Batch Sterilization

You must appreciate the differences between wet and dry sterilization because the antifoam oil for the fermenter requires more severe sterilization conditions.

At the bottom of the introduction page for sterilization is a Java applet that lets you specify some parameters for sterilizing a big fermenter. This supersedes our BASIC exercises, BIGFERM.BAS, that drew the next figure that illustrates some problems with sterilization in large fermenters. A typical simulation run with this exercise is:

The green line shows temperature versus time. The slopes of the rising and descending portions of this curve can be changed by the design of the fermenter and by the specifications of the steam supply and the supply of cooling water. The fermenter contents are heated by a jacket, coils, and injection of live steam. The areas for heat transfer and the sizing of the steam lines determine the rate of heating. The cooling depends on heat transfer area and on driving force which is best with very cold water. Some additional cooling can come from venting the steam from the fermenter as the contents are above this temperature and will boil. This provides evaporative cooling but requires care because foaming can carry the nutrient medium away with the steam.

There are delays in reaching the temperature for sterilization and delays in getting back down to temperatures at which there is no overcooking of the medium. The period at which there is both killing and cooking is shown in red. It would be desirable to have this as a rectangle with little time above the blue line except at the sterilization temperature. This is explained by the effect of temperature on the rate coefficients because the ratio of killing to cooking becomes more favorable at higher temperature. Time at lower temperature is bad because there is cooking without the best amount of killing.

Continuous sterilization is used to almost eliminate these undesired times because there can be more rapid heat transfer to flowing medium in pipes.

while on sabbatical leave in Porto, Sept. 1996