Sugar-cane bagasse is a fibrous waste-product of the sugar refining industry, which can be recycled as a raw material for paper manufacture. IRD researchers have elaborated a new bioprocess that transforms the bagasse into paper pulp and also produces an industrially useful enzyme, laccase. The process is based on the metabolism of a filamentous fungus. Preliminary laboratory trials show that this integrated bioprocess can be adapted to other potential fibre-yielding materials, opening up promising applications for the paper industry.
The principal raw material used for manufacturing paper pulp is wood. However, growing demand in the paper industry, at a time of dwindling forest resources, have compelled the sector to turn to other sources of raw materials, such as cereal straw, reeds, bamboo or sugar-cane bagasse. This residue, obtained after crushing of the cane, is already used as a source of paper-making fibres in producer countries (in South America and India for example, where it represents 20 % of the paper production). The industry absorbs 10% of the world bagasse production. This material offers several advantages: rapid growth of the sugar-cane plant, widespread cultivation, lower energy and bleaching chemical requirements for bagasse refining. Such a process is also a convenient means of usefully clearing this voluminous sugar refinery waste product: indeed, one tonne of refined sugar results in two tonnes of bagasse.However, whatever the raw material used, paper pulp has to undergo processing stages of delignification and bleaching to turn it into high-strength and durable paper. In some countries the chemical processing involved still entail the use of chlorine, dangerous for both health and the environment (2).
Research scientists from the IRD and INRA studied an alternative, biologically based, solution. Laboratory experimentation enabled them to develop a non-polluting process, which at the same time yields a delignifying enzyme, laccase, from a culture of a filamentous fungus and effectively recycles the sugar-cane bagasse. Its principle lies in the specific metabolic characteristics of this fungus, Pycnoporus cinnabarinus, which produces laccase naturally. This enzyme breaks down the lignin in the fibres of bagasse used as substrate in these trials, transforming this waste product, after mechanical refining, into paper pulp. As the lignin progressively disappears, the pulp obtained becomes bleached. This pulp can be used as it is to make cardboard, but it must undergo additional treatment using hydrogen peroxide in order to yield paper for printed and writing.
P. cinnabarinus naturally sythesizes only small amounts of laccase when it grows on bagasse. It is necessary to add volatile agents such as ethanol, in order to increase production of the enzyme under these conditions (3). Ethanol was chosen as a laccase-inducer in this study because of its abundance, its low toxicity and low production cost. The research team moreover showed that if it was put into the system by forced convection at a rate of 7 g of ethanol per m3 (concentration equivalent to 3? of alcohol in the liquid phase), laccase production increased, to a maximum level (90 U per g of dry bagasse support). This amounts to 45 times the yield obtained without ethanol. Moreover, it appeared that little or no ethanol introduced was consumed by the fungus which preferentially uses other sources of carbon, resulting from the bagasse (saccharose) or put in with the substrate (maltose, yeast extracts and so on). It can therefore be recycled in the system or eliminated in a second system associated with it (4).
Replication of the fermentation trials at a larger scale, in an 18 litre bioreactor, confirmed the efficiency of the laccase production obtained using bagasse and ethanol (90 000 U per kg of dry bagasse after 30 days, representing the quantity needed for processing, without input of fungus, an extra 4 kg of bagasse). This bioprocess resulted in a 50% saving in energy consumption required for paper pulp refining, compared with that recorded for refining pulp from bagasse that had not been biologically treated. Another benefit came in the form of a 35% improvement in the paper’s mechanical characteristics (tensile strength and tear resistance) without appreciable loss of material.
The results as a whole emphasize the potential for applications of this bioprocess in the paper industry. Retrieval of the laccase at the end of the cycle, after washing and pressing of the bagasse, allows additional quantities of the substrate to be processed and, in this way, raise the profitability of the operation. Furthermore, this process can be adapted to the processing of other raw materials (wood, cereals). Investigation of the use of methanol as laccase inducer can, similarly, be envisaged as a way of recycling this compound, which constitutes one of the main pollutants emitted by the paper industry.