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Chemical products from temperate forest tree species—Developing strategies for exploitation
Industrial Crops and Products 24 (2006) 238–243
Chemical products from temperate forest tree species—Developing strategies for exploitation D.B. Turley a,∗ , Q. Chaudhry a , R.W. Watkins a , J.H. Clark b , F.E.I. Deswarte b b
a Central Science Laboratory, Sand Hutton, York YO41 1LZ, UK Clean Technology Centre, University of York, Heslington, York Y010 5DD, UK
Abstract Forest trees are integral to the landscape and rural economy. Unfortunately, the economic returns from timber production have declined significantly across Europe. Only 25% of felled wood is converted to timber, the remaining material is a rich composite of primary and secondary metabolites and plant fibres, a relatively unexplored and unexploited resource for potentially novel products that could compliment revenue from traditional market outlets. Wood from temperate forest trees has traditionally been used as a source of tannins, terpenes, rosins and aromatic phenolic compounds. Existing information on such chemical groups, and other secondary metabolites was collated for a range of temperate forest tree species including, alder (Alnus glutinosa L. Gaertn.), ash (Fraxinus excelsior L.), aspen (Populus tremula L.), beech (Fagus sylvatica L.), birch (Betula pendula Roth., Betula pubescens Ehrh.), cherry (Prunus avium L.), Corsican pine (Pinus nigra Arnold), Douglas fir (Pseudotsuga menziesii Mirib. Franco), larch (Larix deciduas Mill., Larix kaempferi Sarg.), oak (Quercus robur L., Quercus petraea Mattuschka, Liebl), poplar (Populus nigra L., Populus gileadensis Rouleau, Populus alba L., Populus canescens Ait. Sm.), Scots pine (Pinus sylvestris L.), sitka spruce (Picea sitchensis Bong. Carr.) and willow (Salix alba L., Salix fragilis L.). Over 37,000 records were extracted from phytochemical databases, research papers, conference proceedings, books, unpublished reports and company literature, covering identified metabolites, the tissues from which they were extracted (e.g., bark, leaves, heartwood, roots), reported yields, properties and hazards. Very little data exists on the yield or variability of individual metabolites limiting the ability to assess economic potential. The information sourced is collated in a database, available to view at http://treechemicals.csl.gov.uk. Traditional and new markets for exploitation of tree metabolites are reviewed along with possible methods of extraction. Computeraided Quantitative-Structure Activity Relationship Modelling (QSAR) augmented the search for novel applications for the tree metabolites. By this method, monoterpenes with useful anti-microbial properties were identified. Application of green chemical technologies also show promise in adding value to tree metabolites including the modification of cellulose and the benign extraction of valuable chemical products. Opportunities for possible future routes of exploitation of wood biomass are presented and discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Metabolites; Wood extractives; Tree extractives; Phytochemicals
1. Introduction There are 2.7 million hectares of woodland in Great Britain. Conifer species account for 59% and ∗
Corresponding author. Tel.: +44 1904 462791. E-mail address: [email protected] (D.B. Turley).
0926-6690/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2006.06.016
broadleaved species 41% of the forest area. Current annual production of wood from UK conifers is in the region of 10 million cubic metres (overbark standing). Outputs from broad-leaved species are small in comparison. The total volume (standing biomass) of wood produced in Great Britain has been increasing over the past decade and future predictions suggest a continuing
D.B. Turley et al. / Industrial Crops and Products 24 (2006) 238–243
increase over the next 20 years. Trees planted during the boom of the 1970’s and 1980’s will come to maturity in the next decade, but prices for timber have slumped and the decline is continuing. Increasing supply at a time of low prices is likely to exacerbate current problems and many plantations are unlikely to be worth extracting for timber or pulp. Finding ways of adding value to such wood resources may help in maintaining rural enterprises. 1.1. UK wood resource for extractive industries Only around 54–58% of the green wood delivered to sawmills is converted to saleable timber, leaving significant volumes of timber co-product. The majority of this is used in the pulp and board industry. The UK timber industries have been effective in adding value to the wood supply and co-product chains. Any potentially new products from forest trees, therefore, either have to compete with wood board and panel industries, add significant value or utilise other process residues (e.g. bark). Another possible means of increasing the market value is by adding value to the by-products of pulp processing. Other sources of wood currently not utilized include branches and other materials derived from forest management. Such resources are more widely scattered and require more effort to collect and transport, so high value outlets are likely to be required to justify costs of collection. 1.2. Current added value uses An example of the ways in which value can be added to wood processing is described by Harms (1998). The Austrian company Lenzing extracts cellulose fibres by a chemical pulping process for use in textiles (Viscose and Lyocell (derived from Fagus sp., and Eucalyptus and Pinus sp., respectively)) and industrial processes. This typically utilises around 40% of the tree biomass. Lenzing has sought to utilise the remaining biomass as a source of fine chemicals; using a further 10% of the harvested biomass with the remainder used to produce energy (Harms, 1998). This is a significant improvement on typical tree biomass utilisation rates of around 35–40%. Further industrial use of products from chemical pulp is constrained by the relatively low-cost of alternatives, such as petrochemically derived synthetic polymers. Other approaches also show promising perspectives. In the US (NREL, 2003), work is progressing to develop phenol-formaldehyde resins from sawdust and bark which have particular uses in heat-set adhesives used for wood board production (ply-wood and
