Natural compounds: a review of their use for wood protection

Full text

A review on natural products as wood protectant
Tripti Singh
*
and Adya P. Singh
Scion
PO Box 3020, Rotorua, New Zealand
*
Corresponding author
Tel. +64 7 3435329. Fax. +64 7 3435507.
Email: tripti.singh@scionresearch.com

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Abstract
Traditional wood protection methods employ chemicals that are considered toxic and can adversely
affect human health and the environment. Fortunately, serious efforts are being made globally to
develop alternative protection methods based on natural products with little or no toxicity, but the
progress in implementation of the technologies has been slow because of certain limitations, including
discrepancies between laboratory and field performance of natural products, variability in their efficacy
related to exposure/environmental conditions, and legislation difficulties due to disagreements globally
on setting standards defining the quality of their performance and use.
The focus of this review is to present information on the natural compounds that have shown promise
for wood protection and the information is presented under defined interactive categories. In closing,
some thoughts are presented on potential use of rapidly evolving technologies, such as nano-and gene-
technologies that can lead to significant advances, particularly from the consideration of specificity of
natural products and their economic value.
Keyword index: Bioactives, Essential oils, Heartwood extractives, Organic biocides, Mode of action,
Natural compounds, Wood deteriorating fungi, Wood preservation

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1. INTRODUCTION
Extending the service life of wood and wood products using natural compounds as bioactives is
proving to be an attractive approach for wood protection from the perspectives of human health and
environmental protection (Laks 1989; Freitag et al. 1991; Preston 2000; Schultz and Nicholas 2002;
Evans 2003). However, completely replacing such effective but toxic preservatives as chromated
copper arsenate (CCA) will take time (Schultz and Nicholas 2007), and require considerable
investment into the exploration of suitable sources of potent bioactives and development of formulation
and treatment processes based on fundamental knowledge of how the target natural compounds control
wood deteriorating organisms, such as fungi, bacteria and termites. Steady advances are being made in
the exploration of new organic biocides as well as in the development and refinement of processes to
employ them for wood protection.
There are a number of factors that are impeding progress necessary to develop technologies to
effectively deploy natural products for wood protection, and the issues have to be addressed and
solutions found. Retention of organic biocides within impregnated wood tissues and their
susceptibility to biodegradation are two main issues. In addition, refinements have to be also made
with formulations and treatment methods in order to achieve penetration of biocides into wood cell
walls to enhance their efficacy and reduce the cost. Certain additives when combined with organic
biocides can enhance efficacy of biocides and their retention within wood, and several approaches have
been taken in this regard. In environments where treated wood products are exposed to moisture,
considerable leaching of organic biocides can occur, and the level of reactive components remaining in
the wood may not be sufficient enough to discourage or prevent microbial attack on wood. Fixation of
organic biocides using additional agents as co-impregnants, that can cross link biocides to polymers in
the wood cell wall, may prove to be an effective way of retaining biocides within the impregnated
wood. For example, condensation polymerisation of chitosan-melamine co-polymers has been
successfully achieved by reacting chitosan oligomers with hexamethyl methylol melamine (Torr et al.
2006). Co-impregnation with water-repellants can also be an effective means of keeping moisture out
of organic biocide treated timber placed in service (Panov and Terziev 2009). There are a number of
water repellants, such as wax based formulations, waterborne and oilborne silica-based formulations,

