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In plywood plants, the bark of a birch tree is a readily accessible and already concentrated feedstock for further processing. It consists of two distinct layers: outer bark and inner bark. Up to 25.7 % of biologically active compounds (betulin, lupeol, betulinic acid) are concentrated in outer bark, with a broad spectrum of applications in the chemical, pharmaceutical, cosmetic and food industries. The inner bark must be separated from outer bark as well as possible because it causes a decrease in the yield and purity of the prepared ethanol extractives. Therefore, it is very important to predict the content of inner bark in the feedstock taken for the extraction process. A novel method for the characterization of feedstock was developed using the higher heating value (HHV) as a reference. The developed method for birch outer bark quality control is very useful in birch outer bark extraction plants. Thus, it would be possible to control the purity of the feedstock and to predict the potential yield of extractives as well as the amount of the solvent to be taken for the extraction process. Pure enough (≥90 % of outer bark) feedstock for biologically active extractives production can be obtained by the floating method after 5 h if the HHV is more than 32-33 MJ/kg.
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Environment. Technology. Resources, Rezekne, Latvia
Proceedings of the 11
th
International Scientific and Practical Conference. Volume III, 282-285
ISSN 1691-5402
© Rezekne Academy of Technologies, Rezekne 2017
http://dx.doi.org/10.17770/etr2017vol3.2550
A Novel Method for Birch Outer Bark
Quality Control Using Higher
Heating Value
Janis Rizhikovs
1
, Aigars Paze
1
, Ance Plavniece
1
, Kristaps Stankus
2
, Inguss Virsis
2
Latvian State Institute of Wood Chemistry, 27 Dzerbenes street, Riga, LV-1006, Latvia
1
Latvijas Finieris AS, 59 Bauskas street, Riga, LV-1004, Latvia
2
Abstract. In plywood plants, the bark of a birch tree is a readily accessible and already concentrated feedstock for
further processing. It consists of two distinct layers: outer bark and inner bark. Up to 25.7 % of biologically active
compounds (betulin, lupeol, betulinic acid) are concentrated in outer bark, with a broad spectrum of applications in the
chemical, pharmaceutical, cosmetic and food industries. The inner bark must be separated from outer bark as well as
possible because it causes a decrease in the yield and purity of the prepared ethanol extractives. Therefore, it is very
important to predict the content of inner bark in the feedstock taken for the extraction process.
A novel method for the characterization of feedstock was developed using the higher heating value (HHV) as a
reference. The developed method for birch outer bark quality control is very useful in birch outer bark extraction plants.
Thus, it would be possible to control the purity of the feedstock and to predict the potential yield of extractives as well as
the amount of the solvent to be taken for the extraction process. Pure enough (90 % of outer bark) feedstock for
biologically active extractives production can be obtained by the floating method after 5 h if the HHV is more than 32-33
MJ/kg.
Keywords: Birch bark, feedstock quality, polysaccharides, higher heating value.
I. INTRODUCTION
Birch wood in the Northern hemisphere is widely
used in the furniture, pulp and plywood manufacture.
2 % of veneer blocks’ mass is made up of birch outer
bark (BOB) [1]. It is readily accessible and already
concentrated for further processing. In the concept of
the circular bio-economy, it is necessary to upgrade
the by-products generated in the processing of forest
products. In this context, birch plywood, furniture or
pulp plants are well established industrial enterprises
that generate known amounts of birch bark residues,
which could have considerable value as a feedstock
for the production of higher added value products, for
example, triterpene rich extractives [2], [3].
Pentacyclic lupane type triterpenes (betulin, lupeol,
betulinic acid) are promising starting materials for the
synthesis of biologically active compounds with a
broad spectrum of medical applications [4].
