fungal growth and selects against most bacterial and yeast contaminants. Also,
most filamentous fungi produce spores. Spore inocula are easy to prepare and
can be stored for longer periods than vegetative cells. This makes processes involving fungi more flexible since the synchronization of inoculum production
with the rest of the process is not so crucial.
Despite the predominance of filamentous fungi, there are also several SSF
processes which involve bacteria and yeasts. Yeasts generally grow on solid
substrates only as minor members of the microflora, as is the case in the
traditional processes of ensiling and tape manufacture [4]. However, some
modern processes have involved monocultures of Saccharomyces cerevisiae for
the production of ethanol (Table 2) and Endomycopsis fibuligera and Schwanniomyces castelli for protein enrichment of starchy solid substrates [65].
Bacteria play roles as the major or minor microorganisms in various SSF
processes. Lactobacilli are the major microorganisms during ensilage and
thermophilic bacteria predominate early during composting before being succeeded by thermophilic actinomycetes and fungi. Natto is a Japanese fermented
food in which Bacillus subtilis is the major organism and forms a sticky coat on
soybeans. In the production of many Asian fermented foods by traditional
methods, in which neither the inoculum preparation nor the fermentation are
carried out under aseptic conditions, bacteria may enter the process as contaminants. One example where this is advantageous is in the production of
tempe in which an as yet unidentified bacterium is present and produces
vitamin B 12 .
1.2
Culture Methods
A range of different practices is used in SSF with respect to the organisms used
and the use of aseptic procedures. These may be referred to as monoculture,
defined mixed culture, sequential culture, and undefined mixed culture.
In monoculture the substrate is pasteurized or sterilized and inoculated with
a pure culture. During the fermentation aseptic procedures may be used to
prevent contamination. In some cases the conditions may select against contaminants and the process organism may grow vigorously so that any contaminants cannot compete. In these cases the process may be able to be carried
out by unskilled operators.
In defined mixed culture the substrate is pasteurized or sterilized and
inoculated simultaneously with more than one pure culture. This can be beneficial for complex substrates and where the various strains use different carbon
sources. For example, mixed cultures of Trichoderma reesei or Chaetomium cellulolyticum with Candida lipolytica resulted in increased protein production
from wheat straw because the yeast uses glucose and prevents catabolite
repression of the fungal cellulase [66, 67].
Sequential culture is a type of mixed culture in which the second organism
is added after growth of the first organism has ceased. This method can be used
to maximize utilization of the nutrients in the substrate. For example, the
protein content of wheat straw fermented by Chaetomium cellulolyticum or
72
D.A. Mitchell et al.
most filamentous fungi produce spores. Spore inocula are easy to prepare and
can be stored for longer periods than vegetative cells. This makes processes involving fungi more flexible since the synchronization of inoculum production
with the rest of the process is not so crucial.
Despite the predominance of filamentous fungi, there are also several SSF
processes which involve bacteria and yeasts. Yeasts generally grow on solid
substrates only as minor members of the microflora, as is the case in the
traditional processes of ensiling and tape manufacture [4]. However, some
modern processes have involved monocultures of Saccharomyces cerevisiae for
the production of ethanol (Table 2) and Endomycopsis fibuligera and Schwanniomyces castelli for protein enrichment of starchy solid substrates [65].
Bacteria play roles as the major or minor microorganisms in various SSF
processes. Lactobacilli are the major microorganisms during ensilage and
thermophilic bacteria predominate early during composting before being succeeded by thermophilic actinomycetes and fungi. Natto is a Japanese fermented
food in which Bacillus subtilis is the major organism and forms a sticky coat on
soybeans. In the production of many Asian fermented foods by traditional
methods, in which neither the inoculum preparation nor the fermentation are
carried out under aseptic conditions, bacteria may enter the process as contaminants. One example where this is advantageous is in the production of
tempe in which an as yet unidentified bacterium is present and produces
vitamin B 12 .
1.2
Culture Methods
A range of different practices is used in SSF with respect to the organisms used
and the use of aseptic procedures. These may be referred to as monoculture,
defined mixed culture, sequential culture, and undefined mixed culture.
In monoculture the substrate is pasteurized or sterilized and inoculated with
a pure culture. During the fermentation aseptic procedures may be used to
prevent contamination. In some cases the conditions may select against contaminants and the process organism may grow vigorously so that any contaminants cannot compete. In these cases the process may be able to be carried
out by unskilled operators.
In defined mixed culture the substrate is pasteurized or sterilized and
inoculated simultaneously with more than one pure culture. This can be beneficial for complex substrates and where the various strains use different carbon
sources. For example, mixed cultures of Trichoderma reesei or Chaetomium cellulolyticum with Candida lipolytica resulted in increased protein production
from wheat straw because the yeast uses glucose and prevents catabolite
repression of the fungal cellulase [66, 67].
Sequential culture is a type of mixed culture in which the second organism
is added after growth of the first organism has ceased. This method can be used
to maximize utilization of the nutrients in the substrate. For example, the
protein content of wheat straw fermented by Chaetomium cellulolyticum or
72
D.A. Mitchell et al.