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resin acids, that have been tried with encouraging results or are commercially available. For example,
a trial combining waterborne resin acids with organic biocides proved to be an excellent water
repellency system (Schultz et al. 2006).
Another approach that can enhance the efficacy of organic biocides is to use additives that can
scavenge free radicals generated by wood degrading microorganisms as a mechanism to degrade wood
cell walls. Aggressive wood degraders such as white and brown rot fungi generate and employ free
radicals, in addition to enzymes, to degrade wood cell walls (Backa et al. 1993; Hirano et al. 2000;
Hammell et al. 2002). Scavenging generated radicals can hamper fungal decay activity. Some natural
biocides possess antioxidant activity, but for others, addition of an oxidant or radical scavenger can
enhance their efficacy.
Combined use of certain metal chelators and organic biocides can also enhance biocide efficacy.
Certain metals are a component of fungal enzymes and have an important function as co-factors, and
metal chelation may prove to be an effective way of minimising wood decay by fungi. For example,
the use of metal chelator, EDTA (ethylenediaminetetraacetate) in combination with 2-HPNO (2-
hydroxypyridine-N-oxide) had a synergistic effect (Mabicka et al. 2005). Bioproducts from industrial
operations, such as resin acids produced as tall oil resin from kraft pulping, that can complex with
metals, can be potentially employed as an economical green technology for metal chelation, as has
been suggested (Schultz and Nicholas 2008). However, it would be important to use appropriate
combinations to promote a synergistic action, as a metal chelator may work in combination with only
certain biocides.
Organic biocides are prone to degradation by wood decay as well as non-wood decay microorganisms
(Briscoe et al. 1990; Wallace and Dickinson 2006) and may also be susceptible to photodegradation
(Evans 2003; Ruddick 2008) and have to be co-impregnated with agents that can shield them from
degradation.

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Interest in the exploration and use of natural products as biocides is rapidly growing worldwide, and
this review is an attempt to bring together information from selected areas of work. Following are the
major topics covered:
• A brief review of the work done on natural products as organic biocides for wood protection.
• A tabulated summary of information on natural products, their use as organic biocides and
references to studies done.
• Mode of action of selected natural compounds.
• Possibilities and limitations of using natural compounds as wood protectant.
2. REVIEW OF NATURAL PRODUCTS
2.1 Plant extracts, essential oils and other derivatives
Plant produces a range of aromatic and non-aromatic compounds, some of which are recognised
antimicrobial agents. Phenols, terpenoids, alkaloids, lectins and polypeptides are the products that have
been extensively utilized in various applications (Geissman 1963). Phenolic compounds possessing a
C3 side chain at a lower level of oxidation and containing no oxygen are classified as essential oils
(Cowan 1999).
Plant derivatives, such as oils, have been used for generations in certain parts of the world to enhance
appearance and to extend the service life of wooden products, such as furnitures, walking sticks etc.
However, use of plant derived products became less attractive for wood protection when synthetic and
inorganic compounds were introduced, as they proved more effective against wood deteriorating
organisms. But now there is a pressing need to replace synthetic and inorganic compounds with
organic biocides because of their toxicity to human health and detrimental impact on the environment.
Derivatives from a range of plant taxa and from various plant parts, such as bark, wood, leaves, seeds
and fruits, have been examined for their wood protection properties in many studies and the
information has been recently reviewed (Yang 2009). For example, cinnamon extracts, linseed oil and
extracts from certain fruits all have shown promising results. Extracts from cinnamon leaves have
proved highly effective against wood decay fungi and termites and can potentially be developed into