239
stranded boards). Resins derived from pyrolysis of softwoods cost around 25% less than synthetic resins and also reduce setting times, speeding up the manufacturing process. These developments reflect a determination to focus leading-edge expertise on high value markets to justify the costs of extraction. 1.3. Secondary metabolites from trees Flavonoids, terpenes, phenols, alkaloids, sterols, waxes, fats, tannins, sugars, gums, suberins, resin acids and carotenoids are among the many classes of compounds known as secondary or special metabolites (cellulose, hemi-cellulose and lignin being classed as primary metabolites). The range of compounds is extensive, with wide ranging chemical, physical and biological activities. The concentrations of these metabolites in trees vary between species (they may contain as little as 1% or as much as one-third of their dry weight as secondary metabolites), within species, between tissues (higher concentrations occur in bark, heartwood, roots, branch bases and wound tissues) and from year to year. They thus present numerous challenges to utilisation. The production and accumulation of a wide variety of organic chemicals is one of the major mechanisms by which plants defend themselves against herbivores and attacks by microbial pathogens and invertebrate pests. Most of these chemicals are products of secondary metabolism, originally thought to be the waste products not needed by plants for primary metabolic functions. It is well known that their presence in different parts of the plant (root, leaves, bark, etc.) deters feeding by slugs, snails, insects and vertebrates, as well as attacks by viruses, bacteria and fungi (Winks and Schimmer, 1999). For example, essential oil extracted from Pinus sylvestris has been shown to possess strong antibacterial activity (Schales et al., 1993). Such plant metabolites have been used for centuries for dyeing, fragrances, flavouring and as a source of medicinal compounds. The potential use of plant metabolites in health care and personal care products and as lead compounds for the development of novel drugs has led to a huge interest in their isolation and characterisation. Trees possess a number of potentially useful metabolites. Polyphenolic flavonoids have pharmacologically useful antioxidant properties. For example, quercetin, found in a number of forest trees (and in particular, the bark of Quercus spp.), has been reported to block the sorbitol pathway (linked to problems associated with diabetes) (Dhanawat et al., 2005). It may also protect beneficial low-density liposomal cholesterol (LDL) in the body (Morand et al., 1998; Chopra et al.,
240
D.B. Turley et al. / Industrial Crops and Products 24 (2006) 238–243
2000). Such flavonoid molecules commonly occur in foliage, bark, sapwood and heartwood in trees. As a first step towards identifying possible routes of exploitation, a review was undertaken to collate data on documented reports of metabolites found in relevant tree species, to create a searchable database resource. 2. Metabolite database Information was collated on the primary and secondary metabolites of alder (Alnus glutinosa), ash (Fraxinus excelsior), aspen (Populus tremula), beech (Fagus sylvatica), birch (Betula pendula, Betula pubescens), cherry (Prunus avium), Corsican pine (Pinus nigra), Douglas fir (Pseudotsuga menziesii), larch (Larix decidua, Larix kaempferi), oak (Quercus robur, Quercus petraea), poplar (Populus nigra, Populus gileadensis, Populus alba, Populus trichocarpa), Scots pine (P. sylvestris), Sitka spruce (Picea sitchensis) and willow (Salix alba, Salix fragilis). Information was extracted from bibliographic databases, phytochemical databases, research papers, conference proceedings, books, unpublished reports and company literature. In total, over 37,000 records published over the last 3 decades was gathered and interrogated. Data extracted from these records included the metabolites identified, the tissues from which they were extracted (e.g. bark, leaves, heartwood, roots), reported yields, properties, hazards, CAS numbers (compoundspecific identifier codes), extraction methodologies (if known) and current and future market potential for these tree products. This enabled the creation of a searchable database accessible to all interested parties in the supply chain from foresters through to industrial innovators. The database is free to access at http://treechemicals.csl.gov.uk. 2.1. Key findings There are numerous examples where tree-derived products have found commercial markets. For example, xylitol (used as a low calorie sweetener) derived from Finnish birch. Resins, waxes and oils can be extracted from living trees by ‘tapping’, or from the bark and wood during pulping. The most important by-products of wood pulping are turpentine and tall oil (used to produce a variety of resins (rosin) and which also has the potential to provide useful fatty acids). Tree-derived turpentine is still widely used in industry as a solvent in paint and varnish (though these sectors are moving to other solvent bases) and it can be fractionated into other useful
