Biomedical Engineering Reference
In-Depth Information
especially before fermentation when a strain grows exuberantly because of the poor
heat transfer efficiency in solid-state fermentation. The temperature of the substrate
is rising rapidly. Sometimes, it is as high as about 2
ı
Ccm
1
. The growth and
metabolism of the strain will be seriously affected if the heat generated cannot
be dissipated in time. Sometimes, burning medium even happens, leading to a
large amount of dead strains and complete failure of the fermentation. This is
the critical and difficult point for solid-state fermentation enlargement. To reduce
the product temperature, proper turning of the medium is necessary in addition to
increased ventilation and spraying of sterile water. In the summer, cooling is difficult
(especially in southern China, with high summer temperatures and humidity). It is
desirable to use liquid ammonia refrigeration or air conditioning for refrigeration
cooling for a short time.
⑤
pH
Similar to the effects of temperature on microorganisms, there is an optimum pH
for microorganisms. Most fungi are less sensitive to the pH of the environment.
Fungi can grow cells typically from pH 3 to pH 9. Compared with bacteria and
actinomycetes, it is easier for fungi to live in an acidic environment; the optimum
pH is 5-8. For some strains, however, maintaining a certain value is necessary, such
as the pH for
Trichoderma reesei
needs to be 4.5 to produce cellulase. Although pH
is an important parameter, in the fermentation process, pH is difficult to control
effectively because of the lack of online measurement of pH in moist material.
So far, pH changes in the solid-state fermentation process have been studied
extensively; however, pH regulation is reported less. Commonly, a substance with
buffer capacity is used as a substrate to eliminate the adverse effects brought by
the change of pH. Another novel method is to use a nitrogen-containing inorganic
salt (such as urea) as a nitrogen source to offset the negative impact of the acid
generated during the fermentation. Lonsane [
123
] proposed that, as long as there
is a good initial pH in the fermentation process, it is not necessary to measure and
control pH in the process of fermentation. In a great deal of solid fermentation,
the pH of the fermentation process, however, has a characteristic change. But the
conventional method (pH electrode method and colorimetric method) is difficult to
detect effectively, limiting the feasibility of pH as an important control parameter.
⑥
Humidity
Humidity refers to the humidity of the ambient air in the fermentation tank. If air
humidity is too low, the material can easily become dry because of evaporation
of water, which has an impact on strain growth. Air humidity affects the oxygen
content in the air. The material surface usually becomes wet by condensation,
affecting strain growth or infection and final product quality. The air humidity
should be maintained at an appropriate value. It is generally maintained in the range
of 85-97 %.
⑦
Fermentation time
The end of fermentation is important for enhancing the amount of product. In the
fermentation process, the concentration of product changes all the time. Generally,
