Fig. 5.12. Continuous sterilization performance chart (Dein-doerfer and Humphrey, 1961).
determined. As pointed out by Banks (1979), this information may not be available for a complex fermentation medium and, therefore, the technique is fairly limited in its practical application. However, provided this information is available the technique may be used in the scale-up of continuous-sterilization processes. As discussed earlier, the Del factor is scale dependent and therefore as the volume to be sterilized is increased so the Del factor should be increased if the probability of achieving sterility is to remain the same. However, the nutrient-quality criterion is not scale dependent so that by changing the temperature-time regime to accommodate the attainment of sterility the nutrient quality may be adversely affected. Examination of Fig. 5.12 indicates that the only way in which the Del factor may be increased without any change in the nutrient quality criterion is to increase the temperature and to decrease the holding time. Although this scale-up could be performed exactly only if the thermal degradation kinetics of the medium were known, the analysis indicates that even without this information the approach would be to reduce the holding time and increase the temperature.
When designing a continuous sterilization process based on spiral heat exchangers it is important to consider the effect of suspended solids on the sterilization process. Micro-organisms contained within solid particles are given considerable protection against the sterilization treatment. If the residence time in the sterilizer is insufficient for heat to penetrate the particle then the fermentation medium may not be rendered sterile. The routine solution to this problem is to 'overdesign' the process and expose the medium to a far more severe regime than may be necessary. Ar-menante and Li (1993) discussed this problem in considerable detail and produced a model to predict the behaviour of a continuous system. Their analysis suggested that the temperature of the particle cores is significantly less than that of the bulk liquid. Furthermore, there is a considerable time lag in heat penetrating to the particle cores, resulting in a very different time-temperature profile for the particles as compared with the liquid medium. Thus, the temperature of the particles may not reach the critical point before they leave the sterilizer and heat penetration into the particles will continue downstream of the sterilize]-. Ar-menante and Li's conclusion is that it is the sterilizer and/or the first cooling exchanger that should be 'overdesigned' rather than the length of the holding coil. Remember that the first cooling exchanger transfers a significant amount of heat from the sterile medium to the incoming medium and increasing its surface area would give more opportunity for the heat to penetrate the particles. This, coupled with increasing the temperature or residence time in the sterilizer, would ensure that the particle cores are up to temperature before the holding coil is reached. Also, this work suggests that it is unwise to use the direct steam injection method to heat a particulate medium because, again, there will be insufficient time for the heat to penetrate the particles.
An example of the scale up of sterilization regimes is given by the work of Jain and Buckland (1988) on the production of efrotomycin by Nocardia lactamdurans. In this case a beneficial interaction appeared to be occurring between the protein nitrogen source and glucose during sterilization, thus making the protein less available but resulting in a more controlled fermentation. When glucose was sterilized separately the oxygen demand of the subsequent fermentation was excessive and the fermentation terminated prematurely with very poor product formation. On scaling up the fermentation it was very difficult to attain the correct sterilization conditions using a batch regime. However, continuous sterilization using direct steam injection allowed the design of a precise process producing sterile medium with the required degree of interaction between the ingredients. The identification of this phenomenon was dependent upon careful monitoring of the small scale fermentation and consideration being given to sterilization as an important scale-up factor.
When a fermentation is scaled up it is important to appreciate that the inoculum development process is also increased in scale (see Chapter 6) and a larger seed fermenter may have to be employed to generate sufficient inoculum to start the production scale. Thus, the sterilization regime of the seed fermenter (and its medium) will also have to be scaled up. Therefore, the performance of the seed fermentation should be assessed carefully to ensure that the quality of the inoculum is maintained on the larger scale and that it has not been adversely affected by any increase in the severity of the sterilization regime.
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