chemical components. Its use depends on chemical composition, determined by both tree species and method of extraction. Monoterpenes (primarily ␣-pinene) are used to prepare synthetic pine oil (the biggest single turpentine derivative), polyterpine-resins, fragrances and insecticides. Demand for use of ‘green solvents’, such as turpentine is also increasing. Larch arabinogalactan is a polysaccharide found in all larch species (5–35% of dry weight in heartwood) used as a replacement for gum arabic in food, cosmetic and pharmaceutical products. The sheer volume of recorded metabolites poses a problem in identifying the most suitable for further assessment and development. In addition, there have been few attempts to quantify the yield of any metabolites found. Most are reported on a ‘presence’ only basis and source materials are not always reported. All of this limits the possibility to assess the market potential of any reported metabolite. In addition, the variability in metabolite yield within and between species also needs to be clarified to identify those metabolites with the greatest opportunities for commercial success. One means of assisting in this search is use of computer-based tools, such as Quantitative-StructureActivity-Relationship modelling (QSAR). 3. QSAR studies Complementary to the traditional approaches of identifying uses for natural products (e.g. on-line searches of phytochemical databases), computational Quantitative Structure-Activity Relationship models have been developed that can predict the activity of a compound based solely on its structure. It is a technique increasingly adopted by food and pharmaceutical companies in the search for new molecules and functionalities. This technique has already been used to identify and predict the activity of tree metabolites as bird repellents that are now the subject of commercial evaluation (Watkins et al., 1999). Using computer-aided molecular design software, 3D-representations for a few select tree-derived metabolites were built. This was limited to an assessment of the antimicrobial and insecticidal activities of monoterpenes. A QSAR model was created using published and in-house generated data for a series of 34 monoterpene compounds with a wide range of insecticidal properties to provide a robust quantitative prediction of insecticidal activity. Insecticidal activity in the tree-derived monoterpenes tested, including pinene, was predicted to be weak (LD50 against houseflies of 190 mg kg−1 ) and, therefore, such materials are unlikely to be good candidates for such uses.
D.B. Turley et al. / Industrial Crops and Products 24 (2006) 238–243
In addition, models for the prediction of antibiotic potency of monoterpenes against Escherichia coli T. Escherich (e.g. gastroenteritis), Staphylococcus aureus Rosenbach (e.g. Staphylococcal food poisoning) and Candida albicans C.P. Robin, Berkhout (e.g. candidiasis) were generated. The potency of three monoterpenes derived from P. sylvestris, l-Carvone, -Myrcene and Citral were evaluated against these micro-organisms. lCarvone from P. sylvestris was predicted to be one of the most potent monoterpenes in killing E. coli (Table 1). Less potent antimicrobial monoterpenes, such as linalool are already being extracted from herbs for incorporation into plastic food wraps (Biever, 2003). QSAR models can assist the development of these product applications
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Table 1 MIC (minimum inhibitory concentrations in parts per million) predicted by QSAR model for three Pinus sylvestris derived monoterpenes for Escherichia coli, Staphylococcus aureus and Candida albicans
l-Carvone -Myrcene Citral
E. coli
S. aureus
C. albicans
1000 10000 4000
6000 11000 5000
3000 3000 1000
by predicting those metabolites that are both effective and safe to consumers. This technique can be used with any biological or chemical activity so long as a dataset of activity is avail-
Fig. 1. Possible technology routes to exploit and add values to Sitka Spruce metabolites through green chemical modification.
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able or can be generated. Expansion of this approach, combined with a database of computer-generated structures for those metabolites presented in the database, would significantly aid the screening and reduce the development costs for natural tree products. By combining existing expertise and these new technologies it is possible to identify potentially useful metabolites without the need for an extensive range of expensive, laborious and technically exacting laboratory screening tests. 4. Applications of Green Chemical Technologies In parallel to the above approaches, the York Clean Technology Centre (CTC) has been carrying out preliminary research on the application of Green Chemical Technologies to exploit and add value to tree metabolites. A potential technology route-map for exploitation of resources derived from UK forest tree species, such as Sitka Spruce is outlined in Fig. 1. Ultrasonic activation provides an energy efficient and low chemical input method for separating the primary metabolites and for physically expanding the structure of cellulose. As has been demonstrated for starches (Budarin et al., 2005), physical expansion of naturally low surface area carbohydrates provides a method for significantly broadening the range of applications (e.g. as chromatographic stationary phases and catalyst supports) and value of the materials without chemical change. Green chemical modification can further add application diversity and value