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excellent organic preservatives (Wang et al. 2005; Cheng et al. 2006; Lin et al. 2007; Maoz et al.
2007). Essential oils from lemon grass, rosemary, tea tree and thyme are effective against mould on
wood (Yang and Clausen 2007). Effective control of moulds on rubber wood was achieved using anise
oil, lime oil and tangerine oil (Matan and Matan 2008). These workers also investigated the effect of
cinnamon oil and clove oil on mould fungi on the surface of rubber wood and found them to be very
effective against mould growth (Matan and Matan 2007). Recently, Rickard et al. (2009), attempted to
isolate antifungal compounds from the leaves of Hinau (Elaeocarpus dentatus), a New Zealand native
plant. In-vitro bioassay tests showed Hinau leaf extracts to possess antifungal activity against two
brown rot fungi, Oligoporus placenta and Coniophora puteana. NMR spectra of Hinau extracts
indicated mixtures of aromatic substances, showing chemical shifts consistent with ellagitannins and/or
gallotannins with minor contributions from flavonoids.
Linseed oil has been used as an important component of protective coatings, such as paints, varnishes
and stains, for a long time, and can also serve as an effective organic biocide, particularly when used in
combination with other organic products with active ingredients. In recent years, some of the oils have
been shown to be effective in retaining the organic biocides in wood, thus increasing the efficacy. For
example, low toxicity compounds such as boron, which are effective against fungi as well as termites,
suffer from susceptibility to leaching from treated wood when in contact with water, such as in outdoor
service of wooden products. Among the methods attempted to retain boron in wood, linseed oil-boron
and tall oil-boron combination treatments have given promising results (Lyon et al. 2007; Temiz et al.
2008), paving the way for future developments to enhance boron retention in wood using a natural
product as an additive, which may also act synergistically in wood protection.
Extracts from certain fruits, such as citruses, contain active ingredients, and can be deployed as organic
preservatives for fungal control (Macias et al. 2005). In this regard, waste products from the industries
processing citrus fruits have the potential to serve as an important source for economically producing
potent industrial natural biocides.
A comparative study (Kartal et al. 2006), involving a range of essential oil compounds based
formulations, which included cinnamaldehyde, cinnamic acid, cassia oil and wood tar oil, revealed

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cinnamaldehyde, cassia oil and wood tar oil to be effective against brown rot fungus Tyromyces
palustris and white rot fungus Trametes versicolor, and cinnamic acid to be effective only against T.
versicolor. The wood treated with all compounds showed resistance against the subterranean termite,
Coptotermes formosanus.
Essential oils from the heartwood of Japanese cedar (Crytomeria japonica) were effective against
several wood decay fungi, and the antifungal activities correlated with the presence of sesquiterpene
compounds (Cheng et al. 2005).
Hsu et al. (2007) while examining the antifungal activity of cinnamaldehyde and several antioxidants,
alone and in combination, against wood decay fungi, discovered that of the antioxidants tested, only
octyl gallate and eugenol possessed antifungal activity. Additionally, combinations of octyl gallate-
cinnamaldehyde and eugenol-cinnamaldehyde had a synergistic effect, as their combinations were
effective to much lower concentrations than when octyl gallate, eugenol and cinnamaldehyde used
alone.
Cheng et al. (2008) evaluated antifungal effectiveness of cinnamaldehyde and eugenol against white rot
fungus Lenzites betulina and brown rot fungus Laetiporus sulphureus, with the aim also of relating the
antifungal activity to chemical structures, and found that cinnamaldehyde, α-methyl cinnamaldehyde,
(E)-2-methylcinnamic acid, eugenol and isoeugenol were very effective against the fungi tested.
Results also showed that when looking at the structure-function activity of these compounds, the
presence of aldehyde and/or acid group, a conjugated double bond and the length of CH chain has an
influence on the activity.
Singh and Chittenden (2008a) screened 12 essential oils to evaluate their antifungal activity against
common mould, stain and wood decay fungi and found activities to vary, with eugenol and
cinnamaldehyde being most effective in inhibiting the growth of test fungi on treated wood blocks.
However, eugenol and cinnamaldehyde extracts leached out when wood blocks were exposed to water
and the growth of tested fungi on the blocks subjected to leaching cycles was not inhibited.

