FUNGAL GROWTH 83
Initially, many potential fungi were screened to find
a suitable organism for commercial use. Then a suit-
able large-scale fermenter system had to be designed
because fungal mycelia have viscous properties in
solutions, so the cultures are difficult to mix to achieve
adequate oxygenation – the fungus uses 0.78 g oxygen
for every 1 g biomass produced. The system used
commercially is a 40 m^3 air-lift fermenter(Fig. 4.17)
about the height of Nelson’s column! Compressed air
is used to aerate and circulate the culture. This avoids
the heating that would be caused by a mechanical
stirrer, saving on cooling costs. There is a potential
problem with the high nucleic acid content of any type
of microbial biomass when used as a human food
source, because our bodies metabolize RNA to uric acid
which can cause gout-like symptoms. So, the nucleic
acid content of the harvested mycelium needed to be
reduced while retaining as much of the protein as
possible. This was achieved by exploiting the higher
heat-tolerance of RNAases than of proteases. The cul-
ture outflow containing the biomass is collected in a
vessel and its temperature is raised rapidly to 65°C
and maintained at this level for 20–30 minutes. The
growth of the fungus is stopped at this temperature and
the proteases are destroyed so that relatively little pro-
tein is lost, but the ribosomes break down and the fun-
gal RNAases degrade much of the RNA to nucleotides
which are released into the spent culture filtrate. Then
the mycelium can be harvested for drying.
A final problem has still not been overcome completely
and it limits the efficiency of commercial production.
During prolonged culture in fermenter vessels the fun-
gus is subjected to selection pressure and it mutates
to “colonial forms” with a high branching density
but relatively low extension rate of the main, leading
hyphae. These forms predominate over the wild-type
after about 500 –1000 hours of continuous culture.
Their hyphal growth unit lengths range from 14 to
174 μm, compared with 232μm for the wild-type, so
they give a significantly less fibrous biomass, which
is undesirable in the end product. The production
runs have to be terminated prematurely to avoid this
problem. Nevertheless, the development of Quorn is
a significant technological achievement as well as a
commercial success.
Cited references
Allan, R.H., Thorpe, C.J. & Deacon, J.W. (1992) Dif-
ferential tropism to living and dead cereal root hairs by
the biocontrol fungus Idriella bolleyi. Physiological and
Molecular Plant Pathology 41 , 217–226.
Anderson, J.G. & Smith, J.E. (1971) The production of
conidiophores and conidia by newly germinated conidia
of Aspergillus niger(microcycle conidiation). Journal of
General Microbiology 69 , 185–197.
Armitage, J.P. & Lackie, J.M., eds (1990) Biology of the
Chemotactic Response. Society for General Micro-
biology Symposium 46. Cambridge University Press,
Cambridge.
Bartnicki-Garcia, S. (2002) Hyphal tip growth: outstanding
questions. In: Molecular Biology of Fungal Development
(H.D. Osiewacz, ed.), pp. 29–55. Marcel Dekker, New
York.
Gow, N.A.R. (2004) New angles in mycology: studies in
directional growth and directional motility. Mycological
Research 108 , 5–13.
Gow, N.A.R. & Gadd, G.M., eds (1995) The Growing
Fungus. Chapman & Hall, London.
Hartwell, L.L. (1974) Saccharomyces cerevisiaecell cycle.
Bacteriological Reviews 38 , 164 –198.
Howard, R.J. & Aist, J.R. (1980) Cytoplasmic microtubules
and fungal morphogenesis: ultrastructural effects of
methyl benzimidazole-2-yl-carbamate determined by
freeze-substitution of hyphal tip cells. Journal of Cell
Biology 87 , 55– 64.
Howard, R.J. & Gow, N.A.R., eds (2001) Biology of the Fungal
Cell. The Mycota, VIII. Springer-Verlag, Berlin.
Jackson, S.L. & Heath, I.B. (1990) Evidence that actin rein-
forces the extensible hyphal apex of the oomycete
Saprolegnia ferax. Protoplasma 157 , 144 –153.
Jackson, S.L. & Heath, I.B. (1993) Roles of calcium ions
in hyphal tip growth. Microbiological Reviews 57 , 367–
382.
Latijnhouwers, M., de Wit, P.J.G.M. & Govers, F. (2003)
Oomycetes and Fungi: similar weaponry to attack
plants. Trends in Microbiology 11 , 462–469.
Lever, M.C., Robertson, B.E.M., Buchan, A.D.B., Miller,
P.F.P., Gooday, G.W. & Gow, N.A.R. (1994) pH and
Ca^2 +dependent galvanotropism of filamentous fungi:
implications and mechanisms. Mycological Research 98 ,
301–306.
Manavathu, E.K. & Thomas, D. des S. (1985) Chemo-
tropism of Achlya ambisexualis to methionine and
methionyl compounds. Journal of General Microbiology 131 ,
751–756.
Mata, J. & Nurse, P. (1998) Discovering the poles in yeast.
Trends in Cell Biology 8 , 163–167.
Mitchell, R.T. & Deacon, J.W. (1986) Chemotropism
of germ-tubes from zoospore cysts of Pythiumspp.
Transactions of the British Mycological Society 86 , 233–
237.
Money, N.P. (1995) Turgor pressure and the mechanics
of fungal penetration. Canadian Journal of Botany 73 ,
S96–102.
Morrin, M. & Ward, O.P. (1989) Studies on interaction
of Carbopol-934 with hyphae of Rhizopus arrhizus.
Mycological Research 92 , 265–272.
Read, D.J. (1991) Mycorrhizas in ecosystems – nature’s
response to the “Law of the Minimum”. In: Frontiers
in Mycology(Hawksworth, D.L., ed.), pp. 29–55. CAB
International, Wallingford, Oxon, pp. 101–130.
Robertson, N.F. (1958) Observations of the effect of water
on the hyphal apices ofFusarium oxysporum. Annals of
Botany 22 , 159–173.
Robertson, N.F. (1959) Experimental control of hyphal
branching forms in hyphomycetous fungi. Journal of the
Linnaean Society,London 56 , 207–211.