Introduction to Fungi, Third Edition

EUROTIALES
303
Cheese production involves complex consortia
of bacteria and numerous different yeast species,
but two Penicillium spp. are important in specialized
cheeses. These are P. roqueforti for blue-veined
cheeses, and P. camemberti for white mould
cheeses. The subject has been summarized by
Jakobsen et al. (2002). The moulds are inoculated
together with enzymes or bacterial starters after
the cooling of the pasteurized milk, and gradually
colonize the maturing cheese. They contribute
significantly to the texture as well as the flavours,
with the characteristic blue cheese or camembert
flavours being due mainly to the activity of extracellular
lipases which break down short-chain
fatty acids. A substantial contribution is also
made by protein and peptide degradation products
resulting from the activities of fungal
proteinases and peptidases (Jakobsen et al., 2002).
Production of enzymes
The prominent role of Aspergillus and Penicillium
species in food production is, of course, due to
their ability to produce large quantities of extracellular
enzymes. This feature has also been
harnessed for industrial purposes, with the
majority of all commercial fungal enzymes produced
by Aspergillus spp. (Oxenbøll, 1994).
Proteases, amylases, lipases and pectinases are
important in many industrial processes, including
the manufacture of dairy, bakery, distillery
and brewery products, juices and leather, and in
the starch industry.
Citric acid fermentation
Citric acid is found in many fruits, and it is used
for flavouring and pH control of food and beverages.
In combination with carbonates and
bicarbonates, it is also used to create the
effervescent effect when medications such as
vitamin preparations or aspirin are dissolved in
water. Initially extracted from citrus fruits, citric
acid has been produced commercially by Aspergillus
niger since about 1923 and this fungus remains
the world’s most important producer. The total
current annual world production of citric acid
is about 9 10 6 tons. Curiously, although production
of citric acid by A. niger is one of the most
efficient biotechnological fermentations with
conversion of up to 95% (by weight) of the sugar
substrate, the biochemistry of it is still only
poorly understood. The uptake of sugar (as
hexose) is followed by glycolysis in the cytosol,
the tricarboxylic acid cycle in the mitochondrion,
and export of citric acid into the cytosol and
thence into the extracellular medium where it
accumulates, creating a pH below 3. Citric acid
production proceeds optimally when an excess of
sugar and aeration is provided, whereas phosphate
and trace elements, especially manganese,
must be limiting. Excellent reviews of citric acid
production have been published by Brooke (1994)
and Karaffa and Kubicek (2003).
Production of antibiotics
The accidental discovery of penicillin by a contamination
of Penicillium notatum growing on a
bacterial agar culture (Fleming, 1929, 1944)
followed by the re-discovery of penicillin by
Florey and Chain and its development into an
antibiotic against Gram-negative bacteria has
been told many times, and the original 1945
Nobel Lectures by Fleming, Florey and Chain can
be found at http://www.nobel.se. In nature,
penicillin (Fig. 11.14a) is produced by P. notatum,
the closely related or identical P. chrysogenum, by
A. nidulans and a few other conidial fungi, whereas
the chemically related cephalosporins are produced
by Acremonium chrysogenum (formerly
Cephalosporium chrysogenum), which probably
belongs to the Pyrenomycetes (p. 348). Together,
penicillin- and cephalosporin-type antibiotics
take a staggering 50% share (approximately
11 billion US$) of the total worldwide sales of
antibiotics (Schmidt, 2002). Aspergillus nidulans has
been useful for studies of the genetics and
biosynthesis of penicillin production because
it is easily manipulated by molecular biological
methods. However, for commercial production
P. chrysogenum has been used traditionally. The
first penicillins were produced commercially
by purification of the final product from
static liquid cultures. Over several decades, highproducing
mutants were generated, resulting
in a 50 000-fold enhanced penicillin yield relative
to that of the original strain, and current yields
are as high as 50 g penicillin l 1 of liquid culture
(Schmidt, 2002). A wide range of penicillin (and
cephalosporin) derivatives has been produced,