Sterilization

THE DESIGN OF CONTINUOUS STERILIZATION PROCESSESThe continuous system includes a time period during which the medium is heated to the sterilization temperature, a holding time at the temperature and a cooling period to restore the to the fermentation temperature.

The temperature of the medium is elevated in a continuous heat exchanger and is then maintained in an insulated serpentine holing coil for the holding period. The length of holding period is dictated by the length of the coil

The hot medium is cooled to the fermentation temperature using two quential heat exchangers - the first utilizing the coming medium as the cooling source and second using cooling water.

The major advantage the continuous process is that a much higher temperature may be utilized, thus reducing the holding and reducing the degree of nutrient degradation.

Required Del factor may be achieved by the combination of temperature and holding time which gives acceptably small degree of nutrient decay.

There are two types of continuous sterilizer which may be used for the treatment of fermentation media:

The indirect heat exchanger and

The direct heat exchanger.

The most suitable indirect heat exchangers are of the double-spiral type which consists of two sheets of high-grade stainless steel which have been curved around a central axis to form a double spiral. The ends of the spiral are sealed by coversTo achieve sterilization temperatures steam is passed through one spiral and medium through the other in countercurrentSpiral heat exchangers are also used to cool the medium after passing through the holding coil. Incoming unsterile medium is used as the cooling agent in the first cooler so that the incoming medium is partially heated before it reaches the sterilizer and, thus, heat is conserved. The major advantages of the spiral heat exchanger

The major advantages of the spiral heat exchanger are:

The two streams of medium and cooling liquid, or medium and steam, are separated by a continuous stainless steel barrier with gasket sealsbeing confined to the joints with the end plates.This makes cross contamination between the two streams unlikely.

The spiral route traversed by the medium allows sufficient clearances to be incorporated for the system to cope with suspended solids. The exchanger tends to be self-cleaning which reduces the risk of sedimentation, fouling and'burning-on'.

Indirect plate heat exchangers consist of alternating plates through which the countercurrent streams are circulated. The plates are separated by gaskets andfailure of these gaskets can cause cross-contamination between the two streams.

The clearances between the plates are such that suspended solids in the mediummay block the exchanger and, thus, the system is only useful in sterilizing completely soluble media.

The Del factor to be achieved in a continuous sterilization process has to be increased with an increase in scale, and this is calculated exactly as described in the consideration of the scale up of batch regimes.

Flow diagram of a typical continuous injector-flash cooler sterilizer.

Flow diagram of a typical continuous sterilization system employing spiral heat exchangers

FILTER STERILIZATIONSuspended solids may be separated from a fluid during filtration by the following mechanisms:

(j) Inertial impaction.

(ii) Diffusion.

(iii) Electrostatic attraction.

(iv) Interception.

(j) Inertial impaction

The particles, because of their momentum, tend to travel in straight lines and may therefore become impacted upon the fibres where they may thenremain. Inertial impaction is more significant in the filtration of gases than in the filtration of liquids.

(ii) Diffusion

Such small particles tend to deviate from the fluid flow pattern due to Brownian motion and may become impacted upon the filter fibres. Diffusion is more significant in the filtration of gases than in the filtration of liquids.(iii) Electrostatic attraction

Charged particles may be attracted by opposite charges on the surface of the filtration medium.

(iv) Interception

The fibers comprising a filter are interwoven to define openings of various sizes. Particles which are larger than the filter pores are removed by direct interception. However, a significant number of particles which are smaller than the filter pores are also retained by interception.

Filters have been classified into two types

Absolute filter (fixed pore filters): Those in which the pores in the filter are smaller than the particles which are to be removed and those in which the pores are larger than the particles which are to be removed.

Depth filters (non-fixed pore filters ): Those in which the pores in the filter are smaller. These are composed of felts, woven yarns, asbestos pads and loosely packed fibre glass.

dN/dx = -KN

If it is assumed that if a particle touches a fibre it remains attached to it, and that there is a uniform concentration of particles at any given depth in the filter, then each layer of a unit thickness of the filter should reduce the population entering it by the same proportion; which may be expressed mathematically as:

The value of K is affected by the nature of the filter material and by the linear velocity of the air passing through the filter.

A typical plot of K and X90 against linear air velocity from which it may be seen that K increases to an optimum with increasing air velocity, after which any further increase in air velocity results in a decrease in K.

dN/dx = -KN

If it is assumed that if a particle touches a fibre it remains attached to it, and that there is a uniform concentration of particles at any given depth in the filter, then each layer of a unit thickness of the filter should reduce the population entering it by the same proportion; which may be expressed mathematically as:dN/dx = -KNwhere N is the concentration of particles in the air at a depth, x, in the filter andK is a constant.The efficiency of the filter is given by the ratio of the number of particles removed to the original number present, thus:E = (No - N)/Nowhere E is the efficiency of the filter.

Filter sterilization of airThe most commonly used sterilization process is filtration.

Fixed pore filters are very widely used in the fermentation industry and several manufacturers produce filtration systems for air sterilization. These systems, like those for the sterilization of liquids, consist of pleated membraneThe most common construction material used for for the pleated membranes for air sterilization is PTFE, which is hydrophobic and is therefore resistant to wetting.

PTFE filters may be steam sterilized and are resistant to ammonia whichmay be injected into the air stream, prior to the filter, for pH control.

The prefilter traps large particles such as dust, oil and carbon and pipe scale and rust . The use of a coalescing prefilter also ensures the removal of water from the air

Fixed pore membrane modules are also used for this application but the system must be able to cope with the sterilization of water saturated air, at a relatively high temperature and carrying a large contamination level.

Some form of pretreatment of the exhaust gas is necessary before it enters the absolute filter. This pretreatment may be a hydrophobic prefilter or a mechanical separator to remove water, aerosol particles and foam.

The pretreated air is then fed to a O.2-m hydrophobic filter.

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