BATCH STERILIZATION PROCESSES

Liquid medium is most commonly sterilised in batch in the vessel where it will be used. The liquid is heated to sterilization temperature by introducing steam into the coils or jacket of the vessel. 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 sterilization temperature is reached, the temperature is held constant for a period of time called holding time. Cooling water in the coils or jacket of the fermenter is then used to reduce the medium temperature to the required value.

For operation of batch sterilization systems, we must be able to estimate the holding time required to achieve the desired level of cell destruction or sterilization. As well as destroying contaminant organisms, heat sterilization also destroys nutrients in the medium. To minimize this loss, holding times at the highest sterilization temperature should be kept as short as possible.

Cell death occurs at all times during batch sterilization, including the heating-up and cooling-down periods. The holding time can be minimized by taking into account cell destruction during these periods.

Although a batch sterilization process is less successful in avoiding the destruction of nutrients than continuous sterilization. The objective in designing batch sterilization is still to achieve the required probability of sterility with the minimum loss of nutritive qualityThe highest temperature which appears to be feasible for batch sterilization is 121°C so the procedure should be designed such that exposure of the medium to this temperature is kept to a minimum. This is achieved by taking into account the contribution made to the sterilization by the heating and cooling periods of the batch treatment.

The following information must be consider for the design of a batch sterilization process:

(i) A profile of the increase and decrease in the temperature of the fermentation medium during the heating and cooling periods of the sterilization cycle.(ii) The number of micro-organisms originally present in the medium.(iii) The thermal death characteristics of the 'design' organism.

By knowing the original number of organisms present in the fermenter and the acceptable risk of contamination , the required Del factor may be calculated.Example 1: As we know that and the acceptable risk of contamination is 1 in 1000, Therefore Nt should equal 10-3 of a viable cell. If the unsterile broth contains 1011viable organisms, then the Del factor may be calculated as:

Therefore, the overall Del factor required is 32.2.

However, the destruction of cells occurs during the heating and cooling period of the broth as well as during the period at 121°C, thus, the overall Del factor may be represented as:

By knowing the temperature-time profile for the heating and cooling of the broth it is possible to determine the contribution for Del factor by these periods. Thus, knowing the Del factors contributed by heating and cooling, the holding time may be calculated to determined the overall Del factor.

Figure (a) Variation of temperature with time for batch sterilisation of liquid medium.

Figure (b) Reduction in number of viable cells during batch sterilisation.

Calculation of the Del factor during heating and cooling

The relationship between Del factor, the temperature and time is given by equation

…………………..1

However, during the heating and cooling periods the temperature is not constant and, therefore, the calculation of Del factor would require the integration of equation (1) for the time-temperature regime observed.Richards (1968) demonstrated the use of a graphical method of integration and this is shown in figure below. In this figure the time axis is divided into a number of equal increments t1, t2, t3, etc.,

For each increment, the temperature corresponding to the mid-point time is noted. It can now be approximated that the total Del factor of the heating-up period is equivalent to the sum of the Del factors of the mid-point temperatures for each time increment.The value of the specific death rate of B. stearothermophilus spores at each mid-point temperature may be deduced from the Arrhenius equation .The value of the

Del factor corresponding to each time increment may then be calculated from the equations:

The sum of the Del factors for all the increments will then equal the Del factor for the heating-up period. The Del factor for the cooling-down period may be calculated in a similar way.

Calculation of the holding time at constant temperature

From the previous calculations the overall Del factor, as well as the Del factors of the heating and cooling parts of the cycle, have been determined.Therefore, the Del factor to be achieved during the holding time may be calculated by difference:

From example 1 where the overall Del factor is 32.2 and if it is taken that the heating Del factor was 9.8 and the cooling Del factor 10.1, the holding Del factor may be calculated:

But V = k t, and the specific death rate of B. stearothermophilus spores at 121°C is 2.54 min -1.

Therefore, t = V/ k

Or t = 12.3/2.54 = 4.84 min.

If the contribution made by the heating and cooling parts of the cycle were ignored then the holding time would be given by the equation:

Therefore, by considering the contribution of the heating and cooling parts of the cycle to the sterilization process a considerable reduction in exposure time is achieved