Philippe Gérardin

Wettability of Heat-Treated Wood

Summary The aim of this work was to study the wettability and chemical composition of heat-treated wood. Heat treatment was performed at 240°C under inert atmosphere on four European wood species (pine, spruce, beech and poplar). Contact angle measurements before and after treatment indicated a significant increase in wood hydrophobicity. Advancing contact angles of a water drop were in all cases systematically higher for heat-treated than for untreated wood. Chemical modifications of wood after heat treatment were investigated using FTIR and 13C NMR analysis. FTIR spectra indicated little structural change which could be attributed either to carbon-carbon double bond formation or to adsorbed water. NMR spectra also revealed little chemical change except for the degree of cellulose crystallinity which was considerably higher in heat-treated wood and could explain the higher contact angles.

Wettability changes and mass loss during heat treatment of wood

A new product called torrefied or retified wood can be obtained by mild pyrolysis of wood in a temperature range between 200 and 2608C and under inert atmosphere. Such heat-treated wood dramatically reduces its hygroscopicity and improves its dimensional stability and durability. The main drawback of the torrefied wood is its high brittleness. Another property, often mentioned but not well investigated, is its hydrophobic character (Pétrissans et al. 2003). In fact, the heat-treated wood becomes rather hydrophobic, which could cause some severe problems during varnish or paint deposition. The aim of our study was to investigate the wettability changes and the mass loss during heat treatment of wood at different temperatures. A comparison of these two parameters could add valuable information regarding wettability changes and chemical degradation resulting from retification. Heat treatments were carried out in the temperature range of 40 to 2608C. Four European wood species – pine, spruce, beech, and poplar – were studied here. The contact angle values were measured by the Wilhelmy technique. Wood wettability is generally difficult to measure. For example, the heterogeneous and porous character of wood causes the so-called contact angle hysteresis, that is, a distinct difference between the advancing (ua) and the receding (ur) contact angles (Liptáková and Kúdela 1994). Moreover, the wood extractives may also contaminate the probe liquids during measurement (Wålinder and Johansson 2001). This hysteresis effect can also be related to the chemical heterogeneity (Menawat et al. 1984) and/or to the solid roughness (Dettre and Johnson 1964). The Wilhelmy method, chosen for this study, is not a conventional technique for wood contact angle measurement but gives good results on this material (Gardner et al. 1991; Wålinder and Johansson 2001; Wålinder and Ström 2001; Pétrissans et al. 2003).

Limitation of XPS for analysis of wood species containing high amounts of lipophilic extractives

Chemical composition of Norway spruce and pine, two softwood species, has been investigated by X-ray Photoelectron Spectroscopy (XPS). Contrary to results previously obtained with beech wood, which allow to obtain information on bulk chemical composition from surface composition analysis, XPS analysis appears to be unsuitable for the characterisation of Norway spruce and pine wood chemical composition. Indeed, chemical compositions calculated from XPS data differ strongly from those obtained from microanalyses which are in good agreement with theoretical composition described in the literature. XPS analysis of both the softwood surfaces indicated high carbon contents explained by migration of lipophilic extractives to the surface under the influence of the vacuum necessary for XPS analysis. Nonvolatile extractives contained in wood were extracted and deposited on glass plates and analysed. Survey and detailed C1s spectra indicated similar signals to those recorded on wood surfaces. This phenomenon was not observed when samples had been previously extracted before analysis. These results strongly evidenced that extractives present in both species are able to migrate through resin canals from the bulk of the sample to the surface when put into ultra high vacuum. XPS presents, therefore, some limits in the case of the analysis of softwood species containing extractives able to migrate to the surface during analysis. This behaviour, difficult to control, could lead to erroneous interpretations due to extractives enrichment of the surface under the effect of vacuum.

Influence of Tropolone on Poria placenta Wood Degradation

ABSTRACT Fenton reactions are believed to play important roles in wood degradation by brown rot fungi. In this context, the effect of tropolone (2-hydroxycyclohepta-2,4,6-trienone), a metal chelator, on wood degradation by Poria placenta was investigated. Tropolone (50 μM) strongly inhibits fungal growth on malt agar, but this inhibition could be relieved by adding iron salts. With an experimental system containing two separate parts, one supplemented with tropolone (100 μM) and the other not, it was shown that the fungus is able to reallocate essential minerals from the area where they are available and also to grow in these conditions on malt-agar in the presence of tropolone. Nevertheless, even in the presence of an external source of metals, P. placenta is not able to attack pine blocks impregnated with tropolone (5 mM). This wood degradation inhibition is related to the presence of the tropolone hydroxyl group, as shown by the use of analogs (cyclohepta-2,4,6-trienone and 2-methoxycyclohepta-2,4,6-trienone). Furthermore, tropolone possesses both weak antioxidative and weak radical-scavenging properties and a strong affinity for ferric ion and is able to inhibit ferric iron reduction by catecholates, lowering the redox potential of the iron couple. These data are consistent with the hypothesis that tropolone inhibits wood degradation by P. placenta by chelating iron present in wood, thus avoiding initiation of the Fenton reaction. This study demonstrates that iron chelators such as tropolone could be also involved in novel and more environmentally benign preservative systems.

