What is wood vinegar ?

Pyroligneous acid (PA) also called wood vinegar is an aqueous liquid produced from pyrolysis of lignocellulose waste and biomass. They are formed by rapidly and simultaneously depolymerizing and fragmenting cellulose, hemicellulose, lignin and other constitutes of biomass. Those biopolymers were broken into smaller molecule with high temperature under air less atmospheres. The crude condensate smoke during pyrolysis was so called bio-oil. In order to prepare woo d vinegar, this liquid is stand in a closed container for at least 3months and decanted from sedimentation tar of bio-oil. The transparent solution in upper phase is practical wood vinegar. It has a special smoky odor and color is light yellow to brown depending on feedstock properties and pyrolysis system used for preparing it. The wood vinegar was very complex solution; the major proportion was water (80–90%) and minor proportion was more than 200 species of organic compounds. Recently, wood vinegar has been wildly applied for various proposes such as medicinal, food and platelet aggregation and anti-dermatophyte activity in pharmaceuticals [1], [2] . In particular, in organic agriculture, a great number of toxic-chemicals were replaced by wood vinegar, a natural product, which has been used to combat disease and pests, stimulate plant growth, improve the quality of fruit, accelerate the speed of plant seed germination and serve as herbicides [3]. However, the physicochemistry and biological activity of wood vinegar are affected by many factors such as chemical composition of biomass, pyrolysis system and refining method

WOOD VINEGAR is 

The crude liquid obtained by condensation of an output, gas phase and smoke, during charcoal production process is called bio-oil. After standing the crude liquid in a closed container at ambient temperature for about 3 months, it will be separated into

2 phases. A clear solution of upper phase is wood vinegar or pyroligneous acid, while the lower phase is tar sediment. The acid has smoky odor, and color may vary from light yellow to dark brown.

 FACTORS INFLUENCING PROPERTIES OF WOOD VINEGAR

Wood vinegar is a complex mixture of polar and non-polar compounds that varies widely in their property among samples that may be observed. These are primarily influenced by the type of carbonaceous feed stocks as well as production techniques.

 FEED STOCK

In general, cellulose (42–45%), hemicellulose (28–35%), and lignin (16–33%) are major constituents of the plant cell wall, which are primary components of wood, while water-soluble and organic-soluble compounds are the minor compounds [4]. The bio polymers and minor compounds are arranged in a complex structure as illustrated in Figure 1. It should be noted that variations in proportion of the constituents are not only depends on the plant species, but are also influenced by other factors such as habitat, age, part of the tree, etc. Hence, it could be deduced that variations in composition of feed stock used for the preparation of wood vinegar might give rise to different properties of the product

PHYSIO CHEMICAL PROPERTIES OF WOOD VINEGAR

Wood vinegar is a complex mixture of polar and non-polar chemicals with varying molecular weights and compositions, which depends upon feed stock and processing condition.  Some  physical  properties  such  as  pH,  specific gravity,  color,  odor, dissolved tar content, ignition residue and transparency have been reported.

Due to the presence of organic acids such as acetic acid, formic acid and propionic acid, the acidity of wood vinegar is often in the range of pH 2–4. Total soluble tar contents were found within a range of 0.23–0.89% wt. The specific gravity and sugar content of aqueous solution was reported within a range of 1.005–1.016 g/mL and 1.7–6.6, respectively .

The major proportion in wood vinegar is water (80–90%). Water is usually miscible with the oligomeric lignin-derivative components because of the solubilizing effect of other polar hydrophobic compounds, low-molecular-weight acids, alcohols, hydroxyaldehydes,  and  ketones,  mostly  originating  from the  decomposition  of carbohydrates.

The minor proportions of 10–20% wt are water-soluble organic compounds. These comprise of acids, alcohols, aldehydes, ketones and sugars. It appears that acetic acid is the highest proportion in this fraction .