Additionally, after triterpene extraction, the
remaining biomass can still be used for the
production of other products from suberin
individual suberinic acids for macromolecular
materials [5] or as a hydrophobic binder for
particleboards [6]. It is known that valuable
triterpenes and suberin are concentrated in BOB, up
to 27 % [3] and 45 % [2], respectively. Birch inner
bark (BIB) has a completely different chemical
composition (Table I), especially regarding the
content of carbohydrates and hot water extractives
[7], [8].
Table I
Chemical Composition of Birch Bark Constituents Calculated on
the Oven-dry Mass
Value BOB BIB Ref.
Carbohydrates
(%) 4.4-10.4 45.2-55.0 [2,7-9]
Ash (%) 0.3-1.0 1.5-2.4 [2,8,9]
Organic solvent
extractives (%) 29.1-40.0 9.0-15.8 [2,8,9]
Hot water
extractives (%) 0.9 19.3 [8]
Suberin - NaOH
soluble extractives
(%)
34.5-45.0 25.2-25.5 [2,8,9]
Lignin (%) 2.2-9.0 9.1-18.1 [2,8,9]
HHV (MJ/kg) 34.1 21.1 Table II
The above-mentioned substances may cause a
decrease in the yield and triterpenes content in the
obtained extractives if BIB and woody admixtures are
present in the feedstock. Therefore, it is very
important to predict the content of BIB in the
feedstock taken for the extraction process.
The main aim of the study was to develop an
appropriate and precise BOB quality controlling
method. Therefore, two different BIB and BOB
analysis methods were used, which could serve on an
industrial scale:
Janis Rizhikovs, et al./ Environment. Technology. Resources, (2017), Volume III, 282-285
283
1) Easily- and hardly hydrolysable
polysaccharides (EHP and HHP) due to the difference
of BIB and BOB in carbohydrates;
2) Higher heating value (HHV) because that for
BOB is 1.5 times higher and HHV for BIB is similar
to that of wood.
II. MATERIALS
AND
METHODS
A. Feedstock
Birch bark, left over at a plywood factory in
Latvia, was selected as a representative industrial
waste with the relative moisture content 35-40 %.
The collected feedstock was dried at room
temperature to a moisture content of 4-7 % and
milled in a cutting mill SM 100 (Retsch GmbH & Co)
to pass the sieve with holes of diameter 2.00 mm.
Milled dry birch bark samples were soaked in
deionized water for 48 h by occasional mixing. Birch
outer bark, floated to the top of the water surface
(BOBF), was collected and used as a reference raw
material for the pure BOB sample. To determine the
optimal floating time and suitability of the above-
mentioned BOB control method based on HHV, a
flotation experiment was carried out, in which dry
birch bark (200 g) was soaked from 15 min to 48 h by
occasional mixing (bark/water hydro modulus 1/5).
After flotation, BOBF was dried to a moisture content
of 4 % for further operations. The BIB, which sank to
the bottom, was collected and dried to a moisture
content of 7 % for further operations. For elemental
and HHV analysis needs, the samples were
additionally ground to the particle size below 0.5 mm
directly prior to analyzing.
For EHP and HHP analysis, the collected birch
bark was separated from the BIB by hand to prevent
the leaching of polysaccharides during the flotation.
B. Analytical Methods
Moisture and ash content was determined
according to EN 14774 and EN 14775 standards,
respectively.
Elemental analysis was performed according to
EN 15104 on an Elementar Analysensysteme
GmbH – vario MACRO CHNS Element Analyzer
an analyzer used for the determination of C, H and N
in solid and liquid samples, using a thermal
conductivity detector.
The content of EHP was determined by mild
hydrolysis of approximately 5.0 g of the sample with
2 % HCl for 3 h. The hydrolyzed sample, after EHP
analysis, was further hydrolyzed by 80 % H
2
SO
4
for 5
h and the content of HHP was determined [10].
HHV was determined according to the EN 14918
standard. Measurements were performed on a “Parr”
Oxygen Bomb Calorimeter, in which approximately
1.0 g of an oven-dried birch bark sample was
completely combusted under a pressurized (3000
kPa) oxygen atmosphere. The rise in temperature of
the cylinder allows the calculation of the calorific
value when the exact weight of the sample is known.
