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Wood Vinegars: Production Processes,
Properties, and Valorization
Hakim Abdel Aziz Ouattara Florence Bobele
´Niamke
´
Jean Claude Yao Nadine Amusant Benjamin Garnier
Abstract
Pyrolysis of lignocellulosic biomass is widely used for the production of charcoal, pyroligneous liquid, and
noncondensable gases. All three are value-added products that are exploited in several fields. However, this review focuses
on three main areas: wood vinegar production methods, its physicochemical properties, and the use of wood vinegar or
pyroligneous acid in agriculture and the environment. Wood vinegar is a liquid derived from wood by the condensation of
gases and vapors released during the carbonization process, which is the transformation of wood into charcoal. It is mainly
composed of aliphatic, aromatic, and naphthenic hydrocarbons and other oxygenated compounds such as alcohols, aldehydes,
ketones, furans, acids, phenols, and ethers. Wood vinegar has antioxidant and free-radical-scavenging properties and is used
in agriculture as an antimicrobial, antifungal, insecticide, and plant germination and growth agent. It is also used in food
preservation, in medicine, and in the ecological preservation of wood. This review also examines the state of the art in
pyroligneous liquid production techniques and factors that could potentially affect its quality.
About 2,400 trees are cut down every minute in the
world. They are used for industrialization, energy sources,
and many other things (Authentic Material 2018). Accord-
ing to the International Energy Agency (2014) and Madon
(2017), wood energy accounts for an average of 70 percent
of the total energy used in Africa (Madon 2017). In Ivory
Coast, the area of natural forest has decreased from 16
million ha in 1960 to less than 2 million ha at the beginning
of the 21st century, representing an annual deforestation rate
of 300,000 ha (Lanly 1969, FAO 2003). Most of this
logging takes place in rural areas, which generate 90 percent
of the wood harvested by industry (Zobi et al. 2009).
Moreover, with the growth of the Ivorian population (from
16.4 million people in 2000 to 29.3 million people in 2022;
Kamgate 2022), wood is in even greater request in
households, particularly as firewood and charcoal. To meet
these needs, some organizations such as the Society of
Forest Development, Reducing Emissions from Deforesta-
tion and Forest Degradation (REDDþand Nitidae), the Food
and Agriculture Organization (FAO), and the Center for
International Cooperation in Agricultural Research for
Development have taken an interest in this area of wood
byproducts recovery. Their objectives are to develop
improved carbonization methods to maximize the yield of
charcoal (solid product). During this process, two other
products are obtained by these same methods but have
remained little used until now. Even if charcoal is an
excellent fuel and a strong CO
2
adsorbent (Creamer et al.
2014), bio-oil (liquid product), a mixture of water and
oxygenated compounds, is obtained from the condensation
of pyrolysis fumes and vapors and can be used in cosmetics,
medicine, agriculture after refining, and many other fields
(Kan et al. 2016). Finally, the noncondensable gases could
be used for drying wood and producing electricity using
thermal reactors (Creamer et al. 2014). For our part, only the
use of the liquid product will be detailed.
The bio-oil, which is a raw smoke condensate obtained
during pyrolysis, can be used to obtain wood vinegar after
storage in a closed container for at least 3 months and then
decantation of the sedimentation tar (Theapparat et al.
2018). Wood vinegar is a dark liquid made up mainly of
water (80% to 90%), with more than 200 natural water-
soluble compounds, the most important of which are acetic
acid, methanol, phenols, and formaldehyde (Laemsak 2010).
Table 1 shows a more comprehensive list of compounds
obtained by gas chromatography–mass spectrometry anal-
ysis in poplar wood vinegar. This table was obtained during
The authors are, respectively, PhD Student, Assistant Professor,
and Researcher, Lab. of the Synthesis Industrial Process and the New
Energies (haa.ouattara48@gmail.com/hakim.ouattara21@inphb.ci
[corresponding author]; bobele.niamke@inphb; yaojc3@yahoo.fr),
Institut National Polytechnic Felix Houphouet-Boigny from Ya-
moussoukro, Ivory Coast; Reasercher, CIRAD, ED Department,
UMR Ecofog, French Guiana (Nadine.Amusant@EcoFog.gf); and
Agricultural Engineer, Toulouse National Agronomy School (b.
garnier@nitidae.org). This paper was received for publication in
April 2023. Article no. 23-00021.
ÓForest Products Society 2023.
Forest Prod. J. 73(3):239–249.
doi:10.13073/FPJ-D-23-00021
FOREST PRODUCTS JOURNAL Vol. 73, No. 3 239
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Table 1.—Main components of poplar wood vinegar.
