On the durability of Burkea africana heartwood: evidence of biocidal and hydrophobic properties responsible for durability
Ann. For. Sci.
On the durability of Burkea africana heartwood: evidence of biocidal and hydrophobic properties responsible for durability
Béli NEYA 0
Mohamed HAKKOU 0
Mathieu PÉTRISSANS 0
Philippe GÉRARDIN 0
0 Laboratoire d'Études et de Recherches sur le Matériau Bois, UMR INRA 1093, Faculté des Sciences et Techniques. Université Henri Poincaré , Nancy 1, Bld des Aiguillettes, BP 239, 54506 Vandoeuvre-lès-Nancy Cedex , France
- Analysis of the extracts contained in Burkea africana heartwood is studied with the aim of better understanding the reasons of the exceptional durability of this type of wood. The results obtained show that in spite of the fungicidal and termiticidal properties of certain products contained in the extracts, these properties do not allow to explain entirely the reasons of the durability. The strongly hydrophobic character of wood as its high dimensional stability would be also significant factors of its resistance. Analysis of the products present in the diethylether extracts indicates the presence of fats, waxes and squalene which could be the reason of the preceding results. Burkea africana / extract / durability / dimensional stability / wettability Résumé - À propos de la durabilité du bois de coeur de Burkea africana. L'analyse des matières extractibles contenues dans le Burkea africana est étudiée dans le but de mieux comprendre les raisons de la durabilité exceptionnelle de ce type de bois. Les résultats obtenus montrent que malgré les propriétés fongicides et termiticides de certains produits contenus dans les extraits, ces dernières ne permettent pas d'expliquer entièrement à elles seules les raisons de la durabilité. Le caractère hydrophobe du bois ainsi que sa forte stabilité dimensionnelle seraient également des facteurs importants de sa résistance. L'analyse des produits présents dans les extraits éthérés indique la présence de graisses, de cires et de squalène pouvant être à la base des résultats précédents.
Burkea africana Hook, also known under the common name
of Mgando or Mkarati, is a tree distributed in dry savana forests
throughout tropical Africa belonging to the family of
Leguminosae. Its wood is hard and heavy particularly useful for
constructional work such as bridges, sleepers, fences or for
toolhandles. The heartwood is dark brown or reddish brown and
described to be very resistant to fungi [
]. However, in spite
of its exceptional decay resistance, no studies had been reported
on the reasons of its natural durability and justify the studies
on the nature and on the influence of extractives on durability
of Burkea africana heartwood. Indeed, conventional
preservation methods involve the impregnation of broadly active
biocides into the wood to prevent fungal decay and insect attack
]. However, in spite of their positive contribution for the
forest products industry, wood preservation is coming under
increasing scrutiny due to its negative influence on the
environment justifying the development of new wood preservation
] or development of new plantations with naturally
durable species [
2. MATERIALS AND METHODS
Burkea africana heartwood was collected in a natural forest located
in Leo in south of Burkina Faso. Solvents used for extraction were
distilled before use excepted toluene purchased from Merck at a 99.8%
purity which was used without further purification.
2.1. Determination of the amount of extractives
Heartwood was ground to fine sawdust powder, passed through a
115 mesh sieve and dried at 80 °C before extraction with different
solvents in a Soxhlet extractor. The temperature of 80 °C was chosen
instead of the normalised 103 °C in order to avoid the possible
degradation of some of the extracts. About 50 g of wood powder was
extracted with 150 mL of toluene/ethanol mixture (2/1, v/v),
diethylether or acetone for 16 h at the rate of 8–12 cycles per h. After
extraction the solvent was evaporated under reduced pressure and the residue
dried over P2O5 in a dessicator before weighing.
2.2. Decay resistance
One white rot, Coriolus versicolor (CV) (Strain FPRL 28), and
three brown rots Poria placenta (PP) (Strain FPRL 280),
Gloeophyllum trabeum (GT) (Strain BAM Ebw 109) and Coniophora puteana
(CP) (Strain BAM Ebw 15) were used throughout this study.