for example, by improving stability towards degradation. Microwave activation is another energy efficient technology that can be used in the processing of tree primary metabolites, for example, by controlled decomposition of the lignin into small and valuable aromatic chemical products. Controlled carbonisation can lead to new mesoporous carbonaceous materials with value in catalysis, remediation and composites. The use of volatile organic solvents to directly extract chemicals from biomass is known, but the method is not compatible with low environmental impact technology. Supercritical carbon dioxide can be used to efficiently extract and even fractionate chemical products directly from tree components or after pre-processing of the plants (e.g. using microwave activation or alcohol extraction). The valuable products that can be extracted include waxes with uses in cosmetics, and wax components that can be used as nutraceuticals and insect semiochemicals (Deswarte et al., 2005). In this way, it should be possible to develop environmentally and economically viable methods for converting low value tree components into valuable and sustainable chemical
products, replacing conventional petroleum derived products which are becoming increasingly expensive or in some cases are insufficient to meet market demands. 5. Final comments There are clearly interesting opportunities for secondary metabolites found in tree species that need to be further evaluated. There is currently a lack of information on potential yields and yield variability associated with secondary metabolites in tree species. This is a particular problem for those tree tissues that are normally discarded, such as leaves, roots and bark. Metabolite yield can also be affected by factors, such as the delay between harvest and processing (an important factor if the tree species is sparsely distributed) and processing methods can also have an effect. For example, pilot studies at the Oregon State University showed that valuable good quality waxes could be extracted from fresh, mature Douglas fir logs. However, waxes produced from mill-run bark were heavily contaminated with resinous extracts from the underlying wood. The additional processing required to ‘clean-up’ the wax ensured that the product failed to be commercialised (Hergert, 1989). To help commercial development, more information is required on the effect of species and environmental factors (e.g. climate, geology, season) as well as extraction methods on the metabolite composition and hence the quantity and quality of tree products. Metabolite extraction processes (typically involving organic solvents) often produce low yields, are relatively non-selective (complex downstream processing is required to remove contaminants), can be destructive to labile tree metabolites, environmentally damaging and hazardous to operators. Using such techniques is often found to be uneconomic or of borderline viability. Traditional aqueous technologies leave saturated wood residues unsuited for combustion in energy intensive wood processing plants. Supercritical fluid extraction (SFE) has potential as an efficacious means of recovering secondary metabolites from the wood, such as waxy lipids, terpenes and phenolics. SFE methods can extract plant products more effectively and faster than conventional organic solvents. SFE using carbon dioxide, at temperatures above 31 ◦ C and pressures beyond 73 atm, has been the subject of increasing interest. It is used on an industrial scale for coffee decaffeination and smaller plants are also operating in the UK. The main drawback is the cost of the equipment. Experimental continuous sub-critical water extraction is also showing promise for essential oil extraction (Lang and Wai, 2001).
D.B. Turley et al. / Industrial Crops and Products 24 (2006) 238–243
The scattered distribution of some species may make metabolite extraction prohibitively expensive unless high-value markets can be developed. Investment costs for new developments and production facilities could be minimised by co-locating extractive and timber processing facilities. Technical developments in both chemistry and extraction technologies can offer new means of market exploitation. The development of QSAR type techniques can also help speed up the identification of potentially useful compounds. In addition, as the number of similar databases grows then it should also be possible to help identify whether any particular chemical metabolite is unique to a particular species and therefore less likely to be affected by competition. The creation of this database of metabolites is the start of a process to help identify potentially useful secondary metabolites in temperate forest tree species. Initial work by the authors to add value to both major (cellulose and lignin) and minor metabolites in Sitka Spruce trees has involved evaluation of different extraction methods (including supercritical CO2 and controlled pyrolysis methods) on freshly felled foliage and wood derived from tree-tops and evaluation of the resulting extracts for various activities of interest. This is being run alongside work to develop added value uses for cellulose and waste pulp liquor that are also showing promise. This broad outlook on possible means of exploitation will be supported by an economic and market analysis to assess the potential for commercialisation and ability to diversify market outlets, with the overall aim of adding value to the rural community which relies on forestry income. Acknowledgments Funding for this and ongoing work was provided by the UK Forestry Commission and the Department for the Environment, Food and Rural Affairs.
