Traditionally, and regardless of the assumed kinetics (see previous chapters), the database for calculating microbial survival has been a set of isothermal inactivation curves from which the microorganism’s or spore’s survival parameters have been obtained. This requires that

• A sample of food or other medium that contains a known number of the cells or spores being studied be heated instantaneously to the desired lethal temperature

• The sample be held at this temperature for a specified time • The sample be cooled instantaneously to a temperature low

enough to stop the destruction and also to disallow resumed growth of the survivors before they are counted

An instantaneous isobaric increase or decrease of a real object’s temperature is physically impossible, of course, because of heat transfer considerations. (It can be


accomplished by very rapid pressurization or pressure release; see Chapter 6. However, the temperatures reached due to the mere application of ultrahigh pressure are usually still sublethal if the initial temperature is ambient and the high pressure’s own effect needs to be considered in such a case.) By external heating and cooling, it is only possible to approximate a “step temperature increase” to be followed by a “step temperature decrease” by shortening the come-up and cooling times of the sample as much as possible relative to the holding time. In contrast, to maintain a small specimen of cells’ or spores’ suspension at a constant temperature is usually fairly easy. This can be accomplished by placing the small specimen in a large volume of hot oil or water already at the desired temperature. Cooling is done by placing the small specimen

in ice water, which is also at a constant but low temperature. Because even a very small volume of liquid (much less than even 1 ml) can still hold a huge number of microbial cells or spores, reducing the treated specimen’s size is not considered a problem in most cases. To accomplish the rapid heating and cooling, the sample containing the cells or spores must be held in or passed through a capillary, to assure the shortest heat penetration distance and largest surface area to volume ratio.