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Fermentation reaction of
microbial metabolic reactions is the
reoxidation of
pyruvate into alcohols and other by-products. The metabolic
reactions are of two types: Anabolic reaction and Catabolic
reaction; and the catabolic reactions are the reaction subset that
generate the pyruvates and other the energy bond substances from the
utilization of a microbe-specific substrate, while the anabolic
reactions are the consumer of such energy bond substances to the end
of producing energy and cellular maintenance functions, hence
microbial growth obtains from from the results anabolic reactions.
As with the catabolic reactions utilization of substrates as
reactants, the anabolic reactions also require its own
reactants and catalysts. Of course,
fermentation reactions are a subset of the catabolic reactions.
In general, the mash or
broth fed any microbe to support fermentation usually is a
mixture of
the reactants, primarily the substrate, for the catabolic
reactions as well as the reactants and catalysts for the anabolic
reactions. The reactants and catalysts that must be blended with the
substrate to form a Mash or Broth, however, is microbe-specific, as
is exemplified by the set of reactants and catalysts for the
anabolic reactions in Saccharomyces Cerevisiae for the
fermentative glucose utilization producing ethanol. Obviously,
therefore the mash for the fermentative utilization by every microbe
is unique, and must be well-mixed and maintained precisely at the
right concentrations everywhere in the mash.
One of the reactants for
the fermentation reaction however, is oxygen, though every microbe
requires the gas at different levels of concentrations: Some
microbes require oxygen at high concentrations at all times, and are
called aerobic, while other microbes require the gas at low
concentrations and are called anaerobic. Yet it is generally
true that in the presence of oxygen, all the microbes preferentially
grow and suffer cell divisions. Experimental support of such include
the studies underlying the Monod Equation.
Clearly then for microbes
to simply effect fermentative utilization of the substrate and not
suffer growth, but only support cellular functions support while being fermentative, the anabolic
reactants concentration, including the oxygen concentration, must be
just enough to simply support cell-maintenance facilities. This
requirement must hold, particularly for anaerobic microbes which
cease being fermentative under high concentrations of oxygen, -
preferring instead to support cellular growth - except during high
concentrations of substrate when the microbes are also
simultaneously fermentative.
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However, for purposes of fermentation reactor performance
reproducibility as well as predictability, from a reactor
engineering perspective though, both performance-related
objectives are easier met when the reactor operating conditions do
not support the growth of microbes: Having the same microbe counts
through out the reaction time is, in fact, the ideal preferred
situation; for reasons that are evident from the
mass balance
analysis of bioreactors.
The reactants for the
anabolic reactions which must also be present in the Mash feed for
the fermentation reaction accounts in some sense for reactor
heterogeneity, given that the requirement - of the mash being
well-mixed at maintained precisely at the right concentrations
everywhere - is impossible to accomplish, and as such microbes in
the regions of the mash with higher oxygen concentrations will
definitely support growth but may or may not possibly support
fermentation.
Currently, the
mash-related cause of heterogeneity in a homogeneous bioreactor,
stems from the adopted method of separately feeding the mash
constituent substrates and the reactants for the anabolic reactions
and then stirring the mixture: Research on mixing shows that
generally, the mixing of liquids poured into a vat together before
mixing, attains the state of well-mixedness only
over an exponential time span, because the heterogeneity of the
state of mixedness decays over that span. Moreover, these
bioreactors have the required oxygen being dissolved into the mash
from the liquid top surface as per Henry's Law; and therefore the mash
being stirred, exposes the mash to different
concentrations of oxygen depending on whether the fluid particle
moves to the surface or continues to remain below and far below the
surface.
The same explanations
applies in heterogeneous bioreactors as in homogeneous bioreactors
except that with respect to the oxygen concentration, the microbes closes to the
mash entry-point of the bioreactor get exposed to more
oxygen than microbes farther away. Rational this
would also explains the immobilized microbes preferential growth
near the feed entrance of the
experimental ICR, as also analyzed in
immobilized microbes dynamics
while the empirical observation simultaneously offers credence to
the assertion of heterogeneity. Generally then preparing Mash, such as may be
ideal for performance analysis of heterogeneous bioreactors should
be quite complex,
and particularly so for such bioreactors of the
continuous flow class,
including even
batch reactors
which that by design and object stipulates the
feeding of the substrate into the reactor in a real-time instead of
batch feed.
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Evidently, preparing a mash, meeting these requirements can be
accomplished only under conditions of stirring pre-mixed mash or
Broth of the required concentration for an extended period of time
before being fed into the bioreactor before the temperature is
raised to induce the reaction at a perceptible rate. In effect, the
effluent substrate of each of substrate processes must be fed into a
special feeder, Mash Feeder, that enables the simultaneous
feeding of the reactants and catalysts for the anabolic reactions as
well as oxygen, under conditions of well-mixedness or at least very
close..
The designing of such
Mash Feeder must adopt as the specification for performance
the de facto experimental observations: for each microbe with
respect to the reactants and catalysts that support the anabolic
reactions. Even without regard for the moment about the
configuration details of the Mash Feeder Design, the design and
functional component of the feeder based on the general
functionality specifications are as follows:
With respect to the
reactants and catalysts for the anabolic reactions, the Mash Feeder
must have as many feed port as there
are anabolic nutrients count; and an efficient stirrer that leads to
well-mixed conditions - even if admittedly, the stirrer type and
design will
depend on the preferred inlet viscosity of the mash: The chosen
method of mixing of the substrate and nutrients as well as the
length of mixing [Mash Feeder length] depends intimately on the viscosity of the mash:
The more viscous the mash the the longer should be the mixing length,
and the more rotational momentum will be required, as the
mixer impeller design invariably depends on the viscosity.
The Mash Feeder design,
irrespective of the configuration for effecting the preparation of
the mash, must explicitly allow for the dissolving of needed gases
into the mash prior to discharge into the reactor, hence a well-designed oxygen
dissolution port must be present; after all, as well determined, both anaerobic and
aerobic fermentation reactions require oxygen. require oxygen, though
differing only in concentration. Even then, dissolving the gas, the
oxygen, is more tasking though. A design engineer therefore
must configure the Feeder in such a manner such that enough oxygen
is dissolved into the mash as to enable the microbes support the
anabolic reactions over the length of the reactor before which the
reactor design provides needed oxygen.
In view of the need to
eliminate the effects of heterogeneity in bioreactor performance,
the feeding of Mash or broth into any bioreactor should be
accomplished with a Mash Feeder, with the design specifications as
synopsized.
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