7
Use of Fungi in Pulping Wood:
An Overview of Biopulping Research
T. Kent Kirk, Richard R. Burgess, and
John W. Koning, Jr.
Introduction
Fresh wood chips destined and stored for pulp production are rapidly colonized
by a variety of microorganisms, including many species of fungi. These organisms
compete vigorously while easily assimilable foodstuffs last, and then their popula-
tions decrease. They are replaced by fungi that are able to degrade and gain
nourishment from the cell wall structural polymers: cellulose, hemicelluloses,
and lignin. Left unchecked, these last colonizers. mostly “white-rot fungi ,"
eventually decompose the wood to carbon dioxide and water. Some of them
selectively degrade the lignin component, which is what chemical pulping pro-
cesses accomplish. Biopulping is the concept of deliberately harnessing white-
rot fungi for pulping.
Pulp is produced from wood by either chemical delignification, mechanical
separation of the cells (fibers), or combinations of chemical and mechanical
methods. Mechanical pulping methods are used increasingly because they give
much higher yields (80% to 90% based on the wood) than chemcial methods
(40% to 50% yields). They also are less polluting than chemical methods. and
mills using these methods are much less expensive to buiId. Currently, about
25% of world pulp production is by mechanical means. The main disadvantages
of mechanical pulping methods are the production of lower quality pulps, which
are unsuitable for fiber products that need high strength properties, and the amount
of energy required for production (and consequent cost).
Chemical pretreatment of wood chips are used to enhance the strength proper-
ties of mechanical pulps. Making such chemimechanical pulps, however, gener-
ates chemical waste streams that mush be treated, and it lowers the pulp yield by
removing wood substance (mainly hemicelluloses and lignin). Biopulping as
studied to date is actually “biomechanical” pulping, the use of fungi to replace
chemicals in pretreating wood for mechanical pulping.
99
100 / Kirk, Burgess, and Koning
Past Work on Biopulping
Early chemical analyses of wood partly decayed by certain white-rot fungi re-
vealed that lignin had often been removed selectively; that is, the cellulose
content had increased. Actually, some naturally white-rotted woods are so heavily
delignified that they resemble chemical pulps and can be made into paper with
excellent properties. Thus, the concept of biopulping was probably obvious to
early investigators.
Perhaps the first serious consideration of fungal delignification for pulping was
by researchers at the West Virginia Pulp and Paper Company (now Westvaco
Corporation) research laboratory in the United States in the 1950s. Their investiga-
tion resulted in a published article (Lawson and Still, 1957) that surveyed 72
lignin-degrading fungi and summarized what was known about how the fungi
degrade lignin. At that time, very little was known. At about the same time, a
study of the effect of natural decay of pine by white-rot fungi on chemical pulping
showed that most paper strength properties increased as the extent of decay
increased (Reis and Libby, 1960: Kawase, 1962).
Any research on biopulping per se that might have been done by various
companies from the 1950s to the present has not been published. Also unpub-
lished, except as an internal report at the Forest Products Laboratory, was a 1972
study of “biomechanical” pulping by T. K. Kirk and Prof. Knut P. Kringstad,
then at North Carolina State University in Raleigh (USA). Aspen wood chips
were partly decayed by
Rigidoporus ulmarius
(Sow.:
Fr. ) Imaz. and mechanically
fiberized to a pulp, and the pulp was made into paper. Pulping these chips required
fewer revolutions in the pulping apparatus than did the pulping of untreated
control wood, suggesting lowered energy consumption; also, the paper from the
biomechanical pulp was stronger.
Similar work was done shortly thereafter at the Swedish Forest Products
Laboratory (STFI) in Stockholm, and the first published paper on biopulping per
se (Ander and Eriksson, 1975) described results very similar to those of Kirk and
Kringstad. In 1976, the Swedish researchers patented a “method for producing
cellulose pulp” (Eriksson et al.,
1976). After the initial study, this group worked
on various aspects of biopulping, primarily with the white-rot fungus
Sporotri-
chum pulverulentum
Novobranova. Mean while, our work at the Forest Products
Laboratory of the USDA Forest Service in Madison focused on the mechanism
of lignin degradation by a white-rot fungus that was tentatively referred to as
Peniophora “G .“
Both fungi were chosen because they grew and degraded lignin
quite rapidly in comparison to other fungi; they also produced copious conidia
and thus were easy to manipulate. It was a surprise to both laboratories when
the two fungi were found to be synonymous, and they are now classified as
Phanerochaete chrysosporium Burds. (Burdsall and Eslyn. 1974).