Retention time (min) Compounds Relative content (%) Formula M (g/mol)
Organic acids 26.97
1.93 Formic acid 0.39 CH
2
O
2
46
3.16 Acetic acid 22.99 C
2
H
4
O
2
60
4.76 Propanoic acid 0.38 C
3
H
6
O
2
74
5.59 2-Hydroxy-2-met-propanoic acid 0.11 C
5
H
10
O
3
118
9.93 Butyric acid 2.55 C
4
H
8
O
2
88
7.21 2-Oxo-n-valeric acid 0.55 C
5
H
8
O
3
116
Ketones 10.53
2.24 2-Butanone 0.37 C
4
H
8
O72
3.58 Acetoin 0.30 C
4
H
8
O
2
88
4.8 1-Hydroxy-2-butanone 0.28 C
4
H
8
O
2
88
5.31 Cyclopentanone 0.62 C
5
H
8
O84
6.15 1-Hydroxy-3-methyl-2-butanone 0.21 C
5
H
10
O
2
102
6.5 2-Cyclopentene 1.61 C
5
H
6
O82
8.81 2-Methyl-2-cyclopenten-1-one 1.44 C
6
H
8
O96
12.47 2,5-Dihydro-3,5-dimeth-2-furanone 0.66 C
6
H
8
O
2
112
13.8 2,3-Dimeth-2-cyclopenten-1-one 1.25 C
7
H
10
O 110
13.96 3-Methyl-1,2-cyclopentanedione 1.14 C
6
H
8
O
2
112
15.85 3-Ethyl-2-methyl-2-cyclopenten-1-one 0.22 C
7
H
10
O
2
126
20.48 2-Hydroxy-3-propyl-2-cyclopenten-1-one 0.38 C
8
H
12
O
2
140
29.38 1-(4-Hydroxy-3-methoxyphenyl)-ethanone 0.48 C
9
H
10
O
3
166
30.66 1-(4-Hydroxy-3-methoxyphenyl)-2-propanone 1.29 C
10
H
12
O
3
180
36.34 1-(4-Hydroxy-3-methoxyphenyl)-ethanone 0.28 C
10
H
12
O
4
196
Esters 3.85
2.37 Ethylacetate 0.67 C
4
H
8
O
2
88
9.24 Gamma-butyrolactone 2.34 C
4
H
6
O
2
86
11.32 Methy-l2-furoate 0.09 C
6
H
6
O
3
126
14.73 2,6-Dimethyl-1-cyclohexen-1-ylacetate 0.75 C
10
H
16
O
2
168
Furan derivatives 1.48
5.51 Tetrahydro-2-furanol 0.41 C
4
H
8
O
2
88
4.33 2-Methoxytetrahydrofuran 0.19 C
5
H
10
O
2
102
8.98 1-(2-Furanyl)-ethanone 0.88 C
6
H
6
O
2
110
Alkane compounds 3.36
5.79 Methoxymethyl-oxirane 0.07 C
4
H
8
O
2
88
9.17 3-Bromo-pentane 0.59 C
5
H
11
Br 150
15.63 Bicyclo-[2.2.2]-octane 2.7 C
8
H
14
110
Aldehydes 1.72
10.85 5-Methyl-2-furancarboxaldehyde 0.19 C
6
H
6
O
2
110
26.77 Vanillin 1.53 C
8
H
8
O
3
152
Phenols and derivatives 43.19
11.95 Phenol 5.91 C
6
H
6
O94
14.64 O-cresol 2.41 C
7
H
8
O 108
15.48 3-Methylphenol 3.74 C
7
H
8
O 108
15.73 Guaiacol 2.75 C
7
H
8
O
2
124
18.03 2,4-Dimethylphenol 0.89 C
8
H
10
O 122
18.11 2,5-Dimethylphenol 0.39 C
8
H
10
O 122
18.65 4-Ethyl-phenol 0.47 C
8
H
10
O 122
18.82 3,5-Dimethylphenol 0.37 C
8
H
10
O 122
19.03 2,3-Dimethylphenol 0.22 C
8
H
10
O 122
19.5 2-Methoxy-4-methylphenol 2.04 C
8
H
10
O
2
138
19.6 3,4-Dimethylphenol 0.27 C
8
H
10
O 122
20.01 Catechol 8.95 C
6
H
6
O
2
110
22.07 4-Methyl-1,2-benzenediol 4.81 C
7
H
8
O
2
124
22.51 4-Ethyl-2-methoxyphenol 1.32 C
9
H
12
O
2
152
25.14 2,6-Dimethoxy-phenol 3.56 C
8
H
10
O
3
154
25.23 2,5-Dimethyl-1,4-benzenediol 0.90 C
8
H
10
O
2
138
25.37 3,4-Dimethoxy-phenol 0.29 C
8
H
10
O
3
154
26.29 4-Ethylcatechol 1.80 C
8
H
10
O
2
138
28.13 1,2,4-Trimethoxybenzene 1.65 C
9
H
12
O
3
168
30.44 1,2,3-Trimethoxy-5-methyl-benzene 0.45 C
10
H
14
O
3
182
Nitrogen compounds 1.46
21.93 Pyridine 1.46 C
5
H
5
N79
240 OUATTARA ET AL.
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the work of Zhu et al. (2021) when determining the
chemical composition of poplar wood vinegar. It is a
product strongly affected by the raw material as well as the
carbonization technique used, as highlighted by Sutrisno et
al. (2014). These authors used different particle sizes of
Macaranga sp. wood residues, and observed differences in
the yield, chemical content (pH, phenol content, total acid),
and color of the wood vinegars (Sutrisno et al. 2014).
Known for more than 2 decades now, wood vinegar has
been the subject of research in East Asia, including
Thailand, China, Japan, Korea, and Cambodia in various
fields such as medicine, pharmaceuticals, food, and
agriculture (Amen-Chen et al. 2001, Loo et al. 2007,
Tiilikkala et al. 2010). However, wood vinegar remains little
used, if at all, in our countries. Wood vinegar is a natural
product used to improve soil quality, stimulate vegetable
growth, reduce the odor of agricultural products, repel
insects on plants, improve the quality of fruits and increase
their sugar content, facilitate composting, and promote
greater resistance to crops and adverse conditions (Uddin et
al. 1995, Shibayama et al. 1998, Yatagai et al. 2002, Mu et
al. 2006, Baimark and Niamsa 2009, Masum et al. 2013,
Mungkunkamchao et al. 2013, Mmojieje and Hornung
2015).
This review article will focus on the different ways of
obtaining wood vinegars, the physicochemical properties of
wood vinegars, the parameters likely to affect composition
and quality of wood vinegars, and the different areas of
application to date.