2.2.2. Growth inhibition
Mycelium was grown in 9 cm Petri dishes filled with 20 mL of
maltagar medium prepared by solubilizing 15 grams of malt and 15 grams
of agar in 1 L of distilled water containing different concentrations of
extract (10, 100 and 1000 ppm). Introduction of the extracts was
carried out after medium sterilization (20 min, 120 °C, 1 bar) by addition
of the necessary quantity of extract solubilized in acetone. Plates were
inoculated by placing a 10 mm diameter plug, cut from the edge of a
starting colony growing on malt-agar medium in the center of each
plate. The cultures were kept 15 days in a growth chamber at 20 °C at
room humidity. Growth was evaluated every 2 or 3 days by measuring
the diameter of the colony estimated from the mean of two
perpendicular diameters and expressed as a percentage of the room available for
growth, i.e. the diameter of the dish. Growth inhibition was calculated
according to the formula:
Growth inhibition (%) = 100 × 1 −
where d0 is the diameter of the control culture and d1 the diameter of
the culture in the presence of extracts. Experiments where stopped
when the diameter of the control culture reached 9 cm. All experiments
are repeated three times. Controls realized with or without acetone
show no difference concerning mycelium development.
2.2.3. Decay test on blocks
Dried blocks of Burkea africana (5 × 2.5 × 1.5 cm in longitudinal,
radial and tangential directions) were used for natural durability
evaluation. Influence of extractives on durability was evaluated after
extraction with toluene/ethanol mixture (2/1, v/v), diethyl ether or
acetone in a Soxhlet extractor for 16 h. All blocks were dried at 80 °C
until stabilization of their mass and weighed before exposition to
fungi. Sterile culture medium (150 mL) prepared from malt (15 g) and
agar (15 g) in distilled water (1 L) was placed in 1 L culture bottle,
inoculated with Coriolus versicolor and incubated at 20 °C and 70%
RH to allow colonization of the medium by the mycelium. Extracted
or unextracted U.V. sterilized blocks (4 replicates) were placed in
separated bottles and exposed to fungus for 16 weeks to evaluate influence
of extractives on durability. After this period, mycelium was removed
and blocks were weighed (m1). Blocks were then dried at 80 °C until
stabilization of their mass and weighed (m2). Weight loss (WL) was
expressed as a percentage of the initial ovendried weight of the sample
according to the formula:
WL% = ((m0 – m2)/m0) × 100,
where m0 is the dried initial weight of the block and m2 the dried
weight after exposition to Coriolus versicolor. Beech blocks (Fagus
sylvatica) were used as controls and treated as described above. Wood
humidity after exposition to fungi was calculated according to the
Wood humidity % = ((m1–m2)/m1) × 100,
where m1 is the wet weight of the block after exposition to Coriolus
versicolor and m2 the dried weight of the block after exposition to
2.3. Termites resistance
Wood blocks (5 × 2.5 × 1.5 cm in longitudinal, radial and tangential
directions) were used for termite resistance tests using Reticulitermes
santonensis de Feytaud. Pine blocks were used as controls. Three
blocks extracted as above or not were placed separately with one
control in a flask on Fontainebleau sand humidified with distilled water
(1 volume for 4 volumes of sand). 256 termites (250 workers, 3 soldiers
and 3 nymphs) were placed in each flask. After eight weeks, wood
samples are classified from 0 to 4 according to the importance of the
] (Tab. I).
Wood wettability was measured by the Wilhelmy method
according to a reported procedure [
]. Wood samples used for advancing
contact angle (θa) measurement were prealably subject to Soxhlet
extraction with different solvents to evaluate the influence of
extractives. Wood samples dimensions used for θa determination are 20 ×
10 × 1 mm in tangential, radial and longitudinal directions
respectively. The measuring unit (tensiometer, processor K12 Krüss society)
consists of a force measuring system (precision 0.02 mN m–1), a
plateform drive system to raise and lower the sample automatically and
determine its position and immersion depth during a test cycle.
Measurements were performed at room condition (20 °C and about 50–60%
relative humidity). The wood sample was immersed in water along the
radial direction. The velocity of immersion was 6 mm m–1.
2.5. Dimensional stability
Influence of extractives on dimensional stability was estimated
using Anti-Swelling-Efficiency (ASE) measurements. ASE was
determined by measuring the increase in volume of extracted and
unextracted Burkea africana blocks. Blocks were immersed in
distilled water and placed inside a dessicator. A vacuum of 30 mbar was
realized during 30 min before restoration of atmospheric pressure.
