Characterisation and Fractioning of Tectona Grandis Bark in View of Its

Home , Tectona

Industrial Crops and Products 50 (2013) 166–175

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Industrial Crops and Products

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Characterisation and fractioning of Tectona grandis bark in view of its

valorisation as a bioreﬁnery raw-material

a a a,b a,∗ a

Isabel Baptista , Isabel Miranda , Teresa Quilhó , Jorge Gominho , Helena Pereira

Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal

Centro das Florestas e Produtos Florestais, Instituto de Investigac¸ ão Cientíﬁca Tropical, Tapada da Ajuda, 1349-017 Lisboa, Portugal

r t a b

i s

c l e i n f o t r a c t

Article history: The anatomy and chemical composition of Tectona grandis bark from mature trees in East Timor

Received 6 March 2013

are described as well as the characterisation of fractionation by grinding and granulometric

Received in revised form 5 June 2013 separation.

Accepted 2 July 2013

Teak bark is composed of secondary phloem, periderm and a narrow rhytidome that included various

periderms with phloem tissues between them. The layer of phellem cells in each periderm was thin.

Keywords:

The phloem showed an orderly stratiﬁcation with tangential bands of ﬁbres in concentric rings that

Tectona grandis

alternated with thin bands of axial parenchyma and sieve tube elements. Abundant prismatic calcium

Teak bark

Anatomy oxalate crystals were present.

The bark fractured easily into clean particles. The yield of ﬁnes was low and 64.4% of the particles were

Chemical composition

Fractionation over 2 mm.

The mean chemical composition of teak bark was: ash 18.5%, total extractives 10.7%, lignin 20.0%

and suberin 1.9%. The polysaccharides, corresponding to approximately 47%, showed a predominance

of glucose (60.3% of total neutral monosaccharides) and an important content of xylose (20.0%). The

content of rhamnose was also comparatively high (4.9%). The content of soluble phenolics was 1.6%.

Ash elemental composition showed the predominance of calcium, representing about 93% of the total

inorganics, followed by potassium (4.8%) and magnesium (1.9%).

Extractives were present preferentially in the ﬁnes, with about 30% more extractives than the coarse

fraction. Lignin content and monosaccharide composition were similar in the different bark fractions. A

difference between fractions was found in relation to suberin content which was lower in the ﬁnes: 0.6%

and 3.5% in the ﬁne and medium fractions, respectively.

1. Introduction depend on the particular species’ anatomy and physical character-

istics (Miranda et al., 2012a, 2012b).

Valorisation of residual biomass is a strategic issue in line with Bark is structurally complex and comprises different tissues:

present preoccupations regarding sustainability and overall eco- phloem, periderm and rhytidome (Evert, 2006). The phloem

logical footprint of materials and energy. Tree barks are interesting includes a functional non-collapsed phloem in the inner part and

resources that have large potential since are available in great a non-functional collapsed phloem to the outside; the periderm

amounts, i.e. from forest operations and industrial processing, and includes phellogen, phellem and phelloderm; the rhytidome corre-

at the same time, barks present a structural and chemical complex- sponds to all the dead tissues isolated by the last formed periderm,

ity that make them well suited to integrate bioreﬁnery platforms. i.e. phloem and periderms. In hardwoods, phloem is made up of

Prior to processing, barks usually undergo pre-treatments, different cells: the sieve tube elements and associated parenchyma

namely of physical nature, to facilitate the subsequent component cells, the axial parenchyma and radial parenchyma cells, the ﬁbres

extraction or material use (Wyman, 1999). For instance, mechan- and sclereids.

ical fractioning disrupts the cellular tissues and may be used to Due to their structural complexity, bark sampling, characteri-

separate fractions of differing composition although fraction yields sation and processing have difﬁculties that are not found in wood

processing. Knowledge on the bark structure is also essential for

an estimation of its potential use (Furuno, 1990). Therefore, bark

valorisation, namely if envisaged within a bioreﬁnery platform,

∗ requires a careful examination of composition and processing char-

Corresponding author. Tel.: +351 21 365 3378.

E-mail address: [email protected] (J. Gominho). acteristics.

I. Baptista et al. / Industrial Crops and Products 50 (2013) 166–175 167

Barks are also chemically complex: they are usually rich in sections were mounted on Kaiser glycerine and after 24 h drying,

extractives, including organic solvent and water solubles, and in the lamellas were submerged in xylol for 30 min to remove the

polyphenolics, and they also contain a high amount of inorganic adhesive, dehydrated on 96% and 100% alcohol, and mounted on

material (Fengel and Wegener, 1984; Pereira et al., 2003). Eukitt.