Chemical Composition of Wood Vinegar
The liquid fractions from pyrolysis consist of a complex
mixture of water and an organic phase containing many
chemical compounds such as acids, alcohols, aldehydes,
ketones, ethers, esters, phenols, sugars, alkenes, furans, and
various oxygen compounds (Kan et al. 2016). The chemical
composition of the pyrolysis product is highly dependent on
the variation in the composition of the raw material. The
water content of wood vinegar also depends on the initial
water content of the biomass and the formation of water
during pyrolysis, ranging from ;15 percent by weight to an
upper limit ;30–50 percent (Mohan et al. 2006). It is light
yellow to dark brown in color, composed mainly of water
(80–90%) and more than 200 natural water-soluble
compounds, the most important of which are acetic acid,
methanol, phenols, and formaldehyde (Laemsak 2010). For
wet wood, the water content can exceed 90 percent. The
chemical composition of poplar wood vinegar presented in
Table 1 shows many more compounds. These compounds
are classified by respective families, namely organic acids,
ketones, esters, furan derivatives, alkane compounds,
aldehydes, phenols and derivatives, and nitrogen com-
pounds. Wood vinegar can be considered a microemulsion,
forming a stable single-phase mixture. The aqueous solution
(especially from the decomposition of holocellulose) acts as
a continuous phase that stabilizes the discontinuous phase of
pyrolytic lignin macromolecules by mechanisms such as
hydrogen bonding (Mohan et al. 2006, Bridgwater 2012).
This liquid fraction has various nomenclatures in the
literature, such as pyrolysis oil, bio-oil, liquid smoke, wood
distillates, pyroligneous tar, and pyroligneous acid (Bridg-
water 2003). The nature and chemical composition of wood
vinegar could also depend on residence time, heating rate,
temperature, particle size, and source of raw material
(Mohan et al. 2006, Mathew and Zakaria 2015).
Production Technology of Wood Vinegar
Wood vinegar is produced when the smoke from charcoal
production is cooled by outside air as it passes through a
chimney or flue. The cooling effect of these gases and
vapors causes the condensation of a pyroligneous liquid
(Chalermsan and Peerapan 2009). In accordance with
Laemsak’s (2010) recommendations, this liquid is refined
by a simple method. This consists of letting it be sealed in a
bottle where it will decant for 2 to 4 months to give wood
vinegar (Burnette 2013). There are several techniques for
obtaining pyroligneous liquid. The most commonly used are
production by 200-liter drum ovens, by Casamance
millstones, and by the hydrothermal process.
Production of wood vinegar with a 200-liter
drum oven
The drum kiln generally consists of a 200-liter barrel
serving as a pyrolyzer to which is connected bamboo wood
acting as a smoke condenser. On this bamboo, several holes
are made to recover the liquid smoke condensates and then
at the outlet, the noncondensable gases (CO, CO
2
,CH
4
, ...)
are evacuated. This process was supported in the literature
by Burnette (2013). Figure 1 shows pyroligneous acid
production by a 200-liter drum furnace. The first step is to
place the wood inside the kiln, then ignite it, then stop
feeding the flames, and then observe distinctly the changes
in color of the smoke emitted (from thick white to yellow
and acrid). Then extend a hollow green bamboo pole (far
end elevated to 458C) from the flue. The pyroligneous liquid
can be collected using containers fixed under one or two
holes, about 2 cm wide, drilled in the bamboo pole about 30
cm from the flue pipe connection. Finally, the recovered
condensate is left to settle in a sealed bottle for at least 3
months, during which time the components settle in three
distinct layers. The first layer at the bottom of the container
is black and contains wood tar. The second layer (below the
top layer of liquid) is the usable part of the wood vinegar.
This component will be light yellow to reddish brown in
Figure 1.—Pyroligneous acid production process by 200-liter
drum furnaces.
FOREST PRODUCTS JOURNAL Vol. 73, No. 3 241
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color. The third (top) layer is classified as light oil and will
have a layer of wood tar on top. This process has been used
by several authors on various biomasses. These include the
work of Ratnapisit et al. (2009) with rubber wood and
Nurhayati et al. (2005) on acacia wood (Acacia mangium).
This technology was also developed by Phineath in 2018
(Keerati 2019). This same technology allowed Jain and
Chavan (2013) to produce charcoal and vinegar from
bamboo pieces of 15 cm split vertically. In total, 16 bamboo
pieces, weight 90 kg, were used for the production of 22.30
kg of charcoal and 2.24 liters of wood vinegar. This
technique shows a low performance for the production of
wood vinegar, with about 2.5 percent by weight (Jain and
Chavan 2013).
Production of wood vinegar with a Casamance
oven
The Casamance oven is a modified traditional type of
millstone. This type of millstone was first assembled in
Casamance, a town in Senegal, during the years 1979 to
1980 (in the United Nations Development Programme-
FAO-United Nations Sudano-Sahelian Office-Senegal proj-
ect 78/002), from which it gets its name. The Casamance
millstone has a chimney that allows for a reverse draught;
the gases heat up inside, circulate in the wood load, preheat
it, and dehydrate it. The gases then flow back down to the
bottom of the grindstone and exit through the chimney, at
which point the charcoal byproducts recombine with the
charcoal (fixed charcoal content 80–90%). Since then, with
the increase in demography in West African countries, some
have taken an interest in developing this millstone locally,
highlighting its performance in optimizing charcoal pro-
duction. These include Benin (Akouehou et al. 2012, Issifou
et al. 2020), Rwanda (Nyampeta 2004), and Cˆ
ote d’Ivoire
(REDDþand Nitidae 2019). However, no specifications on
the amount of pyroligneous liquid during the use of this
process have been obtained in these different countries,
except in Cˆ
ote d’Ivoire during the training of charcoal
makers in the Me
´region (town of Adzope
´) on the use of
improved millstones with chimneys. Several advantages
over the use of the Casamance millstone were listed, notably
the uniform and high temperature of the remaining kiln, and
a high recovery of pyroligneous liquid thanks to the
presence of the chimney. Figure 2 shows a Casamance
millstone created in Togo. The process of obtaining wood
vinegar through such a millstone is illustrated as follows
(REDDþand Nitidae 2019). The assembly of the grinder
consists of laying out stringers in the direction of the length
of the oven, placing wood on the floor, and then positioning
the chimney and covering it with earth. Then comes the
lighting stage (preheating), which is carried out with small
wood, twigs, or embers generally placed in the middle and
then plugging the ignition well. The temperature inside can
reach 6008C; the pyroligneous liquid is recovered at the base
of the chimney between 808C and 1508C. When this phase is
complete, the charge is allowed to cool and the charcoal is
removed from the furnace.