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References Biever, C., 2003. Herb extracts wrap up lethal food bugs. New Sci. 178 (2399), 26. Budarin, V., Clark, J.H., Deswarte, F.E.I., Hardy, J.J.E., Hunt, A.J., Kerton, F.M., 2005. Delicious not Siliceous: expanded carbohydrates as renewable separation media for column chromatography. Chem. Commun. 23, 2903–2905. Chopra, M., Fitzsimons, P.E., Strain, J.J., Thurnham, D.I., Howard, A.N., 2000. Non-alcoholic red wine extract and quercetin inhibit LDL oxidation without affecting plasma antioxidant vitamin and carotenoid concentrations. Clin. Chem. 46 (8), 1162– 1170. Deswarte, F.E.I., Clark, J.H., Hardy, J.J.E., Rose, P.M., 2005. The fractionation of valuable wax products from wheat straw using carbon dioxide. Green Chem. 8, 39–42. Dhanawat, M., Singh, G.K., Paul, A., 2005. Pharmacology and potential therapeutic uses of quercetin—a plant flavonoid. Indian J. Nat. Prod. 21 (2), 3–11. Harms, H., 1998. Wood, a versatile chemical material. In: European Conference on Renewable Raw Materials, Gmunden. Hergert, H.L., 1989. Lignans. In: Rowe, J.W. (Ed.), Natural Products of Woody Plants. Springer-Verlag, Berlin, Germany, pp. 349– 511. Lang, Q., Wai, C.M., 2001. Supercritical fluid extraction in herbal and natural product studies—a practical review. Talanta 53, 771– 782. Morand, C., Crespy, V., Manach, C., Besson, C., Demigne, C., Remesy, C., 1998. Plasma metabolites of quercetin and their antioxidant properties. Am. J. Physiol. 275 (1), 212–219. NREL, 2003. US National Renewable Energy Laboratory. http://www.nrel.gov. Schales, C., Gerlach, H., Koster, J., 1993. Investigation on the antibacterial effect of conifer needle oils on bacteria isolated from faeces of captive capercaillies (Tetrao urogallus). J. Vet. Med. 40, 381– 390. Watkins, R.W., Lumely, J.A., Gill, E., Bishop, J., Langton, S.D., MacNicoll, A., Price, N.R., Drew, M.G.B., 1999. Quantitative structureactivity relationships (QSAR) of cinnamic acid bird repellents. J. Chem. Ecol. 25, 2825–2845. Winks, M., Schimmer, O., 1999. Modes of action of defensive secondary metabolites. In: Wink, M. (Ed.), Function of Plant Secondary Metabolites and their Exploitation in Biotechnology. Sheffield Academic Press, Sheffield. UK, pp. 17– 133.
Chemical products from temperate forest tree species—Developing strategies for exploitation D.B. Turley a,∗ , Q. Chaudhry a , R.W. Watkins a , J.H. Clark b , F.E.I. Deswarte b b
a Central Science Laboratory, Sand Hutton, York YO41 1LZ, UK Clean Technology Centre, University of York, Heslington, York Y010 5DD, UK
Abstract Forest trees are integral to the landscape and rural economy. Unfortunately, the economic returns from timber production have declined significantly across Europe. Only 25% of felled wood is converted to timber, the remaining material is a rich composite of primary and secondary metabolites and plant fibres, a relatively unexplored and unexploited resource for potentially novel products that could compliment revenue from traditional market outlets. Wood from temperate forest trees has traditionally been used as a source of tannins, terpenes, rosins and aromatic phenolic compounds. Existing information on such chemical groups, and other secondary metabolites was collated for a range of temperate forest tree species including, alder (Alnus glutinosa L. Gaertn.), ash (Fraxinus excelsior L.), aspen (Populus tremula L.), beech (Fagus sylvatica L.), birch (Betula pendula Roth., Betula pubescens Ehrh.), cherry (Prunus avium L.), Corsican pine (Pinus nigra Arnold), Douglas fir (Pseudotsuga menziesii Mirib. Franco), larch (Larix deciduas Mill., Larix kaempferi Sarg.), oak (Quercus robur L., Quercus petraea Mattuschka, Liebl), poplar (Populus nigra L., Populus gileadensis Rouleau, Populus alba L., Populus canescens Ait. Sm.), Scots pine (Pinus sylvestris L.), sitka spruce (Picea sitchensis Bong. Carr.) and willow (Salix alba L., Salix fragilis L.). Over 37,000 records were extracted from phytochemical databases, research papers, conference proceedings, books, unpublished reports and company literature, covering identified metabolites, the tissues from which they were extracted (e.g., bark, leaves, heartwood, roots), reported yields, properties and hazards. Very little data exists on the yield or variability of individual metabolites limiting the ability to assess economic potential. The information sourced is collated in a database, available to view at http://treechemicals.csl.gov.uk. Traditional and new markets for exploitation of tree metabolites are reviewed along with possible methods of extraction. Computeraided Quantitative-Structure Activity Relationship Modelling (QSAR) augmented the search for novel applications for the tree metabolites. By this method, monoterpenes with useful anti-microbial properties were identified. Application of green chemical technologies also show promise in adding value to tree metabolites including the modification of cellulose and the benign extraction of valuable chemical products. Opportunities for possible future routes of exploitation of wood biomass are presented and discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Metabolites; Wood extractives; Tree extractives; Phytochemicals
1. Introduction There are 2.7 million hectares of woodland in Great Britain. Conifer species account for 59% and ∗
Corresponding author. Tel.: +44 1904 462791. E-mail address: [email protected] (D.B. Turley).
