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.
Marie
Guillaume-Signoret – IRD
1.
Each of these teams is a partner of
the Universities of Provence and the Mediterranean . They are grouped together
within the research entity IFR 86-BAIM.
2.
In Europe, however, the paper
industry is turning increasingly towards completely chlorine-free processes.
3.
Lomascolo et al.- Overproduction of
laccase by a monokarryotic strain of Pycnoporus cinnabarinus, using ethanol as
inducer, J. Appl. Microbiol. 2003, 94, p. 1-7. Other research conducted by the
IRD, working jointly with the UAM (Autonomous University of Mexico) of Mexico
City and the ICIDEA (Cuban Institute of Research on Sugar-cane derivatives) of
Cuba, have shown that a yeast, Candida utilis, can be used to produce biomass
on the bagasse. It can thus provide a protein-rich feed for animals, while
eliminating ethanol in the process (air pollution removal). See the scientific
bulletin n°155, May 2002, on line on www.ird.fr/fr/actualites/fiches/2002/fiche155.htm
Key-words:
sugar-cane bagasse, biotechnology, industrially applicable enzyme, Pycnoporus
cinnabarinus, paper industry.