Production of wood vinegar by hydrothermal
carbonization
During the hydrothermal treatment of biomass, two
products are obtained, namely hydrochar and wood vinegar.
Sztancs et al. (2020, 2021) found that hydrochar cofired with
coal could reduce CO
2
greenhouse gas emissions and
promote low-carbon electricity generation. Hydrothermal
carbonization is carried out at high temperature (1208Cto
3508C) and pressure (2 MPa to 16 MPa) in the presence of
water (Gonza
´lez et al. 2005). The carbonization temperature
plays an important role in the hydrothermal treatment
process. A study by Wang et al. (2020) showed the influence
of temperature on the yield of wood vinegar during
hydrothermal carbonization. Indeed, the yield of wood
vinegar decreases from 70.6 to 68.8 percent when the
treatment temperature increases from 200 to 2308C and then
increases to 72.4 percent at 2608C (Wang et al. 2020). This
shows that the optimum temperature for maximizing the
yield of wood vinegar via the hydrothermal process was
2608C. The process of producing hydrochar and wood
vinegar from hydrothermal carbonization was supported by
Hernandez-Soto et al. (2019) in their studies on the
production of a material with an organic carbon content
suitable for use as a soil improver or as a substrate
component. First is the calculation of the appropriate
amounts of water and biomass for the reaction mixture.
The water content of the mixture should vary between 70
and 85 percent by weight. Then comes weighing and
introducing the previously calculated biomass and water
into the autoclave. After this, pressurization with nitrogen at
2 MPa is carried out. The agitator is started with a
temperature of 2158C for 30 minutes and maintained. The
carbonization process can last from 4 hours to overnight and
is then left to cool to room temperature after the heating is
switched off. Finally comes the recovery of the raw
hydrochar. All residual pressure is carefully released and
then the autoclave is opened to recover the hydrochar, which
is separated from the liquid phase by vacuum filtration
through a B¨uchner funnel and dried in an oven at 1058C for
2 hours. The liquid phase is left to rest for months to extract
the pyroligneous acid or wood vinegar.
Physical and Chemical Properties of Wood
Vinegar
Quality of wood vinegar
The quality of wood vinegars depends mainly on several
parameters such as pH, color, odor, dissolved tar content,
ignition residue, transparency, and specific gravity (Wada,
1997). Wood vinegars have a pH between 2 and 4 because
of the presence of acetic acid, formic acid, and propionic
acid. The total soluble tar content is between 0.23 and 0.89
Figure 2.—Production of pyroligneous acid by a Casamance
millstone.
242 OUATTARA ET AL.
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percent by weight. The specific gravity and Brix are
between 1.005 and 1.016 g/mL and between 1.7 and 6.6
respectively (Mun et al. 2007). The odor of wood vinegar is
smoky. Density and viscosity could also be important in the
physicochemical properties of vinegar. To standardize the
quality of vinegar, seven parameters have been adopted by
the Japan Pyroligneous Liquor Association, which is an
industrial organization for the pyroligneous liquor trade.
These standards are as follows (Wada 1997):
1. pH of the vinegar should be about 3.
2. Standard specific gravity should be between 1.010 and
1.050 g/mL.
3. The product should have a pale yellow, light brown, or
red-brown color.
4. The product should have a marked smoky odor.
5. The product must have a dissolved tar content of less
than 3 percent.
6. The ignition residue must be less than 0.2 percent.
7. The product should have a transparency with no
suspended solids.
A study by Theapparat et al. (2014) on the physical and
chemical properties of eight wood vinegars from five wood
species (Leucaena leucocephala [katin], Azadirachta indica
[sadao], Eucalyptus camaldulensis,Hevea brasiliensis
[rubber tree], and Dendrocalamus asper [bamboo]) grown
in Thailand, produced by heating wood samples to 4008C.
Table 2 highlights the parameters for assessing the quality
of these wood vinegars. Only two wood vinegars, those
from rubber tree and from bamboo, unanimously met all the
standards of the Japan Pyroligneous Liquor Association.
Also, the pKa of 4.7 was an indicator of the major presence
of acetic acid in the extracts, which was responsible for the
pH values. The specific gravity showed good correlations
with total soluble tar and Brix level (R¼0.87 for both); in
turn, Brix level showed a good correlation with total soluble
tar (R¼0.87; Theapparat et al. 2014). Thus, the easily
determined Brix degree could be used as a general indicator
of total soluble tar. The amount of total soluble tar signified
the presence of phenolic compounds. In addition, phenolic
compounds were confirmed by the ultraviolet absorption
maximum (k
max
at 268 to 274 nm; Theapparat et al. 2014).
Parameters affecting the quality of wood
vinegar
Wood vinegar, being a product with a composition of
several molecules with different chemical functions, is
dependent on a few factors including the raw material, the
carbonization process used (type of pyrolysis), and the
decanting method.