In this study we address the case of teak bark. Teak (Tectona Individual specimens were taken sequentially from the cam-

grandis L.) grows naturally in South East Asia and is now one of bium towards the periphery and macerated in a solution of 30%

◦

the most important species for tropical plantation forestry, mostly H2O2 and CH3COOH 1:1 at 60 C for 48 h and stained with astra

under intensive short rotation management. Teak is one of the most blue. Light microscopic observations were made using Leica DM LA

famous timbers in the world, renowned for its dimensional stabil- and photomicrographs were taken with a Nikon Microphot-FXA.

ity, extreme durability and strength. The terminology follows mainly in Trockenbrodt (1990) and Richter

Teak bark was considered an important ﬁbre resource (Soni et al. (1996).

et al., 1980). The bark is rich in tannins and phenolic compounds Teak bark samples were observed by scanning electronic

(Babu et al., 2010) and extracts have shown the presence of sterols, microscopy (SEM). The samples were vacuum dried and gold was

anthraquinones, triterpenic, hemi-terpenic and naphthalene com- vapour sprayed making up an approximately 450 A thick coating.

pounds (Gaikwad et al., 2011; Khan and Mlungwana, 1999). Teak The surfaces were observed in an electron scanning microscope

bark is traditionally valued and used as a sweet, acrid astringent to Hitachi S-2400 at magniﬁcations ranging from 50 to 1000×, and

treat various anthropogenic ailments such as diabetes, bronchitis, the images were recorded in digital format. The scanning electron

constipation and skin diseases, in line with an ayurvedic function microscope was attached to a Bruker EDX (Energy Dispersive X-

(Ghosh, 2006; Khan and Mlungwana, 1999; Chopra et al., 1956). Ray Spectroscopy) detector using an acceleration voltage of 20 kV

The pharmacognostic and phytochemical use of teak bark were at magniﬁcations of 50–1000×. The images were recorded in digital

related to its anatomical structure (Akhtar et al., 2011; Gaikwad and format.

Mohan, 2011). The anatomy of teak bark was studied in relation

to particular structural aspects (Inamdar and Gangadhara, 1974;

2.3. Fractioning

Ghouse et al., 1977), taxonomy (Gottwald and Parameswaran,

1980) and to differentiate from other closely related species

The bark with 12% of moisture was ground in a knife mill (Retsch

(Goswami et al., 2010). The seasonal development of phloem in

SM 2000) using an output sieve of 10 mm × 10 mm and posteri-

different regions was also investigated (Lawton and Lawton, 1971;

orly sieved using a vibratory sieving apparatus (Retsch AS 200

Rajput and Rao, 1997, 1998; Rao and Rajput, 1999; Dié et al., 2012).

basic) with U.S. standard sieves with the following mesh sizes:

The bark of T. grandis trees from East Timor has not been char-

80 (0.180 mm), 60 (0.250 mm), 40 (0.425 mm), 20 (0.850 mm), 15

acterised. In this paper we describe the anatomy and chemical

(1.0 mm) and 10 2.0 mm. After sieving, the mass retained on each

composition of T. grandis bark from mature trees in East Timor for

sieve was weighed and the corresponding seven mass fraction

which the wood was already studied (Miranda et al., 2011) aim-

yields were determined.

ing at a full resource valorisation. We also study the fractionation

by grinding and the granulometric separation of teak bark. The

2.4. Bark density

fractions with different particle sizes were characterised with the

objective to analyse the potential of granulometric separation for

Bark basic density (p) was determined using oven-dry weight

selective component enrichment within a bioreﬁnery route of bark

and green saturated volume determined by the water immersion use. method: mp

p =

2. Materials and methods Vp

2.1. Samples where mp is sample mass (kg), and Vp is sample volume (m ).The

determination was made in 5 barks samples.

The bark samples were collected from three teak (T. grandis L.)

trees harvested in a pure teak stand with 50–60 years of age located

2.5. Bulk density and inter-granular porosity

in the northeast of East Timor. A description of site and stand are

presented elsewhere (Miranda et al., 2011; Sousa et al., 2012).