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Li et al. (2008) examining the efficacy of cinnamon oil against mould and sapstain fungi on ponderosa
pine sapwood found that for surface treatment, the cinnamon oil was highly effective when used in
ethanol, but its activity declined when mixed with water. The addition of surfactants to cinnamon
oil/water combination failed to produce completely stable solution. Further exploration into more
suitable co-solvents is needed for developing a more commercially viable method for mould and stain
control based on cinnamon oil.
An US patent (Glassel and Mellema 2006), claims to have developed an effective technology for wood
protection against fungi using a combination of oils and a reactive silicone polymer as water repellent.
A study (Voda et al. 2003) that tested antifungal activity of oxygenated aromatic essential oil phenols,
phenol ethers and aromatic aldehydes on white rot fungus Trametes versicolor and brown rot fungus
Coniophora puteana, found significant differences in the activity of compounds used, dependending
upon their chemical structure-functional group.
A recent study by Zhou et al. (2007) found that combination of thymol and acetic acid killed bacteria
more effectively than thymol or acetic acid or other organic acids on their own. The study found that
organic acids boost the effectiveness of essential oils; they convert the active components from their
dissociated form to their molecular form. The molecular form is freely permeable across the bacterial
cell wall and thus is able to enter and damage bacteria. On the other hand, damage to cell walls caused
by thymol permits organic acids to enter bacteria and disrupt them.
2.2 Waxes, resins and tannins from bark
Bark from many tree species is a rich source of antioxidants and antimicrobial agents, such as waxes,
resins, tannins and other extractives. Tannins have been widely used as adhesives, and have also been
employed as wood preservatives for some time (Mitchell and Sleeter 1980; Laks et al. 1988; Lotz and
Hollaway 1988; Lotz 1993). One major problem with tannins and tannin derived compounds is that
they are difficult to fix in wood after treatment, although attempts have been made to retain them using
additives, such as ferric chloride (Mitchell and Sleeter 1980) and metallic salts (Laks et al. 1988; Lotz

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and Hollaway 1988; Laks 1991; Lotz 1993), with satisfactory wood protection achieved. Other
products from bark such as bio-oils obtained using pyrolytic methods have also been tested as wood
preservatives (Suzuki et al. 1997; Mourant et al. 2005). When considering bark as a source of organic
biocides, it should be remembered that bioactivity of the extracts from bark from different sources will
vary, as has been shown in studies involving evaluation of antifungal properties of bark from several
tree species (Yang et al. 2004; Yang 2009).
Bark products, including waxes, resins and phenolic extractives, have also served as adhesive
components. For example, Brandt (1953), reported mangrove tannin-formaldehyde resin as a strong
water resistant adhesive. Wattle tannin has also been recognised as waterproof adhesive (Plomely
1966). Waxes and resins from bark of various pine species, including radiata pine and ponderosa pine,
have been used as bonding agents in the manufacture of wood products (Anderson et al. 1961; Hall et
al. 1960). The characteristics of natural waxes extracted from Aleppo pine leaves and bark were
studied by Passialis and Voulgaridis (1999); wood specimens treated with these waxes showed
hydrophobic properties, with bark extracts showing greater hydrophobicity than needle extracts.
Bultman et al. (1991) investigated the anti-microbial activity of guayule resin. When impregnated into
wood, resin from wood and stem of guayule (Parthenium argentatum Gray), provides protection
against wood destroying organisms, including decay fungi, termites and marine borers (Nakayama et
al. 2001).
Wood particle boards impregnated with Pinus bruita bark extracts showed improved performance in
decay resistance (Nemli et al. 2006). Other board characteristics, such as thickness swell, were also
enhanced. Recently (Si et al. 2011), phenolic glucosides extracted from the bark of Populus
ussuriensis have been shown to possess antioxidant properties. The use of these compounds and
antioxidants from the bark of other tree species (Zhang et al. 2006) should be explored for their
possible use in wood protection.