Raw material.—Several biomasses are used, but gener-
ally these biomasses are lignocellulosic; they are composed
mainly of three major polymers: cellulose, hemicellulose,
and lignin, and other inorganic minerals as well as organic
extracts such as alkaloids, resins, sugars, starches, lipids,
proteins, and essential oils (Balat et al. 2009). The main
high-molecular-weight structural components are cellulose
and hemicellulose (65% to 75%) and lignin (18% to 35%).
There are other components that are generally inorganic
minerals and low-molecular-weight organic extractables in
the plant biomass. The mass percentage of these chemical
components varies among plant species and explains the
differences in the composition of wood vinegar. In general,
agricultural biomass consists of 40 to 50 percent cellulose,
20 to 30 percent hemicellulose, and 10 to 25 percent lignin
(Iqbal et al. 2011). In the literature, many types of biomass
are used for the production of pyroligneous acid. These
include woody plants such as eucalyptus wood (Eucalyptus
globolus; Mungkunkamchao et al. 2013), birch (Betula sp.;
Murwanashyaka et al. 2001), oak (Quercus accutissima; Lee
et al. 2011), beech (Fagus sp.; Beaumont 1985), hairy -
leafy molave (Vitex pubescens; Oramahi and Yoshimura
2013), rubberwood (Ratanapisit et al. 2009), Japanese cedar
(Cryptomeria japonica; Young-Hee et al. 2005), walnut
twigs (Juglans sp.; Wei et al. 2010), East Asian cherry
(Prunus jamaskura), Japanese chestnut (Castanea crenata;
Kimura et al. 2002), and mangrove plant (Rhizophora sp.;
Loo et al. 2007, Zulkarami et al. 2011). We also have
agricultural biomasses that include cotton stalks (Gossypium
hirsutum;P¨ut¨un et al. 2005), hickory hull (Carya sp.), and
rice (Oryza sativa) straw (Lee et al. 2005); almond (Prunus
dulcis; Sztancs et al. 2021), hazelnut (Corylus avellana),
and wheat (Triticum aestivum; Demirbas 2004) hulls;
sugarcane (Saccharum officinarum) and pineapple (Ananas
comosus) wastes (Mathew and Zakaria 2015); and sugar-
cane bagasse (Garc`
ıa-P`
erez et al. 2002), walnut hulls (Wei
et al. 2010), and tea (Camellia sinensis) wastes (Demirbas
2007).
Type of pyrolysis.—The different pyrolysis methods are
classified according to heating rates expressed in degrees
Celsius per second and residence times. In general,
conventional pyrolysis, which can be either slow or fast,
is performed mainly for biochar and liquid and gaseous
products. In contrast, fast pyrolysis is recommended to
maximize the yield of liquid and gaseous products.
Feedstock, heating rate, temperature, and production
technology are important factors for pyrolysis products
(Mathew and Zakaria 2015). High temperatures lead to the
decomposition of large molecules of liquid and residual
solids into small molecules enriched with fractional gases
Table 2. — Physical and chemical properties of eight wood vinegars.
Sample pH
Specific gravity
(g/mL)
Total acid content
(% by weight) pKa
Total soluble tar
(% by weight)
Degree
Brix
k
max
(nm) A
max
Water content
(% by weight)
Leucaena leucocephala 1 3.40 1.006 60.000 4.47 60.01 4.70 0.37 60.02 3.80 60.10 274.50 154.00 65.00 91.13 60.27
L. leucocephala 2 3.10 1.007 60.000 4.61 60.02 4.70 0.57 60.01 4.60 60.10 270.80 375.50 66.20 84.54 60.02
Azadirachta indica 1 3.40 1.004 60.000 3.28 60.03 4.70 0.37 60.02 3.00 60.10 273.40 150.50 65.00 91.50 60.06
A. indica 2 3.20 1.005 60.000 3.16 60.04 4.70 0.58 60.03 3.40 60.10 271.00 298.00 66.50 93.48 60.15
Eucalyptus camaldulensis 1 3.10 1.006 60.000 4.62 60.04 4.70 0.57 60.02 4.60 60.10 272.60 221.00 65.00 89.44 60.25
E. camaldulensis 2 3.50 1.008 60.000 3.62 60.03 4.70 0.49 60.02 3.40 60.10 270.80 393.00 68.00 90.37 60.20
Hevea brasiliensis 2.90 1.012 60.000 4.92 60.02 4.70 0.96 60.03 6.00 60.20 268.00 467.00 67.00 85.30 60.50
Dendrocalamus asper 2.90 1.010 60.000 4.92 60.02 4.70 0.71 60.01 5.60 60.20 269.20 470.00 66.00 81.44 60.26
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(Demirbas 2007). The description of these two types of
conventional pyrolysis is detailed below.
Slow pyrolysis: Traditionally, charcoal kilns, pit kilns,
charcoal heaps, or earthen kilns were used, which were later
replaced by permanent kilns in many places (Theapparat et
al. 2018). During slow pyrolysis, the temperature for
biomass decomposition is about 3008C. The vapors in slow
pyrolysis are not released as quickly as in fast pyrolysis and
the residence time of the vapors varies between 5 and 30
minutes (Bridgwater 2008). Plant biomass feedstocks are
stacked around a central channel in the furnace and heated
slowly at low temperatures to remove moisture, then
exposed to higher temperatures. The openings of the kilns
are partially sealed so that complete combustion of the
biomass does not occur. The smoke from the kiln is piped
from the main production unit to allow condensation and
cooling of the smoke. Then a natural purification is carried
out by sedimentation of the condensed water vapor for at
least 3 months. Three layers of liquid are obtained and
depending on their weight, the top layer will be light oil, the
middle layer will be brown crude pyroligneous acid, and the
wood tar will be at the bottom. Slow pyrolysis yields almost
identical proportions of charcoal (35 wt%), liquid (30 wt%),
and gas (35 wt%; Bridgwater 2008).