The bulk density of the granulated samples (at 12% of mois-

The trees were randomly selected from dominant trees with

ture) was determined for each sieve fraction using a cylindrical

40 cm DBH diameter class, a straight stem and the absence of appar-

container (29.8 mm height × 28.1 mm diameter) as the ratio of the

ent defects. The harvested trees had the following dimensions: BH

sample mass in the container to the volume of the container. The

diameters of 40.5, 37.9 and 43.5 cm and heights of 25.0, 22.7 and

determination was made in triplicate.

29.7 m, respectively. The harvested trees were cut into logs, brought

The inter-granular porosity (ε) was calculated from the mea-

to Portugal, and maintained at ambient conditions. The harvested

sured values of bulk density ( b) and particle density ( p) as

logs with bark were stored in indoor conditions with low light follows:

and good ventilation. The bark samples were collected by manual

ε =

debarking of the bottom log (0.5–1.3 m of stem length). 1 −

2.2. Microscopic observations

2.6. Chemical characterisation

The bark samples were impregnated with DP 1500 polyethyl-

ene glycol and transverse and longitudinal microscopic sections Ash content was determined according to TAPPI Standard T 211

◦

␮

of approximately 17 m thickness were prepared with a Leica om-02 using 2.0 g of material that were incinerated at 450–500 C

SM 2400 microtome using Tesaﬁlm 106/4106 adhesive for sam- overnight and the residues weighed.

ple retrieval (Quilhó et al., 1999). The sections were stained with The general chemical composition of the bark included deter-

a triple staining of chrysodine/acridine red and astra blue. The mination of extractives, suberin, lignin and monosaccharides.

168 I. Baptista et al. / Industrial Crops and Products 50 (2013) 166–175

Extractives were determined by successive Soxtec extractions

with dichloromethane, ethanol and water, for 1.5 h with each sol-

vent. The solvents were recovered and the extractives content

◦

determined from the mass of the solid residue after drying at 105 C,

and reported as a percentage of the original samples.

The extractive-free bark sample was used for determination

of suberin by use of methanolysis for depolymerisation (Pereira,

1988). A 1.5 g sample of extractive-free material was reﬂuxed with

100 ml of a 3% methanolic solution of NaOCH3 in CH3OH during 3 h.

The sample was ﬁltrated and washed with methanol; the residue

was reﬂuxed with 100 ml CH3OH for 15 min and ﬁltrated again.

The combined ﬁltrates were acidiﬁed to pH 6 with 2 M H2SO4 and

evaporated to dryness. The residues were suspended in 50 ml water

and the alcoholysis products recovered with dichloromethane in

three successive 50 ml dichloromethane extractions. The combined

extracts were dried over anhydrous Na2SO4, and the solvent was

evaporated to dryness. The suberin extracts, that include the fatty

acid and fatty alcohol monomers of suberin, were quantiﬁed gravi-

metrically, and the results expressed in percent of the initial dry

mass.

Klason and acid-soluble lignin, and carbohydrates contents

were determined on the extracted and desuberinised materials.

Sulphuric acid (72%, 3.0 ml) was added to 0.35 g of the material

◦

sample, and the mixture was placed in a water bath at 30 C for

1 h after which the sample was diluted to a concentration of 3%

◦

H2SO4 and hydrolysed for 1 h at 120 C. The sample was vacuum-

ﬁltered through a crucible, washed with boiling puriﬁed water and

Klason lignin was determined as the mass of the solid residue after

◦

drying at 105 C. The acid-soluble lignin was determined on the

combined ﬁltrate by measuring the absorbance at 205 nm using a

UV/vis spectrophotometer.

The polysaccharides were calculated based on the amount

of the neutral sugar monomers released by total hydrolysis,

after derivatisation as alditol acetates and separation by gas

chromatography with a method adapted from Tappi 249 cm-

Fig. 1. Bark of T. grandis: rhytidome (Rhy), periderm (Pm) and phloem (Phm) (trans-

00. The alditol acetates were separated by GC (HP 5890A gas

verse section). Scale bar = 125 ␮m.

chromatograph) equipped with a FID detector, using helium as

carrier gas (1 ml/min) and a fused silica capillary column S2330

(30 m × 0.32 mm i.d. × 0.20 ␮m ﬁlm thickness). The column pro- spectrophotometer in a Pye Unicam SP-9 apparatus (Cambridge,

◦ ◦

gramme temperature was 225–250 C, with 5 C/min heating UK) equipped with a graphite furnace GF95.

◦

gradient, and the temperature of injector and detector was 250 C.