The other components are also found, for instance, phenols and monolignols (quaiacol and syringol) which are degradation products from lignin. Levoglucosan, levoglucosenone, furfural, substituted furan, are derived from cellulose. The simpler organic molecules are converted from fats, mucilage, wax, alkaloids, and terpenoids. For inorganic salts and metallic complex are commonly reduced to ash.

Wood vinegar is unstable during prolong storage or aging process. The viscosity of the mixture gradually increases and phase separation is slowly occurring.

Apart from sedimentation of a high-density particle and colloidal matter, this might be a result from the series of chemical reaction such as an aldol condensation reaction between aldehydes and alcohols, and self-aggregation of aldehydes with those of reactive oligomers. In addition, the metallic components may also play important role as reaction catalysis .

QUALITY OF WOOD VINEGAR

As mentioned earlier, there are wide variations in the property of wood vinegar due to its mixture of many compounds with different concentrations. However, Japan Pyroligneous Liquor Association, an industrial body for pyroligneous liquor trader sets 7 parameters in order to standardize a good quality of the product. These are included pH value of around 3.0, standard specific gravity around 1.010–1.050, color should be a pale yellow, bright brown or reddish brown, has a marked smoky odor, the dissolved tar content should not more than 3%, ignition residue should not more than 0.2%, and exhibit transparency without suspended solid matter . However, quality is widely varying. Accordingly, attention should be paid to obtain knowledge base on the preparation processes and quality control using a practical and cheap procedure.

The quality assessments of wood vinegars in Thailand were determined according to criteria from the Japan standard with slightly modification. Eight wood vinegars from the  carbonization  of  five  wood  species—Leucaena  leucocephala  (Katin), Azadirachta indica (Sadao), Eucalyptus camaldulensis, Hevea brasiliensis (rubber wood) and Dendrocalamus asper (bamboo) grown in Thailand, which produced by heating wood samples up to 400C in a Thai-Iwate kiln showed that all wood vinegar samples appeared to be good quality in terms of odor, color and transparency.

An acetic acid concentration from the eight samples whose presence was indicated by pKa at 4.7 was mainly responsible for the pH values a good correlations of plots between pH and acetic acid concentrations (Correlation coefficient, R = 0.92). The specific gravity showed good correlations with total soluble tar and degree Brix (R

= 0.87 for both); in turn the degree Brix showed good correlation with the total soluble tar (R = 0.87).

Thus, the degree Brix which was easy to determine could be used as a general indicator of total soluble tar. The amount of total soluble tar signified the presence of phenolic compounds, of which previous studies suggested antifungal activity and usefulness as wood preservatives. In addition, phenolic compounds were also confirmed by the ultraviolet absorption maximum wavelength at 268–274 nm

PURIFICATION OF WOOD VINEGAR

The crude wood vinegar, bio-oil, is very complex solution contained with at least

200 constituents. Some of those components can be prone to such complex reaction as oxidization and polymerization. Physicochemical characteristics of wood vinegar produced from different sources even between producing batch in the same source have very high variability. It will bring to the inconsistency on the utilization efficiency. Therefore, if wood vinegar is to be a future source of natural chemicals production with consistence activity, an effective separation method must be developed to generate semi-purified bioactive components. Several methods such as standing, filtering, distillation and solvent extraction have been developed for semi- purified and classified components. In order to obtain more specific and consistent property of  product,  the  wood  vinegar  may be  fractionated  into  semi-purified product. These could be achieved by several techniques such as sedimentation, filtration, chromatography, distillation and solvent extraction

STANDING METHOD

This is used to prepare wood vinegar from crude bio-oil. It is simplest and most highly efficient method that keep crude bio-oil standing in a container. When left standing, the unstable constituents in raw wood vinegar are oxidized or polymerized to precipitate, suspend or adhere to the inner wall of the container.