Five water soaking cycles were thus applied (changing the water every
day) before determination of the water-saturated volume. Blocks were
then removed from water, wiped, and measured. The volumetric
swelling coefficients were calculated according to the formula [
S (%) = ((Vw – Vd)/Vd) × 100,
where Vw = volume of water saturated wood and Vd = volume of wood
dried at 80 °C.
The percentage of swelling was calculated from the wet and
ovendried volumes of unextracted and extracted blocks according to:
ASE (%) = ((Se – Su)/Se) × 100,
where Se = volumetric swelling coefficient of extracted samples and
Su = volumetric swellling coefficient of unextracted samples.
2.6. Chemical analysis
FTIR spectra were recorded as film between NaCl plates or KBr
disks on a Perkin Elmer FTIR spectrometer Spectrum 2000. 1H NMR
spectra were recorded on a Bruker AM 400 spectrometer. Extracts
were analyzed by combined gas chromatography-mass spectrometry
(GS-MS) on a Fisons MD 800 at an ionizing potential of 70 e.V. and
maintaining a source temperature of 200 °C. Products were separated
on a DB-5-fused silica capillary column (length: 15 m; diameter:
0.25 mm, J & W Scientific). The temperature program started at 50 °C
and held for 2 min, then raised at 10 °C/min to 250 °C and held at
250 °C for 30 min. Helium was used as the carrier gas. Identification
of products was performed using NIST library and reference samples
used to confirm attribution.
3. RESULTS AND DISCUSSIONS
The quantities of extractives contained in the wood of
Burkea africana are reported in Table II. The two methods used
to quantify these latter, namely the direct method based on the
weight of the extracts after evaporation of the solvent and the
indirect method based on the mass loss of sawdust, give very
near results indicating that practically no products are lost
during vacuum evaporation. The quantity of extracts increases
with the solvent polarity and is in agreement with the strong
contents of extractives often observed in some tropical wood
]. Figures 1, 2 and 3 report effects of the different extracts
on the growth of brown rot and white rot fungi. In all cases, a
significant effect was observed on the growth of the mycelium
at 100 and 1000 ppm of the different extracts tested. The
acetone and toluene/ethanol extracts lead to similar results on the
four fungi tested indicating probably the presence of the same
molecules. The diethylether extract has a behavior a little
different from the two precedents and seems particularly active on
the development of Gloeophyllum trabeum. No difference is
observed between white rot and brown rot fungi.
To confirm the influence of extractives on the natural
durability of Burkea africana heartwood, we studied the behavior
of blocks extracted or not exposed to Coriolus versicolor during
16 weeks. The results are reported in Table III. Independently
of the nature of the extraction, weight losses observed on
Burkea africana blocks are weak, while beech blocks are
strongly degraded. These results suggest that the durability of
Burkea africana heartwood is not only due to the presence of
extractives but depends also of other factors. Among these, the
wood hydrophobicity could be an important reason of its
exceptional durability. Indeed, wood humidity measured after
16 weeks is lower than 30%, which could create unfavourable
conditions approaching the 22% reported in the literature as
being necessary to the good development of most wood rotting
]. Contact angle measurement confirms also the
hydrophobic character of Burkea africana wood compared to
beech wood (compare 62° to 0°, Tab. IV). Influence of
extractives on the θa value indicates that diethylether extracts are
strongly hydrophobic and lead after extraction to a smaller
contact angle, while toluene/ethanol and acetone extracts, which
contain polar and apolar compounds, have a lesser effect on the
contact angle value. Explanations of the limited effect of
extractives on the ASE and θa values is probably due to the
difficulty to remove these latter one from solid wood as
demonstrated by additional experiments performed on Burkea
africana blocks (results not shown).
Hydrophobic compounds contained in diethylether extracts
take an important part in the dimensional stability of Burkea
Weight loss a (%) Wood humidity (%)
africana wood, which was demonstrated by the high values of
swelling coefficient and anti swelling efficiency measured.
Results reported in Table V indicate also that Burkea africana
heartwood is very durable toward termites.