Fast pyrolysis: Fast pyrolysis is a more efficient, high-
temperature process in which biomass is rapidly heated and
converted into biofuel. In this process, the biomass is
heated to a high temperature (5008C) with a very short
vapor residence time (,5 s). At the end of the process, the
vapors or aerosols are rapidly cooled to produce bio-oil
(Tiilikkala et al. 2010). In fast pyrolysis, a finely ground
biomass feed is required because of the high temperatures
and heat transfer rates (Bridgwater 2003). Fast pyrolysis
produces 60 to 75 percent by weight bio-oil, 15 to 25
percent by weight biochar, and 10 to 20 percent by weight
gas (Mohan et al. 2006). In the past, pyroligneous acid was
produced in charcoal kilns, but nowadays pyroligneous
acid is produced in specialized reactors. The gaseous
compounds are condensed by the condenser to produce
wood vinegar.
Methods of decanting wood vinegar
For the pyroligneous liquid to be a future source of natural
chemicals with consistent biological activities, an efficient
separation method must be developed to generate semi-
purified bioactive components. To obtain more specific and
consistent product properties, wood vinegar can be fraction-
ated into a semipurified product. This could be achieved by
several techniques such as sedimentation, filtration, chroma-
tography, distillation, and solvent extraction.
Natural settling method.—This method is the most
widely used to obtain wood vinegar from raw bio-oil or
pyroligneous liquid. It is the simplest and most efficient
method. It consists of leaving the raw bio-oil in a sealed
container. Over time, the unstable constituents of the raw
wood vinegar will oxidize or polymerize to precipitate,
suspend, or adhere to the inner wall of the container. A thin
film of oil will form on the surface of the liquid and finally,
as an intermediate phase, the wood vinegar. All suspended
matter in the wood vinegar is removed to produce clear
wood vinegar. When the resting and filtering processes are
repeated several times, a transparent and stable wood
vinegar is obtained (Fagern¨
as et al. 2012).
Distillation method.—Distillation is a separation technol-
ogy commonly used in the chemical industry. This method
successively separates components according to their
different volatilities. It is mainly used for the separation of
liquid mixtures. In general, there are two distillation
systems: normal pressure distillation and reduced pressure
distillation. In both systems, compounds are separated
according to their respective boiling points. Charred wood
vinegar has a water content of 80 to 90 percent, with a fairly
low boiling point difference between the remaining 10
percent of organic matter. Therefore, boiling of wood
vinegar starts at less than 1008C under atmospheric pressure;
then distillation continues until 250 to 2808C, after which 35
to 50 percent residue remains (Czernik and Bridgwater
2004). The distillation method is quite effective in
concentrating the wood vinegar and also in removing
substances with low or high boiling points. However, the
distillation process cannot completely remove unwanted
polymers. It is more convenient to use this method after
removing unwanted polymers from raw vinegar by the
natural decantation method. However, care should be taken,
as heating the sample to boiling may induce oxidation and
polymerization, resulting in a loss of bioactivity of the
components.
Liquid–liquid extraction.—The liquid–liquid extraction
method involves the selective transfer of a substance from
one liquid phase to another. Usually, an aqueous solution of
the sample is extracted with an immiscible organic solvent.
Thus, the solute is split between an aqueous solvent and an
organic solvent. Liquid–liquid extraction has been intro-
duced for the semipurification of active compounds from the
wood vinegar mixture. This method is used for the
separation of compounds with a different partition or
relative solubility between the two solvent phases. By
selecting the appropriate polarity of the solvents for
extraction, such as hexane, diethyl ether, ethyl acetate,
acetone, water, etc., the sample is extracted with the
appropriate solvent. To obtain higher purity of bioactive
compounds from wood vinegar, Oasmaa et al. (2003)
suggested that step-by-step extraction on the basis of
polarity order can also be used. Some reports have shown
that phenolic compounds and organic acids have been
extracted from wood vinegar using ethers and dichloro-
methane (Sipil¨
a et al. 1998, Bedmutha et al. 2011, Fagern¨
as
et al. 2012). However, they found that a considerable
amount of volatile and high-polarity compounds were lost
because of coevaporation of the compounds during the
solvent drying step. In another phenomenon, the synergistic
function was characterized as the mode of action of wood
vinegar. It has been suggested in the literature that phenolic
compounds and organic acids are active components.
However, a previous report presented other unidentified
components that could be active compounds (Bedmutha et
al. 2011). We have caffeine, which is an alkaloid presenting,
like phenol and its derivatives, natural pesticidal properties
(Bedmutha et al. 2011). During the production of bio-oil,
part of caffeine is degraded into a pyridine derivative that
could also have pesticidal properties.
Areas of Wood Vinegar Application
Wood vinegar has been studied and continues to be
studied in several fields including agriculture, medicine,
wood preservation and food preservation.
244 OUATTARA ET AL.
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Application of wood vinegar in agriculture
The use of chemical fertilizers can be quite depleting to
the soil, as their long-term application has resulted in
depletion of organic soil resources, poor water and nutrient
conservation, and deterioration of the soil structure.
Excessive chemical fertilization not only contributes to
soil, water, and air pollution, but also some residues are
found in the harvested fruit and vegetables. This contributes
to a decrease in the quality and safety of the food supply. It
was therefore important to find a natural alternative for the
production of vegetables and fruit without consumption
risks. Wood vinegar, which meets this required profile, is
therefore suitable for use in agriculture. Over the last 2
decades, wood vinegar has been the subject of research in
Southeast Asian countries, particularly in Thailand, China,
and Cambodia.
