Important topic

1. S t e r i l i s a t i o n
• Commercial fermentations typically require
thousands of litres of liquid medium and millions of
litres of air.
• For processes operated with axenic cultures, these
raw materials must be provided free from
contaminating organisms.
• Of all the methods available for sterilisation
including chemical treatment, exposure to
ultraviolet, gamma and X-ray radiation, sonication,
filtration and heating, only the last two are used in
large-scale operations.

2. Batch Heat S t e r i l i s a t i o n o f L i q u i d s
• Liquid medium is most commonly sterilised in batch in
the vessel where it will be used.
• The liquid is heated to sterilisation temperature by
introducing steam into the coils or jacket of the vessel;
• alternatively, steam is bubbled directly into the
medium, or
• the vessel is heated electrically.
• If direct steam injection is used, allowance must be
made for dilution of the medium by condensate which
typically adds 10-20% to the liquid volume;
• quality of the steam must also be sufficiently high to
avoid contamination of the medium by metal ions or
organics.

3. • A typical temperature-time profile for batch
sterilisation is shown in Figure 13.35(a).
• Depending on the rate of heat transfer from the
steam or electrical element, raising the temperature
of the medium in large fermenters can take a
significant period of time.
• Once the holding or sterilisation temperature is
reached, the temperature is held constant for a
period of time thd.
• Cooling water in the coils or jacket of the fermenter
is then used to reduce the medium temperature to
the required value.

5. • For operation of batch sterilisation systems, we
must be able to estimate the holding time required
to achieve the desired level of cell destruction.
• As well as destroying contaminant organisms, heat
sterilisation also destroys nutrients in the medium.
• To minimise this loss, holding times at the highest
sterilisation temperature should be kept as short as
possible.
• Cell death occurs at all times during batch
sterilisation, including the heating-up and cooling-
down periods.
• The holding time thd can be minimised by taking into
account cell destruction during these periods.

6. • Rate of heat sterilisation is governed by the
equations for thermal for first-order death kinetics,
in a batch vessel.
• where cell death is the only process affecting the
number of viable cells:
• where
N is number of viable cells,
t is time and
– k d is the specific death constant.
• Eq. (13.95) applies to each stage of the batch
sterilisation cycle: heating, holding and cooling.

7. • where thd is the holding time,
• N 1 is the number of viable cells at the start of
holding, and
• N 2 is the number of viable cells at the end of
holding,
• k d is evaluated as a function of temperature using
the Arrhenius equation:

8. • where
• A is the Arrhenius constant or frequency factor,
• E d is the activation energy for the thermal cell
death,
• R is the ideal gas constant and
• T is absolute temperature.
• N1 and N2 are determined by considering the extent
of cell death during the heating and cooling periods
when the temperature is not constant. Combining
Eqs (13.95) and (11.46) gives:

9. • Integration of Eq. (13.98) gives for the heating period:
• where
• t 1 is the time at the end of heating,
• t 2 is the time at the end of holding and
• t f is the time at the end of cooling.
• We cannot complete integration of these equations
until we know how the temperature varies with time
during heating and cooling.

11. Continuous Heat Sterilisation of Liquids
• Continuous sterilisation, particularly a high-
temperature, short-exposure-time process, can
significantly reduce damage to medium ingredients
while achieving high levels of cell destruction.
• Other advantages include improved steam economy
and more reliable scale-up.
• The amount of steam needed for continuous
sterilisation is 20-25% that used in batch processes;
• the time required is also significantly reduced
because heating and cooling are virtually
instantaneous.

13. • Typical equipment configurations for continuous sterilisation
are shown in Figure 13.37.
• In Figure 13.37(a), raw medium entering the system is first
pre-heated by hot, sterile medium in a heat exchanger;
• this economises on steam requirements for heating and cools
the sterile medium.
• Steam is then injected directly into the medium as it flows
through a pipe;
• The liquid temperature rises almost instantaneously to the
desired sterilisation temperature.
• The time of exposure to this temperature depends on the
length of pipe in the holding section of the steriliser.
• After sterilisation, the medium is cooled instantly by passing it
through an expansion valve into a vacuum chamber; further
cooling takes place in the heat exchanger where residual heat
is used to pre-heat incoming medium.

14. • Figure 13.37(b) shows an alternative sterilisation scheme
based on heat exchange between steam and medium.
• Raw medium is pre-heated with hot, sterile medium in a heat
exchanger then brought to the sterilisation temperature by
further heat exchange with steam.
• The sterilisation temperature is maintained in the holding
section before being reduced to the fermentation
temperature by heat exchange with incoming medium.
• Heat-exchange systems are more expensive to construct than
injection devices;
• fouling of the internal surfaces also reduces the efficiency of
heat transfer between cleanings.
• On the other hand, a disadvantage associated with steam
injection is dilution of the medium by condensate;
• foaming from direct steam injection can also cause problems
with operation of the flash cooler.

15. • As indicated in Figure 13.38, rates of heating and cooling in
continuous sterilisation are much more rapid than in batch;
• accordingly, in design of continuous sterilisers, contributions
to cell death outside of the holding period are generally
ignored.
• An important variable affecting performance of continuous
sterilisers is the nature of fluid flow in the system.
• Ideally, all fluid entering the equipment at a particular instant
should spend the same time in the steriliser and exit the
system at the same time;
• unless this occurs we cannot fully control the time spent in
the steriliser by all fluid elements.
• No mixing should occur in the tubes; if fluid nearer the
entrance of the pipe mixes with fluid ahead of it, there is a
risk that contaminants will be transferred to the outlet of the
steriliser.

16. • The type of flow in pipes where there is neither
mixing nor variation in fluid velocity is called plug
flow
• Plug flow is an ideal flow pattern; in reality, fluid
elements in pipes have a range of different
velocities.