The thin oily film on the surface of the liquid and transparent wood vinegar separate on the middle phase must be discarded and the suspended as well as precipitated matters are filtered to produce transparent wood vinegar. When the standing and filtering processes are repeated several times, stable, transparent wood vinegar is obtained. While the standing method requires a long time, it is easy and inexpensive compared to other methods and good results are assured. In practice, wood vinegar is easily separated from whole bio-oil and a viscous oligomeric lignin-containing fractions setting at the bottom by standing at least 3 months

. FILTERATION

Filter like filter paper is used to remove the precipitated as well as suspended matters. The filtering of freshly recovered bio-oil cannot fully remove unwanted constituents, causing the appearance rely on filtering method. To obtain transparent wood vinegar, combining the filtering with the standing method is necessary. In the filtering process, the oil and suspended matters in wood vinegar gradually clog the filter paper or filter, lengthening the filtering time. In order to prevent this, frequent changes of the filter paper or filter are necessary. The appearance of suspended matter in transparent wood vinegar after future standing means that unstable constituent remains requiring further filtering

COULMN CHROMATOGRAPHY

The principle of column chromatography is that substances are separate based on their different adsorption capabilities on stationary phase. Large polar compounds are contained in wood vinegar. In general, highly polar molecules are easily adsorbed in the stationary phase, while weak polar molecules are not. Thus, the process of column chromatography involves adsorption, desorption, re-adsorption, and re-desorption. Silica gel is commonly used as the stationary phase, and an eluent is selected based on the polarity of compounds from wood vinegar

DISTILLATION

Distillation is a common separation technology in the chemical industry. This method separates the components successively according to their different volatilities, and it is essential for the separation of liquid mixtures. In general, there are two distillation systems i.e. the normal pressure and reduced pressure distillation. In both systems, compounds are separated by mean of the deference boiling points The carbonizing wood vinegar has water content of as high as 80–90% with a rather small difference in boiling point between the remaining 10% of organic matters. Therefore, the boiling of wood vinegar starts below 100_C under atmospheric pressure, and then the distillation continues up to 250–280_C, whereupon 35–50% of residue is left [20]. Distillation method is quite effective to concentrate wood vinegar and also to remove substance with particularly low and high boiling points.

However, the distillation process cannot entirely remove unwanted polymer. It is more practical to use this method after unwanted polymer in crude vinegar removed by standing method., However, it should be carefully because the heating to boil sample may be inducing the oxidation and polymerization in which bring to losing bioactivity of any components.

SOLVENT EXTRACTION

Liquid–liquid extraction method so called solvent extraction, 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 A distributed between an aqueous and an organic solvent:

KD = [A] solvent / [A] aqueous

where square bracket denote concentration and KD is known as the equilibrium distribution of partition coefficient which is independent of total solute concentration. It should be note that constant temperature and pressure are assumed and that A must exist in exactly the same from in both phases. Equilibrium is established when the chemical potentials (free energies) of the solute in the two phase are equal and is usually achieve within a few minutes by vigorous shaking.

The value of KD is a reflection of the relative solubility of the solute in the two phases [21]. Solvent extraction or liquid–liquid extraction has been introduced for semi-purification active compounds from wood vinegar mixture. This technique is used for the separation of compounds with different partition or relative solubility between the 2 solvent phases. Commonly, an aqueous solution of the sample is extracted with an immiscible organic solvent.

By selecting appropriate polarity of the solvents for extraction, such as hexane, diethyl ether, ethyl acetate, acetone, water etc., the desired products may be obtained. In order to obtain the higher purity of bioactive compounds from wood vinegar [22] step-by-step extraction on the basis of polarity order may be employed. Some reports showed that phenolic compounds and organic acids were extracted from wood vinegar using ethers and dichloromethane

UTILIZATION OF WOOD VINEGAR

ORGANIC AGRICULTURE

Utilizing chemical fertilizers was not only imposing heavy loads and pollution on the environment but also threatening our health. Long-term application of chemical fertilizers exposed the following problem: exhaustion of soil organics, lower conservation of water and nutrition, deterioration of the soil structure and heavy losses of water and soil. Excessive chemical fertilization not only polluted the soil, water and air but also kept most residues in vegetables, which decreases the quality and security of our food supply.