The net calorific value was calculated on a dry basis
(MJ/kg) to compare the samples.
All analyses were performed in triplicate, and
their average values did not exceed a 1.0 % variation.
C. Experimental Procedure
At first separated by hand, milled dry birch outer
(BOBH) and inner (BIBH) bark samples were mixed
together in five proportions (0, 30, 50, 70 and 100 %
of the BIB admixture calculated on the oven-dry
mass) for EHP and HHP analysis, and for creation of
a calibration curve. When the more precise HHV
method was chosen, additional points in the curve
were added (10, 20, 40, 60, 80 and 90 % of the BIB
admixture), the prepared barks’ mixture samples were
analyzed and a calibration curve was created.
III. RESULTS
AND
DISCUSSION
A. Easily- and Hardly-hydrolyzable
Polysaccharides
As mentioned in the introduction, valuable
triterpenes are concentrated in BOB. The purity of
BOB depends on the separation methodology, as well
as the starting feedstock’s composition (admixture of
BIB, woody particles). Therefore, it is very important
to predict the content of BIB in the feedstock taken
for the extraction process.
Because BOB has a very low content (4-10 %) of
total sugars (carbohydrates) [2], [8], [9], but BIB has
a relatively high content (45-55 %) of carbohydrates
[7], [8], our first idea was to use this difference
between these two representative parts of birch bark
for determining its purity.
Mixed BIBH/BOBH samples in five proportions,
as described in the experimental section, were
prepared. The content of EHP and HHP was
determined. The obtained results are shown in Fig. 1.
Both EHP and HHP content logically increases with
increasing BIBF proportion in the sample.
Fig. 1. Calibration curves for the content of easily- and hardly-
hydrolyzable polysaccharides in the prepared birch bark mixtures.
It can be concluded that the EHP content shows
better correlation (R=0.988) than the content of HHP
(R=0.967), and the method for determining the EHP
Environment. Technology. Resources, Rezekne, Latvia
Proceedings of the 11
th
International Scientific and Practical Conference. Volume III, 282-285
284
content is less time-consuming. Still, the method has
some disadvantages, which were observed during the
research.
It is very difficult to obtain 100 % pure BOB if
separated mechanically by hand or by winnowing.
Fig. 1 shows that the total content of EHP and HHP
in pure BOBH is already 6.9 % (5.3 % + 1.6 %), and
it is reported in the available literature that the
content of carbohydrates can vary in the range of 4.4-
10.4 % (Table I), which means that there are some
BIB admixtures left after mechanical separation.
It is possible to obtain pure BOB by the floating
method, but then it is difficult to calibrate EHP
because, during the floating in a water environment,
about a half of EHP could be leached off from BIB.
Besides, there are also woody particles in the BOB
and as we know, the EHP content in the birch wood
(up to 28 % [11]) is higher than that in BIB (24.1 %).
Of course, it is possible to make some assumptions
but it will not improve the obtained results, and the
controlling process of the BOB quality will not be
precise enough.
B. Higher Heating Value and Elemental
Composition
The above-mentioned obstacles (leaching of
polysaccharides from BIB, non-homogeneous
admixture composition) enabled us to find some
properties which would not have so many variables
depending on the BOB separating methodology.
During the routine BOB and BIB analysis, a
parameter – HHV – was revealed. It can be seen from
Table II that HHV for pure BOB was more than 1.5
times higher than that for BIB calculated on the oven-
dry mass. In addition, HHV for birch wood was close
to that for BIB. This gave a hope that it would be
possible to obtain a more precise calibration curve for
controlling the BOB purity.
There are some differences between BOB samples
depending on the separation method. The highest
HHV was for BOBF – 34.1 MJ/kg; therefore, this
was chosen as a reference feedstock for pure BOB in
the calibration experiments. BIBH was chosen as a
representative of the 100 % admixture because its
HHV (21.1 MJ/kg) was almost the same as for birch
wood (21.2 MJ/kg). This means that the origin of the
admixture – BIB or wood particles – will not affect
the HHV.