For quantitative analysis the GC was calibrated with pure reference 3. Results

compounds and inositol was used as an internal standard in each

run. 3.1. Bark anatomy

2.7. Total phenolic content The bark of T. grandis is thin (on average 1.8 cm) with a greyish

brown colour, lightly streaked longitudinally, with a shedding of

The total phenolic content of the extracts in ethanol and water long and ﬁbrous strips. The bark included the secondary phloem

was determined using an adapted Folin–Ciocalteu method (Pereira, (with an average thickness of 0.6 cm), the periderm and a narrow

1982). Gallic acid (GA) was used as standard and the total phenolic rhytidome showing shedding of outermost periderms (Fig. 1).

content was expressed as mg GA equivalent/mL. The rhytidome included various periderms and the in-between

An aliquot (100 ␮l) of each extract, or of the gallic acid standard, dead phloem with large quantities of ﬁbres, resulting in a ﬁbrous

was mixed with 4 mL of Folin–Ciocalteu reagent (1:10 v/v). After and brittle bark structure. The periderm was somewhat undulated

3 min, at room temperature, 4 mL of aqueous Na2CO3 (7.5% m/v) with a discontinuous phellogen and comprising a phellem layer

was added, then the mixture was vortexed and further incubated in with 2–8 cells in each radial row (Fig. 2A), and a 1–2 cell phelloderm

◦

a thermostat at 45 C for 15 min and the absorbance of the resulting layer. The phellem cells were usually thin walled (Fig. 2B) but also

blue coloured mixtures was recorded at 765 nm against a blank con- included thickened cells (Fig. 2C).

taining only water (UV-2100 spectrophotometer, Unico, Shanghai, The phloem comprised a narrow non-collapsed phloem (Fig. 3A)

China). near the vascular cambium, followed by the collapsed phloem,

which occupied the largest portion of the bark (Fig. 3B).

2.8. Ash composition The phloem showed a conspicuous and very orderly strat-

iﬁcation with a regular layering of long tangential bands of

The ash determined by combustion in a mufﬂe furnace at ﬁbres in concentric rings that alternated with thin bands of axial

◦

500 C was analysed for macro- and micro-element concentra- parenchyma and sieve tube elements (Fig. 4). The ﬁbre bands

tions. After dissolving the ash in HCl, the concentrations of Ca, Mg, included 3–4 rows lined on each side by a single layer of cells

Fe, Cu, Mn, Zn, Na and K were determined by atomic absorption containing a single crystal, and were crossed by rays forming a

I. Baptista et al. / Industrial Crops and Products 50 (2013) 166–175 169

Fig. 2. Periderm of T. grandis (transverse section). (A) Formation of the periderm (Pm); (B) thin phellem cells (SEM); (C) phellem layer with thin and thick cells (arrow). Scale

bar: A and C = 50 ␮m.

conspicuous rectangular pattern (Fig. 5). The ﬁbre bands were The collapsed phloem was characterised by division and

enclosed by large and dark layers of crushed parenchyma and sieve enlargement of ray parenchyma cells and some expansion of the

tube elements. axial parenchyma cells but without strong ligniﬁcations. This was

The sieve tube elements, dispersed in strands between the axial more obvious in the outer phloem (Fig. 3).

parenchyma cells, appear on the transverse section of the non- Rays (Fig. 6) were non-storied, mainly pluriseriate, heterocel-

collapsed phloem with a rectangular and polygonal form with thin lular with procumbent to somewhat upright cells, comparable to

and unligniﬁed cell walls and one companying cell (Fig. 4A). the xylem rays of the same trees (Sousa et al., 2012; Quilhó et al.,

Fig. 3. Stratiﬁed phloem of T. grandis (transverse section). (A) Noncollapsed phloem; (B) collapsed phloem. Scale bar: A and B = 125 ␮m.

170 I. Baptista et al. / Industrial Crops and Products 50 (2013) 166–175

Fig. 4. Sieve tubes of the T. grandis. (A) Sieve tube elements (st) and companion cells (arrow) (transverse section); (B) sieve tube elements (st) (tangential section). Scale bar:

A and B = 50 ␮m.

2013). The rays followed a more or less straight course but became abundant calcium oxalate crystals as prisms in the axial

distorted around collapsed sieve tube elements towards the periph- parenchyma (Fig. 8A) and raphides in the radial parenchyma cells.

ery, and showed dilatation by enlargement and tangential cell The observations using SEM-EDX conﬁrmed calcium as the mineral

division in the outermost collapsed phloem. element of the crystals. Phenolic compounds, starch and oil were

In teak bark the formation of sclereids (Fig. 7) was scarce. observed by colour staining in axial and ray parenchyma and also

They appeared isodiametric with a polylamellate wall and pit- in the dilatation tissue, as seen in Fig. 8B for starch granules and oil

ted, sometimes enclosing a single crystal. The phloem contained droplets.