The Swedish researchers made a number of contributions to biopulping (review:
Eriksson and Kirk, 1985). They described the growth rates of
P. chrysosporium
Use
of
Fungi in Pulping Wood / 101
through wood, finding that colonization of pulpwood chips is unlikely to be rate-
determining. Scanning and transmission electron microscopy were used to show
the growth patterns in wood and the degradation patterns of the cell walls. The
group conducted studies on biomechanical pulping, showing energy savings
and paper strength improvements. A considerable effort went into developing
cellulase-less mutants of selected white-rot fungi for biopulping (Johnsrud and
Eriksson, 1985). Attempts by the group to scale up the biopulping process were
not notably successful (Samuelsson et al.,
1980). That work, however, was
undoubtedly premature because insufficient information was available on how to
scale up the fungal treatment. Subsequent work on a large scale with bagasse,
done in cooperation with Cuban scientists, gave more promising results (Johnsrud
et al., 1987).
Biopulping received little attention outside of Sweden until our recent investiga-
tions. In one small study, Bar-Lev et al. (1982) reported that treatment of a coarse
mechanical pulp with
P. chrysosporium
decreased the energy required for further
fiberization and increased paper strength properties. Akamatsu et al. (1984) found
that treatment of wood chips with any of 10 white-rot fungi decreased mechanical
pulping energy; with three of the fungi (Trametes sanguinea, T. coccinea, and
Coriolus hirsutus), treatment increased paper strength.
Biopulping Consortium Research
Taken together, the results of these various studies suggested to us in 1986 that
biomechanical pulping merited a comprehensive investigation. Consequently, in
April 1987 a cooperative research program on biopulping was established. involv-
ing the Forest Products Laboratory, the University of Wisconsin Biotechnology
Center, and nine pulp and paper and related companies. The number of companies
in Biopulping Consortium had grown to 20 by April 1990. The overall objective
of the 5-year consortium research effort is to evaluate the scientific and technical
feasibility of using a fungal pretreatment with mechanical pulping to save energy
and/or improve pulp and paper properties. In addition. we have assumed that the
fungal pretreatment will have less environmental impact than have chemical
pretreatment, a significant factor in its own right.
The Biopulping Consortium research group is divided into six closely coordi-
nated teams. The fungal research team screens species and strains of white-rot
fungi from culture collections, as well as new isolates. Screening is based on
growth and wood decay rates and on selectivity for lignin degradation in wood.
The team also works to optimize the fungal pretreatment and, importantly,
produces fungal-treated chips for evaluation by the pulp and paper research team.
The pulp and paper team determines energy consumption required for pulp
production and measures pulp and paper properties. The enzyme team seeks
to determine which of the extracellular enzymes secreted during the fungal
102 / Kirk, Burgess, and Koning
pretreatment are beneficial for pulping and which are not beneficial. Emphasis is
on the components of the lignin- and cellulose-degrading systems. The molecular
genetics team has focused on P.
chrysosporium
and ultimately seeks to engineer
improved strains for biopulping. Lignin- and cellulase-degrading systems again
are the focus. An engineering and scale-up team is looking at the fungal pretreat-
ment as an engineered solid substrate fermentation. and it is working with the
fungal research team to determine critical parameters. Supporting the other teams
is an information group. Using sophisticated computer search strategies, the
information team screens the scientific literature and specializes in retrieving
information from particular sources, such as Japanese patent applications.
The industrial partners partially fund the project and provide input during
semiannual meetings with the researchers. The industrial partners are provided
with both research results and synopses of the expanding world literature of
biotechnology as it affects or might affect the pulp and paper industry. Participa-
tion in the consortium also provides the industrial partners with ready personal
access to biotechnology researchers (most of the companies do not have them in
house) and acquaintance with students, postdoctoral associates, and technicians,
who constitute a potential employee pool.
The consortium has made good research progress. Some of the key published
findings are summarized in the following paragraphs. Details are given in the
cited papers.