Wood vinegar as a stimulator of plant growth and
development.—Wood vinegar is a product that appears to
stimulate cell growth and acts as a catalyst for the growth of
several microbes and the activation of enzymes. The latter
are essential for various physiological and biochemical
processes in plants, such as photosynthesis, nutrient uptake,
and cell growth, but the mechanisms involved are poorly
studied. Many researchers have suggested that foliar
application of wood vinegar increases the chlorophyll
content and glossiness of plant leaves, which will increase
photosynthesis and the synthesis of sugars and amino acids.
In addition, esters such as methyl acetate and methyl
formate, which accelerate plant growth and development,
have been found in wood vinegar. Its application has had a
significant effect on the production of rice (Tsuzuki et al.
2000), sorghum (Sorghum bicolor), and sweet potato
(Ipomoea batatas; Shibayama et al. 1998). It has been
reported that wood vinegar significantly increases the yield
of rock melon (Cucumis melo), French marigold (Tagetes
erecta), zinnia (Zinnia elegans), scarlet sage (Salvia
splendens) and tomato (Solanum lycopersicum). In a study
on tomato conducted by Mungkunkamchao et al. (2013),
wood vinegar increased the total dry weight, the number of
fruits, and the fresh and dry weights of the fruits. In
addition, it effectively increased the total soluble solutes of
the fruits. Masum et al. (2013) revealed that the application
of wood vinegar increased the grain yield of rice. This was
attributed to an increase in tillers per hill, 1,000 grain
weight, and grains filled per panicle. In addition to this,
wood vinegar also increased the yield of soybean (Glycine
max; Travero and Mihara 2016), whereas foliar spray
improved the growth and yield of lettuce (Lactuca sativa),
cucumber (Cucumis sativus), and cabbage (Brassica oler-
acea var. capitata) crops (Mu et al. 2006). In addition, wood
vinegar significantly stimulated plant growth, fruit diameter,
sweetness, and fruit weight in rock melon (C. melo var.
cantalupensis; Zulkarami et al. 2011).
Wood vinegar as an organic fertilizer.—Many studies
have demonstrated the beneficial effects of pyroligneous
acid on the soil when applied as an organic fertilizer for rice
(Tsuzuki et al. 2000), sugarcane (Uddin et al. 1995), and
sweet potato (Shibayama et al. 1998). It was found that 20
percent of pyroligneous acids significantly increased the
growth and yield of watermelon (Citrullus lanatus)in
soilless culture (Zulkarami et al. 2011). In research on tea, it
is reported that the application of pyroligneous acid
increases the level of usable phosphoric acid threefold.
Root exudates in the rhizosphere include organic acids that
dissolve phosphoric acid and make it more available for
uptake by the roots. It has been suggested that the organic
acids contained in pyroligneous acid have a similar effect in
the soil.
Wood vinegar as a biopesticide.—Currently, there is
pressure for sustainable agricultural practices to minimize
overreliance on chemical use. The presence of phenolic
compounds in pyroligneous acids allows it to have
antifungal and other pest control properties (Baimark and
Niamsa 2009). Pyroligneous acid also increases the
permeability of agrochemicals into the leaf tissue. Gener-
ally, these agrochemicals are most effective when used in
combination with other acids such as wood vinegar or
pyroligneous acid at pH 4 to 5. Thus, pyroligneous acid is
known to increase the effectiveness of chemical pesticides
when used in combination. However, alkaline chemicals
cannot be mixed with wood vinegar because of their
negative reaction with acids. Mmojieje and Hornung (2015)
studied the insecticidal effect of pyroligneous acid obtained
from mixed wood biomass against green peach aphid
(Myzus persicae) and red spider mite (Tetranychus uriticae)
in the United Kingdom and found mortality of over 90
percent for both pests. In Thailand, pyroligneous acid has
been widely used as an insecticide in agriculture (Mmojieje
and Hornung 2015). For example, birch tar oil is a good
repellent against slugs (Arion lusitanicus) and snails (Aranta
arbustorum; Tiilikkala et al. 2010). The application of
pyroligneous acid resulted in the mortality of 95 percent of
the aphid population on aubergine (Solanum melongena)
when sprayed at a dilution of 1 percent (Regnault-Roger
1997). Yatagai et al. (2002) reported the termicidal effect of
pyroligneous acid against the Japanese termite (Reticuli-
termes speratus). In some publications, pyroligneous acid
has been found to be effective against houseflies (Pangna-
korn et al. 2012, 2014).
Pyroligneous acid can be used as a bioherbicide and
potentially replace synthetic chemical herbicides. Phenols,
organic acids, carbonyls, alcohols, and organic acids in
pyroligneous acid effect its herbicidal activity (Kim et al.
2001). Acetic acid, the main component of pyroligneous
acid, has been used in agriculture to control weeds. In one
study, pyroligneous acid was found to be effective against
underground weed propagules of freshwater plants such as
Hydrilla spp, Sago pondweed (Potamogeton pectinatus),
and smooth cordgrass (Spartina alterniflora; Spencer and
Ksander 1999).
Wood vinegar as an antimicrobial agent.—Pyroligneous
acids have been explored as antimicrobial agents by
researchers but have not been extensively studied in this
role. The presence of high concentrations of phenolic
compounds and organic acids explains its antimicrobial
properties (Lee et al. 2011, Mmojieje and Hornung 2015).
Pyroligneous acid has been shown to have antibacterial
properties on a selection of plant pathogenic bacteria by
many researchers. For example, Chalermsan and Peerapan
(2009) reported that proligneous acid inhibited Xanthomo-
nas campestris pv. Citri and Erwinia carotovora pv.
Carotovora. These pathogens can cause severe losses to
horticultural crops. The agent causing bacterial wilt on
many crops, Ralstonia solanacearum, was inhibited by
phenol and pyroligneous acid guaiacols synthesized from
Japanese cedar (Cryptomeria japonica; Young-Hee et al.