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2.3 Heartwood and heartwood extractives
Perhaps the most important source of inspiration for exploring the feasibility of using natural products
for wood protection has been the long-existing knowledge that heartwoods of many tropical tree
species are highly durable even when used in hazardous environments, such as in-ground contact.
Some progress has been made on developing protection methods/processes based on the use of plant
extractives, with heartwood extractives particularly showing promise (Schultz and Nicholas 2000).
The natural durability of heartwood in trees is largely attributed to its toxic extractive components
(Scheffer and Cowling 1966; Bamber and Fukazawa 1985; Hillis 1987; Syafii et al. 1987), as studies
have shown that after removal of extractives, durable wood looses its resistance to fungal decay or
becomes less durable, and susceptible wood impregnated with potent extractives gains in durability
(Onuorah 2000; Taylor et al 2002 for review).
Heartwood extracts have been tested in many studies for their effectiveness against mould and fungi
causing sapstain and wood decay (Gripenberg 1949; Chow 1982; Onuorah 2000; Stirling et al. 2007;
Wan et al. 2007). Heartwood extractives contain a mixture of many compounds, and usually only
certain compounds within the mixture are active components, and may be specific to wood species,
such as eusiderin in Eusideroxylon zwageri extractives, which confers high durability to the heartwood
of this tropical tree species (Yatagai and Takahashi 1980; Syafii et al. 1987).
Heartwood extractives from a wide range of plant and tree species show activity against fungi and
insects, and many potentially can serve as wood protection agents alone or in combination (Sen et al.
2009). The heartwood of E. zwageri is highly resistant to brown and white rot, and although tunnelling
bacteria can degrade the wood, Nilsson et al. (1992) found degradation to be confined to only surface
layers after several years of exposure.
The active chemical components of the heartwood of some Nigerian timber species have been
documented and their contribution to the durability of wood has been demonstrated (King and Grundon
1949; Bevan et al. 1965; Onuorah 2000).

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Among the combinations of heartwood extractives from Gmelina arborea (Verbenaceae), a South and
Southeast Asian tree species, several glycosides and lignans tested positive for antifungal activity
against basidiomycetes (Kawamura et al. 2004; Kawamura and Ohara 2005). It was concluded that of
the four lignans isolated from G. arborea, gmelinol is an important antifungal constituent (Kawamura
et al. 2004).
Several studies have explored the potential of heartwood extractives in controlling the activity of wood
attacking termites. For example, antifungal and plant growth inhibitory alkaloids have been isolated
from the leaves of Prosopis juliflora, a fast growing and drought resistant plant originating from South
and Central America (Nakano et al. 2004). Sirmah et al. (2009a) demonstrated that heartwood
extractives of P. juliflora are effective in controlling the growth of various wood inhabiting fungi and
termites. Chemical analysis indicated that P. juliflora extractives mainly consist of -(-) mesquitol, a
rare flavonoid possessing important antioxidant properties (Sirmah et al. 2009b), which could be
responsible for durability.
Eremophilone oil from an Australian native tree Eremophila mitchelli Benth is known to have
termiticidal activity (Scown et al. 2009). TERMILONE® is the registered Trade Mark for this oil
extractive.
The heartwood extractives of white cypress pine are well known for their termicidal activity (French et
al. 1979) and studies of their chemical constituents have revealed that the main class of compounds
responsible for termite repellence are sesquiterpenoids (Watanabe et al. 2005).
The extracts of Taiwania (Taiwania cryptomerioides), an endemic tree species of Taiwan, were studied
for antitermitic and antifungal activity (Chang et al. 2003). The bioassay undertaken revealed
Taiwania heartwood extractives exhibited greater antitermitic activity than sapwood extractives (Chang
et al. 2001) and that α-cadinol was the most dominant compound responsible for antifungal and
antitermitic activities.