0926-6690/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2006.06.016
broadleaved species 41% of the forest area. Current annual production of wood from UK conifers is in the region of 10 million cubic metres (overbark standing). Outputs from broad-leaved species are small in comparison. The total volume (standing biomass) of wood produced in Great Britain has been increasing over the past decade and future predictions suggest a continuing
D.B. Turley et al. / Industrial Crops and Products 24 (2006) 238–243
increase over the next 20 years. Trees planted during the boom of the 1970’s and 1980’s will come to maturity in the next decade, but prices for timber have slumped and the decline is continuing. Increasing supply at a time of low prices is likely to exacerbate current problems and many plantations are unlikely to be worth extracting for timber or pulp. Finding ways of adding value to such wood resources may help in maintaining rural enterprises. 1.1. UK wood resource for extractive industries Only around 54–58% of the green wood delivered to sawmills is converted to saleable timber, leaving significant volumes of timber co-product. The majority of this is used in the pulp and board industry. The UK timber industries have been effective in adding value to the wood supply and co-product chains. Any potentially new products from forest trees, therefore, either have to compete with wood board and panel industries, add significant value or utilise other process residues (e.g. bark). Another possible means of increasing the market value is by adding value to the by-products of pulp processing. Other sources of wood currently not utilized include branches and other materials derived from forest management. Such resources are more widely scattered and require more effort to collect and transport, so high value outlets are likely to be required to justify costs of collection. 1.2. Current added value uses An example of the ways in which value can be added to wood processing is described by Harms (1998). The Austrian company Lenzing extracts cellulose fibres by a chemical pulping process for use in textiles (Viscose and Lyocell (derived from Fagus sp., and Eucalyptus and Pinus sp., respectively)) and industrial processes. This typically utilises around 40% of the tree biomass. Lenzing has sought to utilise the remaining biomass as a source of fine chemicals; using a further 10% of the harvested biomass with the remainder used to produce energy (Harms, 1998). This is a significant improvement on typical tree biomass utilisation rates of around 35–40%. Further industrial use of products from chemical pulp is constrained by the relatively low-cost of alternatives, such as petrochemically derived synthetic polymers. Other approaches also show promising perspectives. In the US (NREL, 2003), work is progressing to develop phenol-formaldehyde resins from sawdust and bark which have particular uses in heat-set adhesives used for wood board production (ply-wood and
239
stranded boards). Resins derived from pyrolysis of softwoods cost around 25% less than synthetic resins and also reduce setting times, speeding up the manufacturing process. These developments reflect a determination to focus leading-edge expertise on high value markets to justify the costs of extraction. 1.3. Secondary metabolites from trees Flavonoids, terpenes, phenols, alkaloids, sterols, waxes, fats, tannins, sugars, gums, suberins, resin acids and carotenoids are among the many classes of compounds known as secondary or special metabolites (cellulose, hemi-cellulose and lignin being classed as primary metabolites). The range of compounds is extensive, with wide ranging chemical, physical and biological activities. The concentrations of these metabolites in trees vary between species (they may contain as little as 1% or as much as one-third of their dry weight as secondary metabolites), within species, between tissues (higher concentrations occur in bark, heartwood, roots, branch bases and wound tissues) and from year to year. They thus present numerous challenges to utilisation. The production and accumulation of a wide variety of organic chemicals is one of the major mechanisms by which plants defend themselves against herbivores and attacks by microbial pathogens and invertebrate pests. Most of these chemicals are products of secondary metabolism, originally thought to be the waste products not needed by plants for primary metabolic functions. It is well known that their presence in different parts of the plant (root, leaves, bark, etc.) deters feeding by slugs, snails, insects and vertebrates, as well as attacks by viruses, bacteria and fungi (Winks and Schimmer, 1999). For example, essential oil extracted from Pinus sylvestris has been shown to possess strong antibacterial activity (Schales et al., 1993). Such plant metabolites have been used for centuries for dyeing, fragrances, flavouring and as a source of medicinal compounds. The potential use of plant metabolites in health care and personal care products and as lead compounds for the development of novel drugs has led to a huge interest in their isolation and characterisation. Trees possess a number of potentially useful metabolites. Polyphenolic flavonoids have pharmacologically useful antioxidant properties. For example, quercetin, found in a number of forest trees (and in particular, the bark of Quercus spp.), has been reported to block the sorbitol pathway (linked to problems associated with diabetes) (Dhanawat et al., 2005). It may also protect beneficial low-density liposomal cholesterol (LDL) in the body (Morand et al., 1998; Chopra et al.,
240
