Sterilisation time

Typical units for productivity are kg m 3 h 1. Factors that influence productivity include the production time of the fermentation, the time required to dean and set up the reactor, the sterilisation time and the length of the lag phase of growth. Figure 2.2 shows how total productivity and maximal productivity can be calculated for a batch fermentation. The dedsion as to when the fermentation is terminated (maximum or total productivity) depends on the operating costs, which include the capacity of the fermentation vessel, energy costs and labour costs. 

Retortable pouches can be made from combinations of foil, polyester, nylon, polypropylene, HD polythene, provided the laminant (or the process of lamination) employed will withstand the autoclaving conditions of temperature and moisture. Due to the flat shape of the pouch and high surface area, sterilisation times can usually be reduced compared with cans. 

Sufficient time must be allowed for the whole of the load to reach the required temperature before measurement of the sterilising time-period is commenced. This time must be determined for each type of load to be processed. 

Chemical indicators are available for heat, ethylene oxide and radiation sterilisation, usually in the form of adhesive tapes or patches, colour spot cards, small tubes or sachets. They change colour as a result of chemical reaction brought about by the sterilisation process, but it is possible for the change to take place before the sterilising time has been completed, and hence, with the exception of plastic dosimeters used in radiation sterilisation, they are unsuitable as proof of sterilisation. 

This is a reactive gas, supplied in pressurised cylinders diluted with an inert carrier such as carbon dioxide (10% ethylene oxide 90% carbon dioxide) of freon. It displays good penetrating power and will, for example, permeate unbroken egg shells. The gas is used at a concentration of 400-1000 mg per litre and RH of 25-50%. The sterilisation time may be reduced at elevated temperatures. 

When the logarithm of the number of surviving organisms is plotted against sterilisation time in a semi-logarithmic graphic, a straight line is obtained (see Fig. 30.1). 

The sterilisation time that is required to obtain sterility at a certain temperature is dependent on the number and the resistance of the micro-organisms present in or on the product. An initial contamination with 10 of a certain microorganism requires twelve decimal reduction times to obtain the SAL of 10 , see equation 30.3. 

The bioburden process is based on the initial contamination before sterilisation, and thus it is a tailored sterilisation process. Bioburden based sterilisation cycles are not commonly used in Europe. Prerequisite is that the number and thermo-resistance of the micro-organisms in the product are documented, based on which the customised sterilisation time can be calculated. An example can be found for parenteral solutions (for injection, for infusion) in reference [4]. In this example. No is 1 (rounded up) and the D-value at 100 °C of the most resistant micro-organism is approximately 2 min. Using equation 30.3, it can be calculated that a SAL of 10 will be obtained after 12 min. 

There are basically two possibilities for determination of the sterilisation time of the overkill procedure ... 

This delayed effect is influenced by the number of sterilisation units (objects) in one batch the load of the steriliser. It is hard to take all the contributing factors into account for the determination of the correct sterilisation time. Therefore, the temperature sensor that provides the input for the control of the sterilising process is placed inside a container in the coldest spot of the load. The coldest spot is to be identified by validation studies in a standardised batch size and loading scheme. 

Shortly before the sterilisation phase, the ethylene oxide is brought into the chamber. Sterilisation conditions that are usually applied are gas concentration 300-1,200 mg/L, temperature 30-55 °C, relative humidity 30-70 %, and sterilisation time h [9]. During the degassing phase, the gas is removed from the sterilisation chamber by alternating the application of vacuum and aeration. 

Humidity can be a problem. Whereas it was shown (284) that 33% RH was best for spore inactivation, and that at least 30% RH was needed for effective sterilisation (285), dried spores are difficult to kill, and the spore substrate material and wrappings compete with the spore for the available moisture (286). Therefore, the relative humidity is adjusted to 50—70% to provide sufficient moisture for the spores to equiUbrate. The exposure time depends upon the gas mixture, the concentration of ethylene oxide, the load to be sterilised, the level of contamination, and the spore reduction assurance requited. It may be anywhere from 4—24 hours. In a mn, cycles of pre-conditioning and humidification, gassing, exposure, evacuation, and air washing (Fig. 9) are automatically controlled. 

True. Batch cultures give lower overall outputs than continuous cultures, as they suffer from non-productive down-time (the time taken to empty, clean, re-sterilise and re-fill the fermentor). After inoculation, considerable time can be taken for biomass to build up to a level where substrates are effectively utilised. Continuous cultures do not suffer such drawbacks once they are in operation. 

The production-scale fermentation unit, with a projected annual capacity of over50,000 tonnes was fully commissioned in 1980. The bioreactor (Figure 4.8) is 60 m high, with a 7 m base diameter and working volume 1,500 m3. There are two downcomers and cooling bundles at the base. Initial sterilisation is with saturated steam at 140°C followed by displacement with heat sterilised water. Air and ammonia are filter sterilised as a mixture, methanol filter sterilised and other nutrients heat sterilised. Methanol is added through many nozzles, placed two per square metre. For start-up, 20 litres of inoculum is used and the system is operated as a batch culture for about 30 h. After this time the system is operated as a chemostat continuous culture, with methanol limitation, at 37°C and pH 6.7. Run lengths are normally 100 days, with contamination the usual cause of failure. 

Fermentation usually occurs in a conventional stirred vessel at 30°C (with cooling) and vigorous aeration. The process from start to finish can take as littie as 24 hours thus absolute sterilisation is not crudal. However, several processes reuse the mycelium many times and in these circumstances dean conditions are a minimum requirement... 

The kinetics of culture media sterilisation describe the rate of destruction of microorganisms by steam using a fust-order chemical reaction rate model. As the population of microorganisms (N) decreases with time, the rate is defined by the following equation ... 

Figure 15.4 shows the linear model for (15.6.3), the loss of cell viability at various temperatures. As the temperature increases from 105 to 121 °C, the value for the slope of the line increases. This means that the number of viable cells at a fixed time of sterilisation will drastically decrease as the temperature increases by 16 °C. 

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