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In addition to the chemical composition of extractives, heartwood durability is related to the amount
and distribution of extractives within wood tissues, and the knowledge on this can serve as a helpful
guide in developing extractive-based wood protection systems. In the highly durable heartwoods,
extractives are not only present in cell lumens but they also impregnate cell walls and pit membranes
(Nilsson et al. 1992; Kleist and Schmitt 1999; Kim et al. 2006), and it is the combination of chemical
and physical factors that collectively determine the durability of wood species. Thus, impregnation
based processes should also target blocking of fungal colonisation pathways within wood tissues to
prevent/discourage fungal entry into and colonisation within wood.
2.4 Other bioproducts (miscellaneous)
A study using organic biocide, metal chelators and antioxidants against basidiomycetes concluded that
the combination application of organic biocide and metal chelators and/or antioxidants gave superior
performance against decay fungi as compared to biocide alone, which was considered to result from
synergistic interactions (Schultz and Nicholas 2002).
Chitosan, a deacetylation product from chitin, which is the second most abundant biopolymer on earth
and is produced economically from crustacean shells, has been explored for use as a potential wood
preservative alone or in combination with other biocides to control wood inhabiting fungi, including
those that decay wood (Kobayashi and Furukawa 1996; Chittenden et al. 2004; Maoz and Morrell
2004; Torr et al. 2005; Eikenes et al. 2005a,b; Singh et al. 2008a & b) because of its antimicrobial
properties (reviewed in Rabea et al. 2003) and cost-effective use. Antimicrobial activity of chitosan
appears to be related to its molecular weight as well as degree of deacetylation (Rabea et al. 2003).
However, opinion is divided as to whether low or high molecular weight chitosan and chitosan
products are most effective. Some suggest that high molecular weight chitosan is more efficient
against wood decay fungi (Eikenes et al. 2005a) but there are also indications that low molecular
weight chitosan oligomers within a certain size range are quite effective against a range of fungi
(Hirano and Nagao 1989; Chittenden et al. 2004; Torr et al. 2005).

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Various silicon compounds have been used to modify wood surfaces to enhance resistance against
fungal attack (Weigenand et al. 2008; Panov and Terzier 2009). Silicon based formulations have also
been evaluated to limit leaching of various compounds from treated wood (Kartal et al. 2009). Silicon
compounds react with hydroxyl groups of wood cell wall forming a covalent bond. However, the draw
back is that Si-O bonds are not very stable. Studies are underway to identify silicon compounds with
enhanced water repellency that may reflect strong bonding with cell wall components (Lin and Chen
2006).
Wood protection based on the use of Biological Control Agents (BCA) and microbial secondary
metabolites have also been attempted (Bruce and Highley 1991; Okeke et al. 1992); reviewed by Yang
(2009). However, confirmation of satisfactory field performances of BCAs or their secondary
metabolites requires more exhaustive testing. To improve the effectiveness of BCA, one possible
approach could be to combine them with environmentally friendly bioactive molecules that can
synergistically interact with the BCA, enhancing BCA’s competitive advantage during fungal
interactions. Recent series of studies by Singh and co-workers have demonstrated that integrated
protection systems, where BCA or secondary metabolites are combined with other natural compounds,
are highly effective in inhibiting growth and discolouration of sapstain fungi, particularly in laboratory
trials (Singh and Chittenden 2008b; Chittenden and Singh 2009). A field trial using an integrated
protection system based on the combination of chitosan with Trichoderma sp. has shown good
potential, prompting further trials and refinements in the system.
2.5 Modes of action
Only through improved understanding of the mode of action of various natural compounds, more
refined and cost-effective treatments can be developed. Targeted studies on modes of action also
provide an opportunity to understand in more detail the physiological stresses caused in wood decaying
microorganisms, thereby providing a sound basis for selecting fit-to-purpose agents as well as tools for
possible integrated approaches involving natural compounds and other agents, such as antagonistic
fungi.