Research was initiated by screening species and strains of white-rot fungi for
selective removal of lignin from wood blocks (Otjen et al., 1987: Blanchette et
al., 1988). Wide variation was found among species and among strains within
certain species. For example. in 12 weeks Peniophora hydnoides (Cke. and
Mass. ) M.P. Chris. [
=Phanerochaete rimosa (Cke. ) Burds. ] removed 26% of
the lignin and 24% of the glucan (cellulose) from birch wood. whereas
P.
chrysosporium
Burds. (strain BKM F-1767) was highly selective and removed
73% of the lignin and only 15% of the glucan. Similarly, in 12 weeks,
Heterobasi-
dion annosum
(Fr. ) Bref. removed nearly equal proportions of lignin and glucan
(26% and 23%) from pine, whereas
Ceriporiopsis subvermispora
(Pil. ) Gilbn. et
Ryv. removed 50% and 3% of lignin and glucan, respectively. Within the species
P. chrysosporium, strain HHB-11741 removed 51% and 48% of the lignin and
glucan from birch wood, wherease strain BKM F-1767, as noted, removed 73%
and 15% of lignin and glucan. respectively, pointing to substantial intraspecies
variation. Based on these initial screenings. several species-and in some cases,
specific strains—were chosen for biopulping studies. Screening continues. how-
ever, and some interesting new fungi have recently been selected for further
study. A total of over 200 strains have been screened. Although selective removal
of lignin does not correlate strictly with efficacy of biopulping pretreatment, the
fungi selected by this method have proved to be effective for biopulping.
Better screening methods are needed. A somewhat faster method for screening
for selective lignin removal was described recently by Nishida et al. ( 1988); that
Use of Fungi in Pulping Wood / 103
method is based on the formation of color during growth of test strains of
guaiacol-wood meal agar plates. The Biopulping Consortium reported recently
on a more targeted biopulping screening procedure based on the effect of fungal
treatment of coarse pulp on pulp trainability (Leatham and Myers, 1990). The
method could be
used
to predict fungal efficacy insofar as improved paper strength
properties were concerned, but it did not predict energy savings.
An introductory study with Dichomitus squalens (Karst. ) Reid and P.
chrysosporium
B KM F-1767 with aspen wood chips showed large improvements
in the paper strength properties of biomechanical pulps in comparison to the
properties of controls (Myers et al.,
1988). The chips in that study—and in other
studies described here-were initial] y supplemented with glucose, glutamate,
and other nutrients prior to introducing the fungi.
Dichometus squalens was
allowed to decay the wood for 7 weeks, and
P. chrysosporium
for 4 weeks. Even
so, total loss in wood weight was less than
270.
The fungal pretreatment decreased
the brightness (whiteness) of the pulps in this and in all studies to date, which is
somewhat surprising because white-rot fungi are so named because they eventu-
ally bleach wood. Although the pulps are not difficult to bleach, the necessity of
bleaching is a negative aspect of biopulping.
Subsequent studies with additional fungi and aspen wood chips confirmed the
enhancement of paper strength properties and also demonstrated that large energy
savings for the pulping are possible (Leatham et al., 1990a, b.
C
).
The fungi varied
greatly in their effectiveness with aspen.
Trametes versicolor
had essentially no
effect, despite good lignin degradation,
whereas C.
subvermispora, Phlebia
tremellosa
(Schrad.: Fr. ) Nakas. et Burds., and Phlebia brevispora Nakas. were
quite effective. The fungi also varied greatly in their effectiveness for pretreating
aspen compared to pine. Interesting] y, there was little correlation between re-
moval of specific components of the wood by the fungi and efficacy of the fungal
pretreatment for either energy savings or paper strength property improvement.
This is unfortunate because such a correlation could have pointed to more rapid
screening methods. There was also little correlation between energy savings and
paper improvement. indicating that the changes in the wood cell walls that provide
the beneficial effects are different for energy savings and for paper strength
property improvement. Fortunately, pretreatment with some fungi, including
P.
chrysosporium, Phlebia subserialis (Bourd. et Galz. ) Donk, and P. brevispora,
resulted in both energy savings and paper improvement.
Properties of paper from aspen wood pulped by six commercial pulping pro-
cesses and by biomechanical pulping were recently compared. Results showed
that the biomechanical process produced a pulp that is comparable to a chemither-
momechanical pulp in overall properties (Wegner et al., in press).
Over 100 biopulping runs have now been completed on a 2–5-kg scale. Most
of the work has been with P. chrysosporium on aspen and C. subvermispora on
southern pine. Some of the most promising data obtained thus far are given in
Table 7.1. These data are not atypical, but such results are not always obtained,