Table II
Higher Heating Value and Ash Content of Birch Log Components
Calculated on the Oven-dry Mass
Sample HHV
(MJ/kg) Ash content
(%)
Birch wood (BW) 21.2 ± 0.0 0.4 ± 0.0
Birch bark (BB) 26.0 ± 0.1 1.0 ± 0.0
Inner bark (floating) (BIBF) 21.5 ± 0.0 2.4 ± 0.1
Inner bark (by hand) (BIBH) 21.1 ± 0.1 2.4 ± 0.1
Outer bark (floating) (BOBF) 34.1 ± 0.0 0.5 ± 0.0
Outer bark (by hand) (BOBH) 32.2 ± 0.1 1.0 ± 0.0
Outer bark from log (BOBL) 33.3 ± 0.0 0.3 ± 0.0
Table III
Elemental Composition of Birch Log Components
Sample
*
C (%) N (%) H (%) O (%)**
BW 48.9 ± 0.1 0.2 ± 0.0 5.0 ± 0.1 46.0 ± 0.2
BB 67.5 ± 0.1 0.5 ± 0.0 6.0 ± 0.1 26.0 ± 0.1
IBF 52.7 ± 0.1 0.5 ± 0.0 5.3 ± 0.3 41.5 ± 0.2
IBH 52.4 ± 0.3 0.5 ± 0.0 5.2 ± 0.2 41.9 ± 0.2
BOBF 71.3 ± 0.1 0.4 ± 0.0 8.5 ± 0.1 19.7 ± 0.1
BOBH 70.4 ± 0.2 0.5 ± 0.0 6.3 ± 0.1 22.9 ± 0.1
BOBL 70.9 ± 0.1 0.4 ± 0.0 7.4 ± 0.1 21.3 ± 0.1
* Abbreviations are explained in Table II
** By difference
The BIBF obtained after floating has a close
enough HHV (21.5 %), but still somewhat higher
because of some BOB particles sediment together
with the BIB fraction.
BOBL was collected from a birch log before the
soaking operation in a plywood factory. This sample
showed that, after soaking and floating operations,
small amounts of EHP were extracted, which led to
an increase in HHV.
The elemental composition shown in Table III
testifies the above-mentioned facts. The HHV of
BOB is higher than that of BIB because it has a
higher content of carbon and hydrogen, as well as a
lower content of oxygen due to a lower amount of
carbohydrates in the structure.
After the decrease of oxygen, also HHV can be
prognosticated because that for BIB samples is
approximately 1.5 times higher than for BOB
samples.
C. Calibration Curve of HHV
In the light of known facts, the calibration curve
for the characterization of the feedstock was
developed using HHV as a reference to determine the
content of the pure BOB (Fig. 2).
Fig. 2. Calibration curves for the HHV in the prepared birch bark
mixtures.
HHV increases with the increase of the BOB
proportion in the sample. Such a method turned out to
be more accurate (R=0.995) and faster (45 min).
Obtained equation of the calibration curve allow
Janis Rizhikovs, et al./ Environment. Technology. Resources, (2017), Volume III, 282-285
285
calculating the amount of impurities, depending on
the calorific values determined experimentally.
D. Impact of the Floating Time on HHV
To determine the optimal floating time, an
experiment was carried out, in which equal dry birch
bark samples were soaked in distilled water and the
HHVs were determined as a reference for pure
BOBF. Experimental results are shown in Table IV.