Therefore, it was very important to find and develop natural material for vegetable production. Wood vinegar was highly suitable for use in organic agriculture. Since wood vinegar were naturally organic compounds. A great number of toxic-chemical in agriculture was replaced by wood vinegar, natural product, which has been used to promote growth and yield for field cultivation crops such as rice, Oriza sativa , sweet potato, Ipomoea batatas , sugar cane, Saccharum officinarum [26], melon, Cucumis melo

In addition, it also used to improve the quality of fruit, combat disease and pests, accelerate the speed of plat seed germination and serve as herbicides 

Plant growth and development

wood vinegar appears to stimulate cell growth and acts as a catalyst for the growth of several microbes and enzyme activation. These are key to various physiological and biochemical processes in plants such as photosynthesis, nutrient absorption and cell growth but the mechanisms involved are understudied. Many researchers have suggested that foliar application of wood vinegar increases the chlorophyll content and glossy

appearance of plant leaves, which will increase photosynthesis and synthesis of sugars and amino acids. In addition to this, esters such as methyl acetate and methyl formate, which accelerate plant growth and development have been found in wood vinegar 


Antioxidant and radical scavenger

It has been reported that WOOD VINEGAR also exhibits antioxidant and radical scavenging activity. The WOOD VINEGAR collected from high temperature pyrolysis exhibited strongest antioxidant properties compared to low temperature pyrolysis

Pesticides

The use of synthetic chemicals is the most common approach to control pests and diseases of plants. These chemicals have boosted agriculture production but their applications have negative impact on the environment, ecological system and the health of all life forms. Currently, there is a push for sustainable and smart agricultural practices and ways to minimize the over-dependence on the use of chemicals. The presence of phenolic compounds in WOOD VINEGAR enables it to have antifungal and other pest-control properties

WOOD VINEGAR  also  helps  to  increase  the  permeability of  agro-chemicals  into  leaf  tissue. Typically, these agro-chemicals are more effective when used in combination with some other acids like WOOD VINEGAR with pH 4–5. Thus, WOOD VINEGAR is reported to increase  the effectiveness of chemical pesticides when used in combination.

WOOD VINEGAR can be used as a bio-herbicide with the potential to replace synthetic chemical herbicides. The phenols, organic acids, carbonyls, alcohol and organic acids in WOOD VINEGAR impacts its herbicidal activity [31]. Acetic acid, the main component of WOOD VINEGAR has been used in agriculture for weed control

in Soil health

WOOD VINEGAR improves soil physical and chemical properties as it is known that WOOD
VINEGAR increases
soil microbial activity. According to [37], WOOD
VINEGAR could have
effect on leaching soluble
salts and decreasing soil pH and therefore, improving crop productivity in saline soils.

Fertilizer

Many studies demonstrated the beneficial effects of WOOD VINEGAR on soil when applied as an organic fertilizer for growing rice , sugarcane and sweet potato . It was found that 20% WOOD VINEGAR significantly increased the growth and yield of rockmelon in soilless culture

Germination

Many studies found that WOOD VINEGAR has a stimulating effect on seed germination of various plants . It was determined that wood vinegar stimulates seed germination and radicle growth of watercress, chrysanthemum, lettuce and honewort .

However, high concentration of WOOD VINEGAR inhibits seed germination due to the adverse effect of low pH on seed germination. Further studies are needed in this area to optimize WOOD VINEGAR rate and treatment method for seeds.

Improve composting efficiency and compost quality

There are few studies that demonstrated the addition of WOOD VINEGAR solution to composting biomass  increased microbial  activity and  thus, accelerated the fermentation  of organic matter during the composting process

It was also reported to have reduced nitrogen (N) loss. WOOD VINEGAR increased the number of useful bacilli, actinomycetes and molds and speed up the breakdown of difficult to breakdown fibers. Thus, WOOD VINEGAR not only accelerate fermentation the process but also improves the quality of compost

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