Applications of biotechnology in the forests products industry

Francois Woldaardt

Biotechnology is the application of biological systems in technology that can only be achieved through an integration of the biological, physical and engineering sciences. The current approach of biotechnology, in the forest products industry is to apply biological products such as enzymes rather than live cultures. Enzymatic action is very specific and also easy to control. This paper considers biotechnological applications in different operations including forestry, wood preservation, pulping, bleaching, deinking, papermaking and water treatment. The benefits of biotechnological applications are: improved product quality, production rate or diminished environmental impact. Utilisation of enzymes will become more widespread as biological products become less expensive through large-scale production.

INTRODUCTION

Biotechnology has been defined as the integrated use of biological, physical and engineering sciences in order to achieve technological application of biological systems. The goal of technological application implies that biotechnology excludes fundamental research and that it must be relevant to industry. Fundamental studies do, however, play a crucial role in the development of the applicable biotechnological processes. Recombinant DNA technology can, for example, be used to exploit or enhance the properties of natural biological systems for industrial purposes.

Most biotechnological processes make use of microorganisms such as bacteria, yeasts and filamentous fungi, but vascular plants, algae and even animal tissue can also be utilised. Classical biotechnological processes have been used for ages in the production of food and beverages such as bread, cheese, beer and wine. These processes are characterised by the direct application of live organisms and the in situ production of enzymes and other products. New biotechnology has a stronger focus on the application of biological products or enzymes and these are often produced ex situ. The advantages of the new biotechnology over classical biotechnology are that mass balances are easier to solve and process control is improved.

The forest products industry is based on the processing of biological raw material and it is, therefore, well suited to the introduction of biotechnology. Biotechnological processes have been developed for most of the unit operations of the industry and a number of them are commercialised. The aim of this paper is to examine the potential of different biotechnological processes and their integration into conventional operations of the forest products industry.

BIOTECHNOLOGY APPLICATIONS

Forestry
Biotechnology can be applied in forests and especially plantations to improve fibre production and to protect trees. When exotic tree species were initially planted in South Africa, it was observed that stunted growth occurred due to the absence of mycorrhizal symbionts. Soil, containing these fungi, was then introduced into the country to improve growth of trees. Today, it is believed that fibre production can be improved by inoculating seedlings with more efficient mycorrhizal fungi. Mycorrhizal fungi enhance plant growth through improvement of mineral and water absorption, protection against pathogens and even secretion of growth enhancing hormones (1).

Tree protection can be achieved by using biological control agents such as fungi or bacteria. The biological control agents (BCAs) are most effective against root diseases and especially where they are applied in an enclosed environment such as seedling trays. These BCAs act by competing with pathogens for nutrients and space, by secreting toxic substances or by direct consumption of the pathogens (2).

Treatment of Timber
After felling, logs can be treated with fungi to improve debarking or to protect the wood against blue staining and decay. Inoculation of softwood logs with the white-rot fungus Phlebiopsis gigantea leads to rapid colonisation of the logs during transport or storage. The fungus is able to loosen bark, reduce pitch and protect the wood against sap staining (3). The loosening of bark is accomplished during the degradation of cambial cells by the fungus. The best-known biological agent to protect timber against blue staining is the fungus Ophiostoma piliferum. A white mutant of this fungus has been selected and the inoculum is produced commercially as Cartapip® (4). The fungus is a primary coloniser of wood and is able to utilise sugars and extractives in the wood without affecting the strength of the wood. These characteristics enable the fungus to out-compete any sap-stain fungi that subsequently infect the wood.

Biopulping
Biopulping is a solid-substrate fermentation (SSF) process where lignocellulosic materials are treated with fungi prior to pulping in order to reduce energy during mechanical pulping or to reduce chemical consumption during chemical pulping processes. Research has focussed on the utilisation of lignin-degrading fungi for biopulping, but the only commercial process currently available, utilises O. piliferum. Wood chips have also been treated successfully with fungal enzymes to improve the penetration of pulping liquor (5).

The aim of bio-thermomechanical pulping is to reduce the energy consumption during pulping and to improve pulp strength (6). The beneficial effects of fungal treatment on mechanical pulping can be ascribed to the reduction of the binding capacity of fibres and improved fibrillation. During bio-chemical pulping, the lignin content of the pulp or pulping time can be reduced (7). These benefits are the result of increased lignin solubility that is caused by fungal degradation (8). Depitching is one application of biopulping where Cartapip® is applied to stored wood chips and the pitch in the wood is reduced by the fungus metabolising it (9). It is evident that different fungi are suited to different pulping processes, but these processes must be adapted to achieve the full potential of an environmentally friendly technology.