Fig. 5. Phloem ﬁbres of T. grandis. (A) and (C) tangential bands of thick ﬁbres (F) with prominent pits (arrow) lined by crystals (c) in the parenchyma cells in transverse and

tangential section, respectively; (B) crystals (SEM); (D) dissociated ﬁbres (F) and crystal (c). Scale bar: A and C = 50 ␮m; D = 25 ␮m.

I. Baptista et al. / Industrial Crops and Products 50 (2013) 166–175 171

Fig. 6. Phloem rays of T. grandis. (A) Pluriseriate rays (R) (tangential section); (B) heterocelular rays (R) with procumbent to somewhat upright cells (radial section). Scale

bar: A and B = 50 ␮m.

Fig. 7. Dilatation tissue of T. grandis. (A) Expanded parenchyma cells (Ex) and sclereids (Sc) (radial section); (B) sclereids (Sc) enclosing large crystals (c) (radial section). Scale

bar: A = 50 ␮m and B = 25 ␮m.

172 I. Baptista et al. / Industrial Crops and Products 50 (2013) 166–175

Fig. 8. (A) Crystals in bark tissues (SEM); (B) starch and oil in ray parenchyma cells of the T. grandis bark (radial section). Scale bar: B = 12.5 ␮m.

3.2. Bark fractionation

The results of the mass distribution by particle size after grinding

of teak bark are shown in Table 1.

The particle size distribution depicts a skewness of distribu-

tion with a low yield of ﬁnes and of particles below 1 mm. The

major fraction corresponded to larger particles, i.e. 64.4% of the

particles were over 2 mm. The anatomical features of T. grandis

bark, as described previously, allowed a clean fracture between the

structural elements, as shown in Fig. 9 with fractures along the

interﬁbrous tissues.

3.3. Bulk density and inter-granular porosity

The basic density of teak bark was on average 618 kg/m . The

bulk density of teak bark particles was on average 209 kg/m and

showed differences between fractions with higher values for the

ﬁner fractions below 0.250 mm, in line with a better compaction

of the particles (Table 1). The porosity (amount of air space in the

Fig. 9. T. grandis bark with fractures along the interﬁbrous, non-ligniﬁed, tissues,

bulk sample) varied between fractions from 53% to 72%.

observed by SEM.

3.4. Ash content and composition

determined as a mass weighed average of all fractions, and no dif-

ferences between fractions were found.

The ash content of the teak bark samples after milling and sep-

The ash composition of the whole bark of teak is reported in

aration into the different granulometric fractions is reported in

Table 2. The results show the predominance of calcium (6.3%),

Table 1. The whole biomass of teak had an ash content of 18.5%,

followed by potassium (0.33%) and magnesium (0.13%). Lower

contents were observed for sodium (0.015%), zinc (12.7 mg/kg),

Table 1 manganese (8.7 mg/kg) and copper (0.1 mg/kg).

Yield % (w/w) (%) after grinding and granulometric separation of teak (T. grandis)

bark and bulk density (kg/m ) and ash content (% of total dry mass) of the different

Table 2

granulometric fractions.

Elemental constituents in ash from bark of T. grandis.

Fraction (mm) Mass yield (%) Bulk density (kg/m ) Ash content (%)

Mineral Amount of element

<0.180 3.1 290.0 19.3

Ca (%) 6.3

0.180–0.250 2.0 244.8 18.6

Na (%) 0.015

0.250–0.425 3.9 217.2 17.9

K (%) 0.33

0.450–0.850 7.2 204.5 18.0

Mg (%) 0.13

0.850–1.00 3.0 188.1 18.4

Fe (mg/kg) 71.7

1.00–2.00 16.4 167.9 18.7

Cu (mg/kg) 0.1

>2.00 64.4 218.5 18.7

Zn (mg/kg) 12.7

Mean 209.2 18.5 ± 0.46 Mn (mg/kg) 8.7

I. Baptista et al. / Industrial Crops and Products 50 (2013) 166–175 173

Table 3

there was enrichment in dichloromethane and water solubles in

Summative chemical composition (% of total dry weight) of the teak (T. grandis)

the ﬁne fraction, while ethanol solubles were similar in the three

bark of three granulometric fractions after milling of teak bark: ﬁne (F, <0.180 mm),

bark fractions.

