3
Biochemical Reactions - A First Look
Having had a brief overlook of the processes, which determine the life processes in a microbial cell
we shall now turn to a quantitative treatment of these processes. The goal of all the remaining
chapters of the book is, indeed, to understand at least at a superficial level how the cell functions,
and by calculations predict what we may expect when living cells are exploited for the production
of valuable products.
The first step in this progress towards a quantitative understanding of cell processes is to make
experiments with cultivation of cells in a reactor, and to extract the rates of the biological reactions
from these experiments. This is the subject of the present chapter. No biological interpretation of
the data is offered - this has to wait until more fundamental knowledge has been obtained using
methodologies described in Chapters 4 to 7, but the basic tools of quantitative analysis of
fermentation data will be presented, and it will be shown how the detrimental effect of experimental
errors and of a less than perfect experimental procedure may be minimized.
3.1 The Continuous Stirred Tank Reactor
A very efficient bioreactor set-up for analysis of overall conversion rates in bioprocesses is the
continuous stirred tank reactor, which is schematically shown in Fig. 3.1. There are two feed ports,
one for liquid feed and one for gaseous feed. In the liquid reaction medium the reactants, called
substrates,
are converted to
biomass
and
metabolic products.
Cells and metabolic products
dissolved in the aqueous medium leave the reactor through the liquid effluent, while gaseous
products such as carbon dioxide leave through the headspace of the reactor (exhaust gas). When
the liquid feed rate v (L h '), the gas feed rate vg (L h'1), the reactor volume
V
(L) and the
concentrations of substrates in the liquid and gas feed streams are independent of time, i.e., when
all the input variables to the system have constant values, we will expect all the output variables,
i.e., biomass concentration and activity of the cells as well as the concentration of metabolic
products in liquid and gas effluents to have constant values. The bioreactor is operating in
steady
state continuous mode.
In a large majority of cases all the output variables have time independent
values when the feed variables are constant, but there are exceptions, and these exceptions can
have considerable scientific and practical interest as will be qualitatively discussed in Note 3.1
and considered again in Chapters 8 and 9.
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