wood discoloration. For example, radio-frequency treatment can be used to eradicate fungi in wood (Tubajika et al., 2007). In their study, they found RF treatment depended strongly on initial wood moisture and the power density used and that eradication of fungi by RF treatment was a function of temperature and heating time. Vacuum drying is interesting because vaporization of liquids takes place at lower temperatures with relation to the temperature range considered in air dryers, but a better understanding of the drying conditions and innovations in heating technology are needed to control discoloration during kiln-drying of wood. Hiltunen et al. (2008) studied drying of European white birch (B. pubescens) during vacuum drying. They established that discoloration extended to a depth of about 7 mm on both faces of boards. In their research, discoloration was linked to the accumulation of phenolic compounds during drying or storage. Charrier et al. (2002) immersed walnut logs in water at temperatures of 80°C to 90°C for up to 51 h and found that these thermal treatments promoted the darkening and reddening of wood, but the darkening was more prominent in heartwood than in sapwood. Vacuum technology shortens drying time to minutes (Chen and Lamb, 2001; Perré and Turner, 2006), while drying temperatures can be very low (between 30°C and 40°C), so that thermal degradation and chemical reactions of discoloration can be diminished. Moreover, vacuum drying enables an important overpressure inside the material to enhance internal moisture migration (Jomaa and Baixeras, 1997). The use of drying agents other than air could be studied. Pang (2006) studied the drying of radiata pine by using methanol and ethanol vapors. His research indicates that using an alternative oxygen-free medium could be an effective way for preventing surface discoloration, but did not investigate its industrial application and costs. Luostarinen (2006) studied the discoloration of birch wood after conventional and vacuum drying processes. He suggested that in less darkened conventional drying, rays kept the color light in spite of darkened phenolics inside the wood, while in badly

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darkened vacuum drying, both the ray and the axial parenchyma took part in darkening; this depended on the more condensed nature of the phenolics in vacuum drying than in conventional drying. Then, changes in cell dimensions may simply affect the color by changing the ratio of lumens to cell walls, which emphasizes the contribution of cell wall compounds to color. Mass and heat transfers have a very important effect on discoloration. In fact all chemicals or substances in wood move and change as a result of moisture transport and heat transfer. Nevertheless, few studies include these facts in analyses of discoloration. The migration of phenolics in wood has not been reported in vacuum drying, but in conventional drying phenolics did not migrate in oak wood (Lavisci et al., 1991). Previous studies proposed to use wood extractives as protection for photo-discoloration, but more studies are needed to understand the chemistry and mechanism of action of these extractives. Furthermore, BASF, a German chemical company, has developed chemicals (coatings) for wood protection (protective products for weathering and discoloration) Their products, such as Basazol©, Lignosatb© and Ludopal©, offer solutions after drying. All the same, a drying and heating technology to alleviate discoloration is necessary to improve high quality wood products. 5.2 Plain vacuum drying by contact In other studies, Sandoval-Torres et al. (2009), Sandoval et al. (2007) have investigated wood discoloration in European oak wood during plain vacuum drying by contact. In this research work, we have proposed vacuum drying by contact as a solution to overcome this problem, In order to ensure an acceptable heat transfer, the boiling temperature of water and oxygen availability should be diminished, enabling an important alleviation of wood discoloration. We have elucidated three mechanisms of discoloration of wood. The first mechanism consists of the emission of volatile components (VOC’s), the second is the degradation of ellagitanins (hydrolysable tannins) and the third method is one of loss or alteration of carbohydrates (hemicelluloses). Still, in the absence of oxygen, depolymerization reactions do occur. Industrial applications must be explored and efforts should focus on the application of new heating technologies on an industrial scale.

6 Conclusion Our review provides vast amounts of information and a large number of references concerning causes of wood discoloration during drying. We have described difficulties and causes of this alteration. Readers can elucidate that wood discoloration depends on several factors. Wood discoloration is a very complex problem, but important for both high quality wood prod-


ucts and decorative purposes. New drying methods, such as radio-frequency drying, radio-frequency vacuum drying, microwave drying, plain vacuum drying and drying by using methanol, ethanol or others vapors as drying agents, can offer new possibilities. More efficient heating systems must be investigated, since large thermal gradients inside wood produce significant differences in wood coloration. We pose that principles of transport phenomena must be included for a better understanding of chemical changes in and appearance of wood.

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