2005). Similar growth inhibition studies using 10 percent
FOREST PRODUCTS JOURNAL Vol. 73, No. 3 245
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pyroligneous acid showed an inhibitory effect on Agro-
bacterium tumefaciens and X. campestris (Mmojieje and
Hornung 2015). Pyroligneous acid produced from pineapple
solid biomass showed growth inhibition of the yellow gum
disease bacterium Corynebacterium agropyri (Mahmud et
al. 2016). As well as antibacterial properties, pyroligneous
acids have also shown an inhibitory effect on phytopatho-
genic fungi. The higher content of organic acids and phenols
in pyroligneous acids from various sources has shown
inhibition of a wide range of plant pathogens (Mourant et al.
2005, Chalermsan and Peerapan 2009, Tiilikkala et al. 2010,
Wei et al. 2010). Wei et al. (2010) reported antipathogenic
effects of pyroligneous acid on plant pathogenic fungi such
as Helminthosporium sativum,Cochliobolus sativus,Valsa
mali, and Colletotrichum orbiculare. Pyroligneous acids
produced at lower temperatures have a lower degree of
antimicrobial activity compared with those produced at
higher temperatures. Pyroligneous acids are effective in a
wide range of dilutions. Jung (2007) showed that pyrolig-
neous acid at a dilution of 1:32 inhibited Alternaria mali,
the causal agent of alternaria leaf spot on apple. It was
assumed that the presence of furaldehydes and phenols in
pyroligneous acid are the main cause of the antifungal
activity. A primary report by Tiilikkala and Set ¨
al¨
a (2009)
indicates that birch tar oil can be used effectively to control
Phtophthora infestans. All these functional activities and
applications suggest that pyroligneous acid could be an
alternative to synthetic chemicals for use as antimicrobial
agents.
Application of wood vinegar in medicine
Wood vinegar from charring has been used in the
preparation of the detoxification pad available in Japan,
the United States, Korea, and China. The method of using
the detoxification pad is to place it under both feet before
going to bed. The detoxification cushion is attached directly
to the reflex points on the feet. It is believed to promote
balance and healing in the body. It is a source of short-chain
fatty acids that help promote acidity in the large intestine,
resulting in the inhibition of the growth of bad bacteria,
enteropathogenic bacteria (Nakai and Siebert 2003), and
protozoa (Cryptosporidium parvum; Kniel et al. 2003), and
stimulates the growth of prebiotics, Enterococcus faecium
and Bifidobacterium thermophilum (Tana et al. 2003). In
addition, wood vinegar also reduces the absorption of
alkaline carcinogens, improves the absorption of calcium
and magnesium, and increases blood circulation. Distilled
wood vinegar could inhibit allergic reactions, especially
type I allergic reactions by oral administation. This solution
was indicated to prevent allergic rhinitis, hay fever, allergic
conjunctivitis, atopic dermatitis, allergic asthma, urticaria,
and food allergy (Imamura and Watanabe 2007).
Application of wood vinegar in food
preservation
Underreporting of foodborne diseases is a common and
sometimes fatal problem for millions of people worldwide
(Vattem et al. 2004). Food additives are used to preserve
foods, improve nutritional value, add or replace color, add
or replace flavor, improve texture, or provide processing
aids (Branen et al. 2002). However, in recent times, a
decrease in demand for synthetic food additives has been
recorded worldwide because of increased consumer aware-
ness. Consequently, natural food additives have become
popular (Deba et al. 2008). Several natural additives and
preservatives have been widely used in foods, such as
spices, herbs, essential oils from plants, and wood vinegar.
Wood vinegar could be an excellent additive as it is
obtained from natural biomass and by natural processes. It
can be used in processed foods to inhibit microbial growth
because of the phenols and short-chain organic acids
contained in the vinegar (Kahl and Kappus 1993). In
addition, smoke flavors extracted from wood vinegar have
been used in foods as a safety product (Mohan et al. 2006).
In addition, the US Food and Drug Administration permits
the use of pyroligneous acid for smoke flavoring and
preserving foods such as ham, bacon, sausages, fish, and
cheese.
Application of wood vinegar in wood
preservation
Recently, some researchers have reported that bio-oil and
wood vinegar obtained from fast pyrolysis have a strong
potential to be used as wood preservatives, and they have
suggested that phenolic compounds, which permeate the
wood matrix, seem to play an important role in fungal
growth inhibition and decay resistance tests (Kartal et al.
2004, Mohan et al. 2006, Nakai et al. 2007). Furthermore,
wood blocks treated with filtrates obtained from the fuel
slurry of several woods such as sugi (Cryptomeria japonica)
and acacia (Acacia mangium) showed resistance to the
brown rot fungus (Fomitopsis palustris) and white rot
fungus (Trametes versicolor). However, the filtrates did not
increase the durability of the wood blocks against
subterranean termites (Coptotermes formosanus; Kartal et
al. 2004).
Conclusion
The carbonization of lignocellulosic biomass is an
important process in the valorization of residues from
agriculture and forestry. This pyrolysis allows the formation
of three exploitable compounds, namely biochar as an
amendment, gases for thermal drying, and pyroligneous
acid, which is used in several fields thanks to its impressive
composition. Pyroligneous acid has antioxidant and antimi-
crobial properties, making it suitable for use as a
biopesticide, organic fertilizer, plant growth stimulator,
food preservative, wood preservative, and even in medicine.
As such, pyroligneous acid could be an ecological and
sustainable tool for farmers and, in turn, for consumers.
Because it is obtained without potentially harmful chemical
additives in the long term, the more global use of wood
vinegar could contribute to a healthier environment and
ecological system, as well as increased socioeconomic and
health benefits.
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