medium (M, 0.250–0.450 mm) and coarse (C, >2 mm) and of a mean bark composi-

tion (weighed mean with fraction yield). For the structural components, the observed differences were

either of small magnitude or without a consistent pattern of vari-

Mean Teak bark fractions

ation. For instance, lignin content was similar in the three bark

F M C

fractions (20.5% and 20.0%, respectively, in the ﬁne and coarse frac-

Extractives, total 10.7 14.5 12.2 10.4 tions). Similar monomeric proportion of polysaccharides was also

Dichloromethane 1.6 2.9 2.1 1.5 observed in the three bark fractions (Table 4).

Ethanol 2.9 2.7 3 2.9

A difference between fractions was found in relation to the

Water 6.1 8.9 7.1 5.9

suberin content which was lower in the ﬁner fraction: 0.6% and

Suberin 1.9 0.6 3.5 1.9

3.5% in the ﬁne and medium fractions, respectively.

Lignin, total 20.0 20.5 18.8 20.0

Klason lignin 16.0 15.8 15.8 17.0

Acid soluble lignin 3.1 4.7 3.0 3.0

Total soluble phenolics 1.6 1.6 1.1 1.6

4. Discussion

Ethanol soluble phenolics 0.9 1.1 0.5 0.9

Water soluble phenolics 0.7 0.5 0.6 0.7

4.1. Anatomical characterisation

The observations using SEM conﬁrmed the presence of calcium The observed anatomical structure of teak bark is in agree-

oxalate crystals as prisms in the axial parenchyma (Fig. 5B and 8A). ment with previous descriptions, namely regarding the regular

ﬁbres stratiﬁcation (Lawton, 1972; Rajput and Rao, 1997). Such

3.5. General chemical composition continuous tangential bands of sclerenchyma cells may serve

as a mechanical protective barrier against collapse of functional

The chemical composition of the teak bark, calculated as a mass parenchyma and sieve tubes (Machado et al., 2005), which may be

weighed average of all granulometric fractions, is shown in Table 3. an explanation why the phloem of teak has not abundant sclereids

Total extractives of teak bark were 10.7%, corresponding mainly (Lawton, 1972; Rajput and Rao, 1998; Inamdar and Gangadhara,

to polar extractives that were removed with ethanol and water 1974; Ghouse et al., 1977).

(9.0% of oven-dry bark, corresponding to 84% of the total extrac- The dilatation growth in teak bark was of reduced dimension

tives). Waxes and other non-polar compounds that are soluble in compared with other species, where prominent expansion of axial

dichloromethane make up the remaining bark extractives, repre- and radial parenchyma leads to conspicuous sclereids, i.e. in Euca-

senting 16% of the total extractives. lyptus (Quilhó et al., 1999) or Quercus spp. (Quilhó et al., 2003, 2013;

The determination of phenolics and polyphenolics in these Sen et al., 2011).

extracts showed that they represent approximately one-ﬁfth (30% Teak bark also showed a conspicuous high amount of calcium

of the ethanol extractives and 12% of the water extractives), corre- oxalate crystals (Fig. 5B and 8A). These crystals should have a sig-

sponding to 1.6% of oven-dry bark. niﬁcant role in the rigidity of bark, probably substituting sclereids

Teak bark contained 1.9% of suberin and 20.0% of lignin. The in this role.

monomeric composition of polysaccharides, which correspond to The layers of ﬁbres and parenchyma cells and sieve tubes ele-

approximately 47% of teak bark, is given in Table 4 in relation to ments (Figs. 1 and 3) might correspond to annual rings in the bark,

the proportion of neutral monosaccharides. It shows a predomi- as referred by Roth (1981) for the bark of some Verbenaceae. This is

nance of glucose (60.3% of total neutral monosaccharides) and an in agreement with our observation on the bark of T. grandis because

important content of xylose (20.0%). The content of rhamnose is the number of ﬁbre rings approximately coincides with the number

also comparatively high (4.9%). of wood annual growth rings in the same trees (44–47 rings, Sousa

et al., 2012).