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Phenolic compounds are well documented as antimicrobial agents (Duke 1985). The site(s) and
number of hydroxyl group on the phenol are thought to be related to relative toxicity of phenolic
compounds, and studies have shown that increasing hydroxylation usually results in increased
bioactivity (Geissman 1963). Also, there are suggestions that oxidized phenols are potent inhibitors,
with enzyme inactivation being one of the possible mechanisms of phenolic toxicity (Mason and
Wasserman 1987).
Essential oils are a very complex mixture of secondary metabolites that are highly enriched in terpenes.
Although it has been known for centuries that essential oils possess biological activity, their mode of
action is not very well understood. However, some specific studies undertaken have identified vital
cell components as the primary target of essential oil. For example, the work of Lambert et al. (2001),
using oregano essential oils and two of its main constituents thymol and carvacrol against bacteria,
suggests that these compounds damage microbial membrane integrity and also nucleic acids.
Carvacrol and thymol also exhibit significant antifungal activity against various fungal species, causing
lesion formation in cytoplasmic membranes and a reduction in ergosterol content (Pinto et al. 2006).
The essential oil extract, eugenol, is considered to function as growth inhibitor against a range of
microorganisms, including fungi (Duke 1985), as it limits protein production and DNA replication.
Eugenol causes lysis of spores and complete inhibition of mycelial growth in seed born fungi
(Thobunluepop et al. 2009).
Lavender oil and one of its main components linalool cause a rise in intracellular cAMP (Lis-Balchin
and Hart 1999); however, the precise mechanism of action underlying this cellular response remains
unclear.
There are several proposals regarding the mode of action of chitosan. Chitosan may bind with DNA
and affect mRNA synthesis by altering the conformation of DNA (Hadwiger and Loschke 1981).
Chitosan affects cell permeability by interacting with cell membranes, such as the plasma membrane
(Leuba and Stössel 1986). Several ultrastructural changes leading to plasma membrane disruption and
cell wall modification have been observed in wood deteriorating fungi treated with chitosan (Vesentini

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et al. 2007; Singh et al. 2008b). Binding to metals may be another possible mechanism by which
chitosan may influence fungal physiology and growth. Chitosan is known to bind with a range of
metals, including trace metals, which are essential growth nutrients (Skjak-Braek et al. 1989; Onsøyen
and Skauggrud 1990). It is however still unclear whether or not there are multiple sub-cellular targets
for chitosan.
3. LIMITATIONS AND POSSIBILITIES OF USING NATURAL COMPOUNDS AS WOOD
PROTECTANT
Although the current focus of wood protection/preservation research globally is on wood protection
using natural environmentally compatible compounds, there is still limited industrial uptake of the
compounds and technologies by wood preservation industry. Following are the main reasons for this.
One, incompatibility of results from laboratory studies with field trials, as well as discrepancies
regarding the efficacy of organic biocides in nutrient medium and on biocide impregnated wood. Two,
the activity of some of these compounds has a narrow range. Three, inconsistencies among legislation
and registration of new compounds.
Any compound with antimicrobial activity, regardless of its origin, can pose a risk to human health and
environment due to its intrinsic biological activity. Hence, registration in most countries is controlled
by regulatory authorities. In New Zealand, the responsibility for issuing registration lies with ERMA
(Environmental Risk Management Authority).
In most countries, regulatory act requires that any new compound or formulation be registered prior to
manufacturing or sale. Generally, for a new compound a risk assessment is conducted to predict the
effect on human health and environment from direct and in-direct exposures, which requires exhaustive
toxicity studies over several years, demanding considerable financial investments. Because of inherent
investment risks and considerable time and cost involved with the development and registration of new
compounds, most companies in wood preservation area are piggybacking on the already registered
compounds. This is a less risky approach, with a saving on time and money compared to registering a
new compound or formulation (Jacoby and Freeman 2008). Risk assessment of a formulation is