Effect of heat treatment on extracellular enzymatic activities involved in beech wood degradation by Trametes versicolor

Effect of heat treatment on extracellular enzymes involved in wood degradation by Trametes versicolor was investigated. Heat-treated and untreated beech blocks were exposed to T. versicolor on malt-agar medium and extracellular enzymatic activities investigated. A strong ABTS oxidizing activity has been detected during the first stage of colonization in both cases, while cellulase activities are mainly detected in the case of untreated beech wood. Further investigations carried out on holocellulose, isolated using sodium chlorite delignification procedure and subjected to heat treatment or not, indicate that commercially available cellulases and xylanases are able to hydrolyse untreated holocellulose, while heat-treated holocellulose was not affected. All these data suggest that chemical modifications of wood components during heat treatment disturb enzymatic system involved in wood degradation.

Transcriptomic Responses of Phanerochaete chrysosporium to Oak Acetonic Extracts: Focus on a New Glutathione Transferase

ABSTRACT The first steps of wood degradation by fungi lead to the release of toxic compounds known as extractives. To better understand how lignolytic fungi cope with the toxicity of these molecules, a transcriptomic analysis of Phanerochaete chrysosporium genes was performed in the presence of oak acetonic extracts. It reveals that in complement to the extracellular machinery of degradation, intracellular antioxidant and detoxification systems contribute to the lignolytic capabilities of fungi, presumably by preventing cellular damages and maintaining fungal health. Focusing on these systems, a glutathione transferase (P. chrysosporium GTT2.1 [PcGTT2.1]) has been selected for functional characterization. This enzyme, not characterized so far in basidiomycetes, has been classified first as a GTT2 compared to the Saccharomyces cerevisiae isoform. However, a deeper analysis shows that the GTT2.1 isoform has evolved functionally to reduce lipid peroxidation by recognizing high-molecular-weight peroxides as substrates. Moreover, the GTT2.1 gene has been lost in some non-wood-decay fungi. This example suggests that the intracellular detoxification system evolved concomitantly with the extracellular ligninolytic machinery in relation to the capacity of fungi to degrade wood.

Selection and validation of enzymatic activities as functional markers in wood biotechnology and fungal ecology

The dead wood and forest soils are sources of diversity and under-explored fungal strains with biotechnological potential, which require to be studied. Numerous enzymatic tests have been proposed to investigate the functional potential of the soil microbial communities or to test the functional abilities of fungal strains. Nevertheless, the diversity of these functional markers and their relevance in environmental studies or biotechnological screening does still have not been demonstrated. In this work, we assessed ten different extracellular enzymatic activities involved in the wood decaying process including β-etherase that specifically cleaves the β-aryl ether linkages in the lignin polymer. For this purpose, a collection of 26 fungal strains, distributed within three ecological groups (white, brown and soft rot fungi), has been used. Among the ten potential functional markers, the combinatorial use of only six of them allowed separation between the group of white and soft rot fungi from the brown rot fungi. Moreover, our results suggest that extracellular β-etherase is a rare and dispensable activity among the wood decay fungi. Finally, we propose that this set of markers could be useful for the analysis of fungal communities in functional and environmental studies, and for the selection of strains with biotechnological interests.

