Lect7(Mode of Fermenter Operation)

8/13/2019 Lect7(Mode of Fermenter Operation)

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used to produce biomass,primary metabolites

In order to produce maximum possiblebiomass at the end of the process,optimization of cultural conditionssupporting growth should be

established.

For primary metabolite production,

conditions to extend the exponentialphase (and hence, product excretion)should be provided


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conditions giving a short exponentialphase and an extended stationary, orproduction phase, or conditions giving adecreased growth rate in the exponential

phase might be preferred for secondarymetabolite formation


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principal advantages of batch cultures are;low contamination risk; the ability to rundifferent succesive phases in the same

vessel; and close control of the genetic

stability of the microorganism

However, the high proportion of unproductive

down time in batch fermenter operation willreduce the overall productivity of the process


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Control systems for batch fermentation arenormally associated with pH, dissolved oxygentension and temperature.

In batch fermentation, if growth is subject tosubstrate inhibition, fermentation has to bestarted with low initial substrateconcentration.

This result in lower maximum biomass andhence, probably, lower maximum

concentration of the required product.


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0

2

4

6

8

10

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0 5 10 15 20 25

TIME

Log(C

ELLCONC.)

Lag

Phase

Exponential Phase Stationary Phase

(a)

(b)

( c )

Curve (a) represents cell mass in the absence of lysis, (b) cell masswhen lysis occurs and is followed by cryptic growth, and (c) viable

cell count when cell lysis occurs.


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The lag phase follows inoculation of thenutrient medium and is a period ofadaptation.

The log phase is characterized by a straightline on a semilog plot of the ln X vs time.

This is a period of balanced or steady state

growth during which the specific growth rate, ,is constant.


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Throughout the fermentation the chemicalcomposition of the broth is changing sincenutrients are being consumed and metabolic

products are being produced.

As a consequence the environment is not atsteady state.

Over a range of nutrient concentration, thegrowth rate is independent of concentration.

Also, during the log phase, the cellmacromolecular composition remainsconstant;


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This stationary phase occurs when all thecells have stopped dividing or when viablecells have reached equilibrium with dead

cells, that is, with the rate of death.

Upon further incubation several things arelikely to happen.

Even though net growth has stopped, theremay still be metabolism and accumulation of

products in the cell or in the broth.

The total cell mass may stay constant


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Fed-batch Fermentation


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One or more nutrients are supplied to fermenter duringcultivation but no removal of culture until end of process.

Characteristic: concentration of nutrients fed intoculture liquid can be controlled by changing feed rate.


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Fed-batch operation, two initialdecisions:

1. Nutrients to be fed

2. Mode of feeding

Purpose: to control substrate concentrationwithin the liquid culture and hence,some of the biochemical parameters can

be controlled as well.


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Application of Fed-batch Culture


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Advantages and disadvantages of fed-batchreactors


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When high concentrations of substrate arepresent, the cells get "overloaded", this is, theoxidative capacity of the cells is exceeded, anddue to the Crabtree effect, products other than

the one of interest are produced, reducing theefficacy of the carbon flux


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Control Techniques for Fed-batch Fermentation


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)$'

Effluent

Light

Valve

Culture medium

reservoir

Photocell

Overflow


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The population density or concentration of cell

in the culture is kept constant by controlling

the flow of medium such that the turbidity of

the culture is set at a certain value, but the

growth rate is controlled by unknown factors

in the population itself, since all nutrients are

in excess.


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In a well-mixed fermenter, a

steady-state results when thespecific growth rate of themicroorganisms balances

exactly the dilution rate.

;


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Lect7(Mode of Fermenter Operation)

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