Introduction to Fungi, Third Edition

302 PLECTOMYCETES
Fig11.13 Conidiophore development in Aspergillus nidulans. (a) Tip of a conidiophore which has swollen to produce a vesicle.The
conidiophore is multinucleate (nuclei not visible). (b) Development of metulae. Each metula contains one nucleus. (c) Production of
phialides, two from each metula.Each phialide contains one nucleus. (d) Production of uninucleate conidia. All images to same scale.
be because the hormone is concentrated in the
cell wall, rather than diffusing into the medium.
In liquid cultures, conidiogenesis in P. cyclopium
can be triggered by the addition of Ca 2þ ions
which are thought to act merely by enhancing the
sensitivity of the fungus to its own hormone
(Roncal et al., 2002). It is not known whether
a similar hormonal system exists in Aspergillus.
11.4.3 The roles of Aspergillus and Penicillium
in biotechnology
Species of Aspergillus and Penicillium are among
the most important organisms used in biotechnology,
second only to S. cerevisiae. Their applications
are diverse, including the production of
enzymes or primary and secondary metabolites,
and direct colonization and modification of foodstuff.
We will briefly consider examples of each
of these applications.
Food production
Species of Aspergillus have been used in the Far
East for food production for many centuries, and
we have already mentioned the role of A. oryzae
in the degradation of rice starch as a first step in
saké production (see p. 276). Similar two-stage
fermentation processes were developed for soy
sauces, although in this case the raw material
consists of a mixture of soy beans and wheat, and
proteolytic as well as amylolytic enzymes are relevant.
The degradation of this substrate is called
the koji process, and it is one of the best examples
of a solid-substrate fermentation. As such,
it requires great skill to find the optimum
moisture level of the soybean cake because
excessive moisture will limit aeration, whereas
low moisture limits growth. The koji fermentation
is completed within 72 h, and it utilizes
mainly A. oryzae and A. sojae. Interestingly, on the
basis of morphological as well as molecular data
(Geiser et al., 2000), these appear to be domesticated
forms of the potent mycotoxin producers
A. flavus and A. parasiticus, respectively (see p. 304).
The partially degraded substrate enriched in
fungal extracellular enzymes is then suspended
in brine, and the main fermentation (moromi) is
carried out with a consortium of bacteria and
yeasts. The Aspergillus enzymes continue to be
active during the moromi fermentation, thus
releasing a steady supply of degradation products.
A readable account of soy sauce production is
that by Aidoo et al. (1994). Other Far Eastern food
types produced with the aid of Aspergillus spp.
have been summarized by Nout and Aidoo (2002).