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2000). Such flavonoid molecules commonly occur in foliage, bark, sapwood and heartwood in trees. As a first step towards identifying possible routes of exploitation, a review was undertaken to collate data on documented reports of metabolites found in relevant tree species, to create a searchable database resource. 2. Metabolite database Information was collated on the primary and secondary metabolites of alder (Alnus glutinosa), ash (Fraxinus excelsior), aspen (Populus tremula), beech (Fagus sylvatica), birch (Betula pendula, Betula pubescens), cherry (Prunus avium), Corsican pine (Pinus nigra), Douglas fir (Pseudotsuga menziesii), larch (Larix decidua, Larix kaempferi), oak (Quercus robur, Quercus petraea), poplar (Populus nigra, Populus gileadensis, Populus alba, Populus trichocarpa), Scots pine (P. sylvestris), Sitka spruce (Picea sitchensis) and willow (Salix alba, Salix fragilis). Information was extracted from bibliographic databases, phytochemical databases, research papers, conference proceedings, books, unpublished reports and company literature. In total, over 37,000 records published over the last 3 decades was gathered and interrogated. Data extracted from these records included the metabolites identified, the tissues from which they were extracted (e.g. bark, leaves, heartwood, roots), reported yields, properties, hazards, CAS numbers (compoundspecific identifier codes), extraction methodologies (if known) and current and future market potential for these tree products. This enabled the creation of a searchable database accessible to all interested parties in the supply chain from foresters through to industrial innovators. The database is free to access at http://treechemicals.csl.gov.uk. 2.1. Key findings There are numerous examples where tree-derived products have found commercial markets. For example, xylitol (used as a low calorie sweetener) derived from Finnish birch. Resins, waxes and oils can be extracted from living trees by ‘tapping’, or from the bark and wood during pulping. The most important by-products of wood pulping are turpentine and tall oil (used to produce a variety of resins (rosin) and which also has the potential to provide useful fatty acids). Tree-derived turpentine is still widely used in industry as a solvent in paint and varnish (though these sectors are moving to other solvent bases) and it can be fractionated into other useful
chemical components. Its use depends on chemical composition, determined by both tree species and method of extraction. Monoterpenes (primarily ␣-pinene) are used to prepare synthetic pine oil (the biggest single turpentine derivative), polyterpine-resins, fragrances and insecticides. Demand for use of ‘green solvents’, such as turpentine is also increasing. Larch arabinogalactan is a polysaccharide found in all larch species (5–35% of dry weight in heartwood) used as a replacement for gum arabic in food, cosmetic and pharmaceutical products. The sheer volume of recorded metabolites poses a problem in identifying the most suitable for further assessment and development. In addition, there have been few attempts to quantify the yield of any metabolites found. Most are reported on a ‘presence’ only basis and source materials are not always reported. All of this limits the possibility to assess the market potential of any reported metabolite. In addition, the variability in metabolite yield within and between species also needs to be clarified to identify those metabolites with the greatest opportunities for commercial success. One means of assisting in this search is use of computer-based tools, such as Quantitative-StructureActivity-Relationship modelling (QSAR). 3. QSAR studies Complementary to the traditional approaches of identifying uses for natural products (e.g. on-line searches of phytochemical databases), computational Quantitative Structure-Activity Relationship models have been developed that can predict the activity of a compound based solely on its structure. It is a technique increasingly adopted by food and pharmaceutical companies in the search for new molecules and functionalities. This technique has already been used to identify and predict the activity of tree metabolites as bird repellents that are now the subject of commercial evaluation (Watkins et al., 1999). Using computer-aided molecular design software, 3D-representations for a few select tree-derived metabolites were built. This was limited to an assessment of the antimicrobial and insecticidal activities of monoterpenes. A QSAR model was created using published and in-house generated data for a series of 34 monoterpene compounds with a wide range of insecticidal properties to provide a robust quantitative prediction of insecticidal activity. Insecticidal activity in the tree-derived monoterpenes tested, including pinene, was predicted to be weak (LD50 against houseflies of 190 mg kg−1 ) and, therefore, such materials are unlikely to be good candidates for such uses.
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In addition, models for the prediction of antibiotic potency of monoterpenes against Escherichia coli T. Escherich (e.g. gastroenteritis), Staphylococcus aureus Rosenbach (e.g. Staphylococcal food poisoning) and Candida albicans C.P. Robin, Berkhout (e.g. candidiasis) were generated. The potency of three monoterpenes derived from P. sylvestris, l-Carvone, -Myrcene and Citral were evaluated against these micro-organisms. lCarvone from P. sylvestris was predicted to be one of the most potent monoterpenes in killing E. coli (Table 1). Less potent antimicrobial monoterpenes, such as linalool are already being extracted from herbs for incorporation into plastic food wraps (Biever, 2003). QSAR models can assist the development of these product applications
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Table 1 MIC (minimum inhibitory concentrations in parts per million) predicted by QSAR model for three Pinus sylvestris derived monoterpenes for Escherichia coli, Staphylococcus aureus and Candida albicans
l-Carvone -Myrcene Citral
E. coli
S. aureus
C. albicans
1000 10000 4000
6000 11000 5000
3000 3000 1000
by predicting those metabolites that are both effective and safe to consumers. This technique can be used with any biological or chemical activity so long as a dataset of activity is avail-
Fig. 1. Possible technology routes to exploit and add values to Sitka Spruce metabolites through green chemical modification.