Determination of the nature of the compounds contained in
the different extracts were investigated by FTIR, 1H NMR and
GC-MS analysis. FTIR analysis are reported in Figure 4. FTIR
spectrum of etheral extract indicates strong absorptions at
2920, 2850 and 1730 cm–1 characteristic of CH and C=O bonds
present in fats and waxes. FTIR spectra of acetone and toluene/
ethanol extracts present additional absorptions at 3350, 1600,
1515 and 1450 cm–1 characteristic of O-H and C=C bonds
corresponding to phenolic compounds which could be extracted
due to the higher solvent polarity. 1H NMR spectrum of etheral
extract confirms the presence of waxes and fats (protons of
alkyl chain between 0.9 and 1.5 ppm, protons attached to alpha
oxygenated carbon between 3.5 and 4 ppm and ethylenic
protons between 7 and 7.5 ppm). Peak at 11.5 ppm indicates also
the presence of carboxylic acid like fatty acids. 1H NMR
spectra of toluene/ethanol and acetone extracts are much more
difficult to exploit due to the poor solubility of the products.
GC-MS analysis are reported in Figure 6. All
chromatograms present the same typical peaks indicating that only a part
of the extract corresponding to the more volatile compounds
could be analyzed. Identification of the products using NIST
library indicates the presence of squalene corresponding to the
peak at 25 min. Presence of this latter was confirmed after
injection of a commercial sample of squalene which appears for the
same retention time. 1H NMR analysis of squalene allows also
to identify the product in the crude etheral extract (peaks
between 1.95–2.2 for protons attached to carbon in alpha
position of unsaturated systems and 5.1–5.2 ppm for vinylic
protons) (Fig. 5). Other peaks correspond to compounds bearing
fatty alkyl chain but were more difficult to identify.
As it was reported in the literature [
], Burkea africana
heartwood is very resistant to termites and fungal attacks.
Different reasons allow to explain this exceptional durability. The
first one concerns the presence of biocidal compounds in
extractives, which are shown to inhibit mycelium growth of
several brown rot and white rot fungi on malt agar medium. The
second reason lies in the strong hydrophobic character of
Burkea africana heartwood. Indeed, the high dimensionnal
stability of Burkea africana characterized by low swelling
coefficient compared to beech wood and its low wettabilitty
diminish humidity uptake leading to low water content which create
unfavorable conditions for the development of fungi.
These results suggest that dimensional stabilization
treatments associated with environmentally acceptable biocidal
compounds could be interesting alternatives to conventional
wood preservation products involving broadly active biocides.
Further studies are also necessary to identify the products
responsible for the biocidal properties.
 Aubréville A. , Burkea africana , in: Flore Forestière SoudanoGuinéenne, Société d'Éditions Géographiques, Maritimes et Coloniales , Paris, 1950 , pp. 244 - 246 .
 Irvine F.R. , Burkea africana , in: Woody Plants of Ghana , Oxford University Press, London, 1961 , pp. 276 - 277 .
 Dalziel J.M. , Burkea Hook, in: The useful plants of West Tropical Africa (An appendix to the Flora of West Tropical Africa), Crown Agents for Oversea Governments and Administrations, London, 1937 , p. 612 .
 Barnes H.M. , R.J. Murphy . Wood protection. The classics and the new age . For. Prod. J . 45 ( 1995 ) 16 - 23 .
 Suttie E. , Novel wood preservatives , Chem. Ind . 18 ( 1997 ) 720 - 724 .
rus atlantica Manetti) wood from southern Italy , Ann. For. Sci. 58 ( 2001 ) 607 - 613 .
 EN 117 , Determination of toxic values against Reticulitermes santonensis de Feytaud (laboratory method ), 1990 .
 Wålinder M. , Johansson I. , Measurement of wood wettability by the Wilhelmy method , Part 1, Holzforschung 55 ( 2001 ) 21 - 32 .
 Wålinder M. Ström G. , Measurement of wood wettability by the Wilhelmy method , Part 2, Holzforschung 55 ( 2001 ) 33 - 41 .
 Stamm A.J. , Wood and Cellulose Science , Ronald Press Co., New York, 1964 .
 Fengel D. , Wegener G. , Chemical analysis and composition of wood in Wood Chemistry , Ultrastructure, Reaction, Walter de Gruyter, Berlin, New York, 1984 , pp. 26 - 65 .
 Ridout B. , Post-harvest changes and decay in Timber decay in buildings . E and FN Spon , London, 2000 , pp. 23 - 36 .
 Brunetti M. , L. De Capua E. , Macchioni N. , Monachello S. , Natural durability, physical and mechanical properties of Atlas cedar (Ced Rayzal , M. , Durabilité et préservation, in: Guide de la préservation du bois, CTBA , 1998 , pp. 4 - 14 .