Table IV
Impact of the Floating Time on the Yield and Higher Heating
Value of Birch Outer Bark
Floating time
(h) Floating yield
(% dry mass) HHV (MJ/kg)
0 (BB) 100 26.0 ± 0.1
0.25 52.3 ± 0.1 29.4 ± 0.1
1 48.0 ± 0.1 30.8 ± 0.0
3 43.2 ± 0.0 31.3 ± 0.1
5 39.5 ± 0.1 32.2 ± 0.0
12 38.7 ± 0.1 32.5 ± 0.1
24 36.9 ± 0.1 33.5 ± 0.0
48 (BOBF) 35.4 ± 0.1 34.1 ± 0.0
It is obvious that, with increasing floating time,
the yield of BOBF decreases while the HHV
increases due to the increase of the BOB proportion
in the obtained sample. To obtain a sufficiently pure
BOBF, the flotation time must be at least 5 h, in
which the yield of the floated BOBF is 39.5 % from
the dry birch bark. The BOBF obtained by the
floating method can be regarded as qualitatively and
sufficiently purified from birch inner bark if its
combustion heat is above 32-33 MJ/kg (BIB content
would be below 10 %).
Therefore, the developed method for the BOB
quality control is very useful in BOB extraction
plants. Thus, it would be possible to control the purity
of the feedstock and to predict the potential yield of
extractives as well as the amount of the solvent to be
taken for the extraction process.
IV. CONCLUSION
If we compare EHP and HHP, then the EHP
content shows better correlations (R=0.988) than the
content of HHP (R=0.967), and the method for
determination of the EHP content is less time-
consuming. Still, the method has some
disadvantages – it is very difficult to obtain 100 %
pure BOB if separated mechanically by hand or by
winnowing.
The HHV for pure BOB was more than 1.5 times
higher than that for BIB. Therefore, the calibration
curve for the characterization of the feedstock was
developed using HHV as a reference to determine the
content of pure BOB, which was more accurate
(R=0.9955) and faster (45 min).
Pure enough (90 % of BOB) feedstock for
triterpene production can be accepted if the HHV is
more than 32-33 MJ/kg.
V. ACKNOWLEDGMENTS
The study was funded in accordance with the
contract No. 1.2.1.1/16/A/009 between "Forest Sector
Competence Centre" Ltd. and the Central Finance and
Contracting Agency, concluded on 13 October 2016.
This study was partly supported by the European
Regional Development Fund (ERDF). Project No.
1.1.1.1/16/A/042.
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Suberin, an aliphatic-aromatic cross-linked natural polymer present in the outer tissues of numerous vegetable species, is discussed in terms of (i) its occurrence, particularly where it dominates the bark composition of some trees, (ii) its macromolecular structure and positioning within the cell wall, (iii) its controlled chemical splicing (depolymerization through ester cleavage), (iv) the qualitative and quantitative composition of the ensuing monomeric fragments, and (v) the exploitation of this mixture of monomers in macromolecular science, both as a possible functional additive and as a source of novel materials. The presence of terminal carboxylic and hydroxy groups and of side hydroxy and epoxy moieties on the long chains of suberin “monomers” makes them particularly suited as building blocks for polymers with original architectures and interesting properties.
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Betulin (lup-20(29)-ene-3beta,28-diol) is an abundant naturally occurring triterpene and it is found predominantly in bushes and trees forming the principal extractive (up to 30% of dry weight) of the bark of birch trees. Presently, there is no significant use for this easily isolable compound, which makes it a potentially important raw material for polymers and a precursor of biologically active compounds. Betulin can be easily converted to betulinic acid, which possesses a wide spectrum of biological and pharmacological activities. Betulinic acid has antimalarial and anti-inflammatory activities. Betulinic acid and its derivatives have especially shown anti-HIV activity and cytotoxicity against a variety of tumor cell lines comparable to some clinically used drugs. A new mechanism of action has been confirmed for some of the most promising anti-HIV derivatives, which makes them potentially useful additives to the current anti-HIV therapy. Betulinic acid is specifically cytotoxic to several tumor cell lines by inducing apoptosis in cells. Moreover, it is non-toxic up to 500 mg/kg body weight in mice. The literature concerning derivatization of betulin for structure-activity relationship (SAR) studies and its pharmacological properties is reviewed.