3.6. Effect of particle size on bark chemical composition

The milled bark fractions were chemically characterised for the 4.2. Fractionation

fractions <0.180 mm (ﬁne), 0.250–0.450 mm (medium) and >2 mm

(coarse) (Table 3). The fractionation of teak bark upon grinding (Table 1) shows a

A particle size effect was observed on the content and compo- trend of particle size distribution similar to what was reported for

sition of extractives. Extractives were present preferentially in the other barks, i.e. Betula pendula (Miranda et al., 2012a), Pinus pinea

ﬁnes: the ﬁnest bark fraction yields about 30% more extractives (Miranda et al., 2013) and Pinus sylvestris and Picea abies (Miranda

than the coarse fraction. As regards the composition of extractives, et al., 2012b), which also showed a predominance of larger particles

over 2 mm and with little formation of ﬁnes.

The anatomical features of T. grandis bark, as described previ-

Table 4

Carbohydrate composition of cell wall in relation to the proportion of neutral ously, allow a clean fracture between the structural elements, as

monosaccharides (w/w) of the different granulometric fractions in T. grandis bark: shown in Fig. 9 with fractures along the interﬁbrous tissues. This

ﬁne (F, <0.180 mm), medium (M, 0.250–0.450 mm) and coarse (C, >2 mm) and of a

is also the explanation for the macroscopic appearance of layer

mean bark composition (weighed mean with fraction yield).

shedding of the teak bark.

Monosaccharide Mean Teak bark fractions In fact, the cellular structure of barks is in relation to their

mechanical behaviour and fractionation pattern. For instance, the

F M C

grinding of Eucalyptus globulus bark (Miranda et al., 2012a) shows

Rhamnose 5.0 5.5 4.8 5.0

a quite different fraction mass distribution. Also in studies of

Arabinose 4.2 4.6 6.2 4.1

breakage behaviour of different biofeed stocks, i.e. switch grass,

Xylose 20.4 19.7 20.9 20.4

Mannose 5.6 6.1 6.1 5.6 corn kernels, soybean seeds, wheat straw and corn stover, it was

Galactose 3.1 3.3 3.7 3.0

observed that under the same conditions each material exhibited

Glucose 61.7 60.8 58.3 61.9

a speciﬁc behaviour (Ogden et al., 2009; Yancey et al., 2009).

174 I. Baptista et al. / Industrial Crops and Products 50 (2013) 166–175

Bulk density, which is important for the handling of the mate- proportion of the phellem tissue in the periderms (Miranda et al.,

rial, depends on its composition, particle shape and size, and the 2012a).

speciﬁc density of individual particles (Lam et al., 2008). The aver-

age bulk density of teak bark particles was 209 kg/m (Table 1) but

4.4. Effect of particle size on chemical composition

there were differences between fractions with higher bulk den-

sity values for the ﬁner fractions below 0.250 mm, resulting from

The chemical composition of the different granulometric frac-

a better compaction of the particles. The high values of porosity

tions (Table 3) showed differences in relation to extractives and

(53–72%) indicate that it will be economically justiﬁed to have some

suberin. Extractives content was higher and suberin content was

form of densiﬁcation for the transportation of such bark biomass

fractions. lower in the ﬁnes. Such differences are due to the different anatom-

ical tissues that condition the distribution of sizes after grinding

Very little information exists on the bulk density of commin-

(Vázquez et al., 2001).

uted lignocellulosics. The values found here are within the range

3 3 In general, the same type of compositional changes with particle

reported for other ground barks, i.e. 277 kg/m and 169 kg/m for

size have been reported: Tamaki and Mazza (2010) and Chundawat

B. pendula and E. globulus, respectively (Miranda et al., 2012a).

et al. (2007) showed that with increasing particle size extractives

content tend to decrease and hemicelluloses and glucan content to

increase, while lignin content did not show clear trends; Ottone and

4.3. Chemical composition

Baldwin (1981) found that extractives increased with decreasing

particle size and depended on the relative amounts of phloem and

An important characteristic of teak bark was the high content

outer bark. Miranda et al. (2012a) also reported a large increase in

of inorganic material (18.5%, Table 1) and the predominance of cal-

extractives content in the ﬁne fraction.

cium (Table 2). This is in accordance with the abundant content in

As regards suberin, the only published reference refers to birch

crystals in the phloem of teak bark and the SEM-EDX observations

bark for which no differentiation in content with particle size was

conﬁrming the calcium composition (Figs. 5 and 8).