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possible without toxicology studies, especially if the safety of active compounds or their decomposition
products has already been established.
Natural compounds established as safe to human health and environment such as chitosan, essential
oils etc., have been investigated with the aim of replacing toxic chemical preservatives currently still in
use. However, the application of many such compounds is hampered due to their leachability from
treated wood when exposed to water. Retention of organic biocides within impregnated wood tissues
is one of the biggest hurdles to developing effective protection technologies based on use of natural
compounds. It is heartening that some progress has been made in recent years towards alleviating the
problem, for example, in situ enzymatic polymerization of biocides to render them water insoluble.
Enzymatic polymerisation of such substances as phenols and their derivatives is an area with immense
opportunities. For example, enzymatic polymerisation with laccase has been used as a means of
binding phenolic preservatives into wood (Ratto et al. 2004).
Other relevant technologies that have the potential to greatly impact wood protection industry are
nanotechnology and plasma added applications. In recent years, significant advances have been made
in the development and use of nano and micro-particles of active components of certain known
preservatives, such as micronized copper based formulations (Cao and Kamdem 2005; Stirling et al.
2008; Matsunaga et al. 2009). A number of patents have also been granted (Leach and Zhang 2004),
including a recent one to Jun and Wenjin (2009) on the formulation comprising micronized copper and
zinc for application in wood preservation. Similar approaches can be extended to developing
nanotechnologies based on natural active compounds. According to Clausen (2007), potentially the
nanocarrier delivery system could be linked with slow release biocides and/or employed to prevent
leaching of soluble compounds from treated wood. However, the potential impact of nanotechnology
is not fully understood (Colvin 2003), hence further investigations are needed to identify opportunities
as well as potential risks (Clausen 2007).
Another exciting area in natural product chemistry is Genome mining (Van Lanen and Shen 2006).
Discovering the new compounds from natural sources is usually a long and laborious process based on
screening of plant and microbial extracts, combined with bioassay guided identification and elucidation

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of the structure of natural products. Genome mining, which involves searching a genome for DNA
sequences that encode enzymes associated with the biosynthesis of particular products can significantly
accelerate progress in identifying natural products to specific targets. Increasingly rapid and more cost-
effective genome sequencing technologies coupled with advanced computational tools have converged
to realise full potential of this line of pursuit (Li et al. 2009).
Consideration of impact that a protection agent may have on human health and the environment at the
end of service life of the treated product is important for its selection and use. One of the main benefits
to be gained from the use of natural compounds for wood protection is their minimal environmental
impact at the end of service life. There is continuing dilemma about how to manage wood materials
that are treated with toxic chemicals. Efforts have been made towards minimising environmental
impact of such treatment agents by employing bioremediation technologies. This includes biological
extraction of preservatives from wood or recycling by chemically extracting toxic compounds.
However, due to economic reasons the majority of treated wood products are still either landfilled or
combusted. Studies to gain an insight into the impact of toxic treated chemicals through their leaching
into ground water or waterways and combustion have raised serious concerns over the use of such
chemicals and current practice of disposing treated wood products. For example, Lebow et al. (2003)
showed that preservative compounds from CCA treated wood are toxic to non-target organisms if
released into the environment in sufficient quantities.
4. CONCLUSION
As we see, limitations are few but potential is enormous for developing wood protection technologies
based on the use of natural compounds as biocides. Great opportunities for achieving advances in the
development of effective environmentally sustainable protection systems lie ahead, and the following
are key points to consider when initiating developments for attaining suitable technologies.
 Some natural compounds are well tested and possess wide-spectrum antimicrobial activities.
 Considerable information on the chemistry and antimicrobial activity of a range of natural
bioactive compounds is available from health, food and agricultural applications.

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 Potential exists for enormous health and environmental benefits when protection systems are
based on suitable natural compounds.
 Some natural compounds can be obtained cost-effectively, for others genetic engineering
approaches can be employed for economic production. Gene transfer into fast growing plants
can facilitate production of specific active compounds in large quantities.
 Opportunities exist for enhancing antimicrobial activity through combining the primary
organic biocide with other substances/agents, eg. antioxidants, metal-sequestering agents,
BCA.
 Opportunities also exist for enhancing bioactivity through gene technology.
 Treatments with appropriate organic biocides can be combined with biological/enzymatic
agents aimed at destruction/removal of pit membranes to enhance penetrability of wood,
which can lead to greater and more uniform impregnation of wood tissues.
 Implementation of a robust appraisal of environmental and health benefits derived from the
use and disposal of organic biocides tailored to meet specific wood protection requirements.
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