Pulpwood Preservation
Cost of wood contributes more than 80 % to the variable cost of unbleached chemical pulp. Contaminating organisms that degrade cellulose or cause staining of wood can, therefore, have a significant impact by reducing yields of chemical pulp (10) or necessitating bleaching of mechanical pulp (11). Biopulping fungi are inoculated at high dosages and incubated under conditions that favour their ability to colonise and compete with undesirable organisms. These fungi are thus able to protect pulpwood against decay and staining organisms.

Biobleaching
Two different approaches have been used to enhance bleaching through enzymes. The first method utilise hemicellulolitic enzymes such as xylanase. These enzymes are unable to degrade lignin, but cause limited degradation of hemicellulose in kraft pulp to expose lignin to further attack by bleaching chemicals. Xylanase pre-bleaching is used at several kraft mills worldwide. In 1994, eight percent of Canada's bleached kraft pulp production was treated with xylanase (12). This technology holds environmental and economic advantages, especially since the enzymes have become less expensive as result of increased market size.

The typical chemical saving associated with the use of xylanase is more than 15 % active chlorine. Unfortunately, the limit of approximately 20 % in chemical savings cannot be exceeded. The second approach to enzymatic bleaching is the utilisation of oxidative enzymes, mainly laccase and manganese peroxidase (MnP) to delignify pulp. Of these enzymes, laccase is preferred, because MnP requires hydrogen peroxide, manganese (II) ions and a chelator. Laccase can cause delignification of pulp under slight oxygen pressure, but is considerably more effective when mediators are added (13). The most important obstacles to commercial application of laccases are the lack of sufficient enzyme stocks and the cost of mediators. Marked progress has been made over the last two years to solve these problems and it is expected that biobleaching with laccase will be able to compete with other TCF bleaching processes.

Enzymatic Deinking
Enzymatic deinking can be applied on old newsprint and office waste, but it is more effective on the latter, because it contains large amounts of laser and xerographic inks that are more difficult to remove by conventional methods (14). The commercial cellulases used, are able to hydrolyse fines to release ink particles. The ink can then be removed by chemical or air flotation. A secondary benefit of the enzyme treatment is that the stripping of fines from fibres results in improved drainage (15) without reduction in strength properties (16).

Improvements in Paper Making
Most of the applications of enzymes to improve the papermaking characteristics of pulp are aimed at the improvement of secondary fibre. Application of cellulases and hemicellulases to modify fibres has resulted in the following benefits (16):

  • Improved beating to save energy;
  • higher freeness;
  • enhanced drainage;
  • improvement of certain strength properties;
  • decreased disintegration time of recycled pulp, and
  • reversal of hornification.

In other applications, enzymes are used to remove the non-fibrous fractions of secondary fibre that hamper drainage. A good example of such material is residual starch that is especially abundant where old corrugated containers are used as raw material. Amylases can be applied to degrade remaining starch that would not contribute to the web characteristics, but would restrict drainage. In one mill, application of amylases resulted in a 6,9 % increase in production of paper (17).

Control of Microbial Fouling
Microorganisms in mill water systems cause slime or biofilm build-up, odour and corrosion. The consequences of the contamination are increased web breakages, holes and spots in paper as well as increased cost of maintenance. Biocides are generally used to control microbial build-up, and deposits are removed during boilouts by sodium hydroxide or dispersants. Inoculation of water systems with competing microorganisms or bacteriophages have been proposed, but the most feasible biotechnological approach appears to utilise enzymes to prevent aggregation of biofilm-producing organisms and degrade extra-cellular polysaccharides (18). These enzymes include proteases and carbohydrate-degrading enzymes and combinations of these enzymes are often applied. The enzymes also expose microorganisms to toxins by reducing protective polysaccharides, and are usually dosed in combination with biocides.

Effluent treatment
The oldest application of biotechnology in the pulp and paper industry is probably that of wastewater treatment. Biological processes are usually employed in secondary or polishing treatments that follow sedimentation or other primary treatment. Biological treatment processes include: Aerated stabilisation basins, activated sludge, oxygen-activated sludge, trickling filters, rotating biological contactors, anaerobic lagoons, upflow anaerobic sludge blankets, anaerobic filters and anaerobic fluidised beds (19). These systems are all characterised by a complex microbial community that is responsible for the improvement of water quality.

CONCLUSIONS

Biotechnology is applied on a commercial scale in many operations of the forest products industry. Application of enzymes has become very prominent recently, because they are highly selective in their action and have a negligible environmental impact. Wider application of these enzymes are restricted by cost and availability of the enzymes. This problem will soon cease to exist, given that the cost of enzymes has decreased considerably over the last decade as the market increased. A number of companies are now also producing enzymes specifically aimed at the pulp and paper market. One remaining hurdle, is the perceived incompatibility of biotechnology with conventional processes. This problem is more difficult to solve since it is largely due to a lack of understanding of biological systems and a lack of acceptance that biological products are expected to work under extreme conditions in mills. It is, therefore, clear that biotechnology can only be developed through integration and understanding of biological as well as physical sciences.

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