found (Miranda et al., 2012a). Suberin exists only in the cell wall of

Teak is a “calcicolous” tree species that requires a relative large

phellem (cork) cells of the periderms and therefore it is to expect

amount of calcium for its growth development and takes up Ca

that fracture and structural composition of particles will depend on

rapidly (White, 1991). Goswami et al. (2010) also reported a high

the phellem development in the periderms. In teak bark, the ﬁne

content of minerals in teak bark, although the ash content was

fraction had substantially lower content of suberin, which may be

lower at 7.3%. Kumar et al. (2009) found the highest concentrations

explained by the differing periderm features of the two species: in

for Ca in teak plantations in dry tropical region of India, although

teak, the periderm is more discontinuous around the tree circum-

the values were also under the present ones: Ca 0.38%, K 0.19%, Mg

ference, therefore allowing the formation of ﬁner particles from the

0.16% and Na 0.025%.

surrounding phloemic tissue.

Overall, the high content of mineral nutrients in the teak bark

The chemical differences of the teak bark fractions may there-

(especially Ca and K) makes this tree component an important

fore be exploited for further processing towards component

reserve of bioelements and suggests its application as a soil or fractionation.

substrate enrichment.

Another important chemical feature of teak bark is the high con-

tent of extractives, especially of polar extractives (Table 3). Similar

5. Conclusions

results were obtained by Goswami et al. (2010) who reported 2.8%

petroleum ether extract, 6.5% ethyl acetate extract, 5.2% ethanol •

The ﬁndings enhance our understanding of T. grandis bark and

extract and 8.9% aqueous extract.

show that the tissue structural complexity is a key feature that

Compared to other hardwood species, the extractives content of

has to be taken into account when envisaging bark uses.

T. grandis bark obtained here was higher than the values reported •

The high content of mineral nutrients in teak bark (especially Ca

for B. pendula (6.5%) and lower than the values for E. globulus (17.6%)

and K) makes this tree component an important reserve of bioele-

(Miranda et al., 2012a). For the barks of Fagus crenata and Quercus

ments. However, the substantial presence of calcium oxalate

mongolica, Kofujita et al. (1999) reported, respectively, 7.5% and

crystals is an important characteristic that may call for an ade-

10.5% ethanol/benzene extractives.

quate processing design.

There are no published data for teak bark regarding lignin con- •

Another important chemical feature of teak bark is the high

tent and polysaccharides composition. The lignin content (20.0%,

content of extractives, especially of polar extractives. Extraction

Table 3) is similar to the values of 19.2% and 18.6% reported by

processes may therefore be considered for a selective component

Vázquez et al. (2008) and Sakai (2001) for E. globulus bark and lower

removal from these barks.

than the 27.9% reported for B. pendula bark (Miranda et al., 2012a) •

T. grandis bark grinding and fractionation by particle size may be

and 34.6% and 24.9% for F. crenata and Q. mongolica, respectively

used to selectively enrich the ﬁne fractions in soluble materials.

(Kofujita et al., 1999). The monomeric composition of polysaccha-

rides is similar to the composition of other barks in relation to the

predominance of glucose and xylose as the second most abundant Acknowledgements

monosaccharide, although the individual compositions vary some-

what with species, i.e. Vázquez et al. (2008) for E. globulus, Bargatto This work was supported by the EU research project “AFORE

(2010) for E. grandis E. urophylla and E. grandis barks, Harkin and – Forest bioreﬁneries: Added-value from chemicals and polymers

Rowe (1971) for B. alleghaniensis and B. papyrifera. by new integrated separation, fractionation, and upgrading tech-

The teak bark contained 1.9% of suberin. This value ﬁts well nologies” under the 7th Research Framework Programme, and by

with the small amount of phellem tissue in the periderms of the Strategic Project (PEst-OE/AGR/UI0239/2011) of Centro de Estudos

rhytidome. In fact the phellem layer contained only 2–8 cells in Florestais, a research unit supported by the national funding of FCT

each radial row with thin and thick suberous cell walls (Fig. 2). The – Fundac¸ ão para a Ciência e a Tecnologia.

amount of suberin has been reported to exist in low amounts in The sampling and wood transport was only possible due to the

bark of several higher plant species like eucalypt (less than 1%) and cooperation of PADRTL (Programme for the Support of Rural Devel-

in higher amount in birch bark (5.9%) in relation with the relative opment in East Timor) from the Portuguese Government and we

I. Baptista et al. / Industrial Crops and Products 50 (2013) 166–175 175

thank all the help from the local ﬁeld team. We thank also Joaquina Miranda, I., Gominho, J., Mirra, I., Pereira, H., 2012b. Chemical characterization of

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