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able or can be generated. Expansion of this approach, combined with a database of computer-generated structures for those metabolites presented in the database, would significantly aid the screening and reduce the development costs for natural tree products. By combining existing expertise and these new technologies it is possible to identify potentially useful metabolites without the need for an extensive range of expensive, laborious and technically exacting laboratory screening tests. 4. Applications of Green Chemical Technologies In parallel to the above approaches, the York Clean Technology Centre (CTC) has been carrying out preliminary research on the application of Green Chemical Technologies to exploit and add value to tree metabolites. A potential technology route-map for exploitation of resources derived from UK forest tree species, such as Sitka Spruce is outlined in Fig. 1. Ultrasonic activation provides an energy efficient and low chemical input method for separating the primary metabolites and for physically expanding the structure of cellulose. As has been demonstrated for starches (Budarin et al., 2005), physical expansion of naturally low surface area carbohydrates provides a method for significantly broadening the range of applications (e.g. as chromatographic stationary phases and catalyst supports) and value of the materials without chemical change. Green chemical modification can further add application diversity and value for example, by improving stability towards degradation. Microwave activation is another energy efficient technology that can be used in the processing of tree primary metabolites, for example, by controlled decomposition of the lignin into small and valuable aromatic chemical products. Controlled carbonisation can lead to new mesoporous carbonaceous materials with value in catalysis, remediation and composites. The use of volatile organic solvents to directly extract chemicals from biomass is known, but the method is not compatible with low environmental impact technology. Supercritical carbon dioxide can be used to efficiently extract and even fractionate chemical products directly from tree components or after pre-processing of the plants (e.g. using microwave activation or alcohol extraction). The valuable products that can be extracted include waxes with uses in cosmetics, and wax components that can be used as nutraceuticals and insect semiochemicals (Deswarte et al., 2005). In this way, it should be possible to develop environmentally and economically viable methods for converting low value tree components into valuable and sustainable chemical
products, replacing conventional petroleum derived products which are becoming increasingly expensive or in some cases are insufficient to meet market demands. 5. Final comments There are clearly interesting opportunities for secondary metabolites found in tree species that need to be further evaluated. There is currently a lack of information on potential yields and yield variability associated with secondary metabolites in tree species. This is a particular problem for those tree tissues that are normally discarded, such as leaves, roots and bark. Metabolite yield can also be affected by factors, such as the delay between harvest and processing (an important factor if the tree species is sparsely distributed) and processing methods can also have an effect. For example, pilot studies at the Oregon State University showed that valuable good quality waxes could be extracted from fresh, mature Douglas fir logs. However, waxes produced from mill-run bark were heavily contaminated with resinous extracts from the underlying wood. The additional processing required to ‘clean-up’ the wax ensured that the product failed to be commercialised (Hergert, 1989). To help commercial development, more information is required on the effect of species and environmental factors (e.g. climate, geology, season) as well as extraction methods on the metabolite composition and hence the quantity and quality of tree products. Metabolite extraction processes (typically involving organic solvents) often produce low yields, are relatively non-selective (complex downstream processing is required to remove contaminants), can be destructive to labile tree metabolites, environmentally damaging and hazardous to operators. Using such techniques is often found to be uneconomic or of borderline viability. Traditional aqueous technologies leave saturated wood residues unsuited for combustion in energy intensive wood processing plants. Supercritical fluid extraction (SFE) has potential as an efficacious means of recovering secondary metabolites from the wood, such as waxy lipids, terpenes and phenolics. SFE methods can extract plant products more effectively and faster than conventional organic solvents. SFE using carbon dioxide, at temperatures above 31 ◦ C and pressures beyond 73 atm, has been the subject of increasing interest. It is used on an industrial scale for coffee decaffeination and smaller plants are also operating in the UK. The main drawback is the cost of the equipment. Experimental continuous sub-critical water extraction is also showing promise for essential oil extraction (Lang and Wai, 2001).
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The scattered distribution of some species may make metabolite extraction prohibitively expensive unless high-value markets can be developed. Investment costs for new developments and production facilities could be minimised by co-locating extractive and timber processing facilities. Technical developments in both chemistry and extraction technologies can offer new means of market exploitation. The development of QSAR type techniques can also help speed up the identification of potentially useful compounds. In addition, as the number of similar databases grows then it should also be possible to help identify whether any particular chemical metabolite is unique to a particular species and therefore less likely to be affected by competition. The creation of this database of metabolites is the start of a process to help identify potentially useful secondary metabolites in temperate forest tree species. Initial work by the authors to add value to both major (cellulose and lignin) and minor metabolites in Sitka Spruce trees has involved evaluation of different extraction methods (including supercritical CO2 and controlled pyrolysis methods) on freshly felled foliage and wood derived from tree-tops and evaluation of the resulting extracts for various activities of interest. This is being run alongside work to develop added value uses for cellulose and waste pulp liquor that are also showing promise. This broad outlook on possible means of exploitation will be supported by an economic and market analysis to assess the potential for commercialisation and ability to diversify market outlets, with the overall aim of adding value to the rural community which relies on forestry income. Acknowledgments Funding for this and ongoing work was provided by the UK Forestry Commission and the Department for the Environment, Food and Rural Affairs.
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