8. The Quest for Sterilization. Thermal Death Time and the F Value.  (TOC)

How do we get around this problem? The most straightforward way is to assume that the bioburden, or the total number of organisms originally present, is greater than it actually is. If it is assumed that the initial bioburden is several orders of magnitude greater than it is likely to be, then we can sensibly speak of reducing the probable number of surviving organisms to a number smaller than the geometric difference between the assumed bioburden and the actual figure. The greater the assumed increase in bioburden, the greater the probability that sterilization will be achieved. This is complicated by the fact that many biological materials are denatured at elevated temperatures even faster than microorganisms are killed. So in most real systems, the balance between killing all organisms and the denaturation of critical biological or other heat sensitive materials must be delicately arbitrated.

You may read in the literature of a so-called "thermal death time, t_D." This is by definition the time required for the complete kill of all organisms in a given suspension. Experimentally, measurements can be made of the number of organisms surviving a particular treatment and then extrapolating to zero. Realistically, we cannot speak of a complete kill except as a probability. Practically, a commonly used way to estimate the thermal death time is to determine the thermal death constant, and calculate the time required to reduce the viable count to one organism. The thermal death time is then slightly more than this time.

You will observe that the thermal death time depends on the initial bioburden. If we define the initial concentration of organisms as 10^B, then ln(N/N₀) = ln(1/10^B) =Kt_D. Therefore, to a good approximation,

There is another term in wide use, the F value. The F value is simply the thermal death time, the time required to kill all organisms in a population, measured at 121^oC. We must emphasize that this concept of thermal death time is empirical, and represents the treatment time for which all organisms will probably be killed. The term is used in the canning industry in particular. To avoid contaminated cans, the material must be over-treated, held at temperature for longer than the estimated necessary time. There will be a trade-off between assured kill and loss of flavor, vitamins, etc.

There remains the problem that for a mixed population of organisms, each type or species of organism will be killed independently of the others (cf. Equations (2 – 5) above), unless the organisms fortuitously have the same value of K. It is generally safest to select a "worst-case" scenario, wherein the most difficult to kill organism likely to be present is treated as though it were the only organism present. If the bioburden is unknown, a worst-case must be assumed for this factor as well.