Design of Continuous Sterilization Systems for Fermentation Media Containing Suspended Solids

A mathematical model was developed to analyze the performance of a continuous sterilizer fed with a liquid fermentation medium containing suspended solids. This type of medium is commonly used in the industrial practice of a number of fermentation processes. The unsteady‐state energy balance equations for the liquid and the suspended solids passing through each component of the system were solved analytically. A computer model was then used to calculate the temperature profiles in the liquid and in the solid particles as they move through the sterilizer, as well as the level of sterility achieved under different operating conditions. It appears that the level of sterility is always lower when solids are present in the system, as one may anticipate. The temperature gradients produced in the solids as a result of the unsteady‐state heat transfer occurring as the media moves through the heat exchangers in the system result in lower temperatures in the solid cores. This reduces the effectiveness of the process by protecting the microorganisms against thermal death. In some cases it was found that the level of sterility was so low that operating under such conditions would fail to produce a sterile product. The model was also used to assess the sensitivity of the sterilization process to changes in the most significant design and operating parameters for the case in which solids are present in the medium. The sterilization temperature, steam temperature, heat transfer coefficients in the heat exchangers, particle size, and residence time in the sterilizer holding section appear to be the most important of these parameters. The model obtained in this work can be used to design continuous sterilizers processing slurry media or to evaluate the performance of existing sterilizers.