Biomedical Engineering Reference
In-Depth Information
Generally, a smaller substrate particle is considered to be the ideal choice
because it can provide a larger surface area for microbial contact, significantly
improving the reaction rate of solid-state fermentation. In many cases, particles that
are too small are likely to cause substrate cluster and low interparticle porosity,
resulting in increased resistance. So, the heat and mass transfer are adversely
affected, preventing microbial respiration or ventilation and subsequently causing
poor growth of the microorganism. At same time, large particles are conducive to
improving mass and heat transfer efficiency because the gap increases. They can
also provide better breathing and aeration conditions but a smaller surface area
for microbial reaction. With the growth of mycelia, the size of the gap will be
reduced during the reaction, and the effective diffusion coefficients of oxygen and
carbon dioxide decrease. For some biological processes, it is essential to select the
appropriate particle.
②
Water activity
The greatest feature of solid-state fermentation is the lack of free water, so substrate
water content changes will inevitably have a major impact on microbial growth and
metabolic capacity. Whether a microorganism grows on the substrate depends on its
water activity
a
w
instead of water content. The water activity factor
a
w
is defined as
P .
saturated vapor pressure of wet material
/
a
w
D
Pa .
pure water saturated vapor pressure under the same temperature
/
The water activity is related not only to the substrate itself but also to the type
and quantity of the solute:
a
w
DVM '=55:5
where
V
is number of ions,
M
is the molar concentration of solute,
is the molecular
osmotic coefficient, and 55.5 is the molar concentration of pure water.
The moisture level of substrate is decided by the property of the substrate, the
type of final product, and the needs of the microorganism. Different microorganisms
have different water activity requirements. In general, bacteria require an
a
w
between 0.90 and 0.99; most yeast require an
a
w
of 0.80-0.90; fungi and a few
yeasts require an
a
w
from 0.60 to 0.70. Therefore, fungi are applied in solid-
state fermentation because of their low water activity requirements and exclusion
of other bacterial contamination. A high
a
w
influences the growth of fungi by
reducing porosity, hindering the diffusion of O
2
and CO
2
, and increasing infection
probability. In the process of fermentation, the
a
w
decreases because of evaporation
and temperature increases. To improve the
a
w
for the normal growth of a strain,
the following methods could be applied: addition of sterile water or humid air, jet
wetting by installating sprayer, and so on. In solid-state fermentation, the integration
of ventilation, cooling, and water content control can effectively adjust the
a
w
.
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