FE 462 BIOCHEMICAL ENGINEERING
Most industrial fermentations are carried out as pure cultures in which only selected strains are allowed to grow. If foreign microorganisms exist in the medium or any parts of the equipment, the production organisms have to compete with the contaminants for the limited nutrients. The foreign microorganisms can produce harmful products which can limit the growth of the production organisms. Therefore, before starting fermentation, the medium and all fermentation equipment have to be free from any living organisms, in other words, they have to be completely sterilized. Furthermore, the aseptic condition has to be maintained.
Sterilization of fermentation media or equipment can be accomplished by destroying all living organisms by means of heat (moist or dry), chemical agents, radiation (ultraviolet or X-rays), and mechanical means (some or ultrasonic vibrations). Another approach is to remove the living organisms by means of filtration or high-speed centrifugation.
Heat is the most widely used means of sterilization, which can be employed for both liquid medium and heatable solid objects. It can be applied as dry or moist heat (steam).
Laboratory autoclaves are commonly operated at a steam pressure of about 30 psia, which corresponds to 121°C. Even bacterial spores are rapidly killed at 121 °C.
Many cellular materials absorb ultraviolet light, leading to DNA damage and consequently to cell death. Wavelengths around 265 nm have the highest bactericidal efficiency.
Sonic or ultrasonic waves of sufficient intensity can disrupt and kill cells.
Filtration is most effectively employed for the removal of microorganisms from air or other gases.
Chemical agents can be used to kill microorganisms as the result of their oxidizing or alkylating abilities. However, they cannot be used for the sterilization of medium because the residual chemical can inhibit thefermentation organisms.
THERMAL DEATH KINETICS
Thermal death of microorganisms at a particular temperature can be described by first-order kinetics:
which shows the exponential decay of the cell population. The temperature dependence of the specific death rate kd can be assumed to follow the Arrhenius equation:
From above equations, the design criterion for sterilization Ñ can be defined as
Sterilization of the medium in a fermenter can be carried out in batch mode by direct steam sparging, by electrical heaters, or by circulating constant pressure condensing steam through heating coil. The sterilization cycles are composed of heating, holding, and cooling. Therefore, the total Del factor required should be equal to the sum of the Del factor for heating, holding and cooling as
The Design Procedure
The design procedure for the estimation of the holding time is as follows:
1. Calculate the total sterilization criterion, Ñ total.
2. Measure the temperature versus time profile during the heating, holding, and cooling cycles of sterilization.
a. For batch heating by direct steam sparging into the medium, the hyperbolic form is used:
3. Plot the values of kd as a function of time.
During the cooling process, the change of temperature can be approximated as