Characterization of a Phanerochaete chrysosporium glutathione transferase reveals a novel structural and functional class with ligandin properties

Background: GSTs are detoxification enzymes poorly characterized in fungi. Results: GSTFuA1 possesses a unique three-dimensional structure and binds wood degradation compounds at or near the glutathione binding pocket. Conclusion: This GST is a new fungal isoform that we name GSTFuA1. Significance: GSTs with binding properties could be of great interest in various biotechnological applications. Glutathione S-transferases (GSTs) form a superfamily of multifunctional proteins with essential roles in cellular detoxification processes. A new fungal specific class of GST has been highlighted by genomic approaches. The biochemical and structural characterization of one isoform of this class in Phanerochaete chrysosporium revealed original properties. The three-dimensional structure showed a new dimerization mode and specific features by comparison with the canonical GST structure. An additional β-hairpin motif in the N-terminal domain prevents the formation of the regular GST dimer and acts as a lid, which closes upon glutathione binding. Moreover, this isoform is the first described GST that contains all secondary structural elements, including helix α4′ in the C-terminal domain, of the presumed common ancestor of cytosolic GSTs (i.e. glutaredoxin 2). A sulfate binding site has been identified close to the glutathione binding site and allows the binding of 8-anilino-1-naphtalene sulfonic acid. Competition experiments between 8-anilino-1-naphtalene sulfonic acid, which has fluorescent properties, and various molecules showed that this GST binds glutathionylated and sulfated compounds but also wood extractive molecules, such as vanillin, chloronitrobenzoic acid, hydroxyacetophenone, catechins, and aldehydes, in the glutathione pocket. This enzyme could thus function as a classical GST through the addition of glutathione mainly to phenethyl isothiocyanate, but alternatively and in a competitive way, it could also act as a ligandin of wood extractive compounds. These new structural and functional properties lead us to propose that this GST belongs to a new class that we name GSTFuA, for fungal specific GST class A.

Control of wood thermal treatment and its effects on decay resistance: a review

Key messageAn efficient use of thermal treatment of wood requires a depth understanding of the chemical modifications induced. This is a prerequisite to avoid problems of process control, and to provide high quality treated wood with accurately assessed properties to the market. Properties and structural anatomy of thermally modified woods are slightly different than un-processed woods from a same wood species. So it is necessary to create or adapt new analytical methods to control their quality.ContextHeat treatment as a wood modification process is based on chemical degradation of wood polymer by heat transfer. It improves mainly the resistance of wood to decay and provides dimensional stability. These improvements, which come at the expense of a weakening of mechanical properties, have been extensively studied. Since a decade, researches focused mainly on the understanding of wood thermal degradation, on modelling, on quality prediction and quality control.AimsWe aimed at reviewing the recent advances about (i) the analytical methods used to control thermal treatment; (ii) the effects on wood decay resistance and (iii) the advantages and drawbacks of a potential industrial use of wood heating.MethodsWe carried out a literature review of the main industrial methods used to evaluate the conferred wood properties, by thermal treatment. We used papers and reports published between 1970 and 2015, identified in the web of science data base..ResultsApproximately 100 papers mostly published after 2000 were retrieved. They concentrated on: (i) wood mass loss due to thermal degradation determination, (ii) spectroscopic analyses of wood properties, (iii) colour measurements, (iv) chemical composition, (v) non-destructive mechanical assessments and (vi) use of industrial data.ConclusionsOne of most interesting property of heat-treated wood remains its decay resistance. Durability test with modified wood in laboratory are expensive and time-consuming. This review displays data from different analytical methods, such as spectroscopy, thermogravimetry, chemical analyses or mechanical tests that have the potential to be valuable indicators to assess the durability of heat treated wood at industrial scale. However, each method has its limits and drawbacks, such as the required investment for the equipment, reliability and accuracy of the results and ease of use at industrial scale.

New substrates and activity of Phanerochaete chrysosporium Omega glutathione transferases.

Omega glutathione transferases (GSTO) constitute a family of proteins with variable distribution throughout living organisms. It is notably expanded in several fungi and particularly in the wood-degrading fungus Phanerochaete chrysosporium, raising questions concerning the function(s) and potential redundancy of these enzymes. Within the fungal families, GSTOs have been poorly studied and their functions remain rather sketchy. In this study, we have used fluorescent compounds as activity reporters to identify putative ligands. Experiments using 5-chloromethylfluorescein diacetate as a tool combined with mass analyses showed that GSTOs are able to cleave ester bonds. Using this property, we developed a specific activity-based profiling method for identifying ligands of PcGSTO3 and PcGSTO4. The results suggest that GSTOs could be involved in the catabolism of toxic compounds like tetralone derivatives. Biochemical investigations demonstrated that these enzymes are able to catalyze deglutathionylation reactions thanks to the presence of a catalytic cysteine residue. To access the physiological function of these enzymes and notably during the wood interaction, recombinant proteins have been immobilized on CNBr Sepharose and challenged with beech wood extracts. Coupled with GC-MS experiments this ligand fishing method allowed to identify terpenes as potential substrates of Omega GST suggesting a physiological role during the wood-fungus interactions.