Thermal Death Time Studies

Low-acid food processes are designed to produce either commercially sterile or public health protected shelf-stable foods. The design and implementation of these processes depends on knowledge of the heat resistances of the target organisms. A processor can opt to have his processes designed using either the “book value” or a custom sterilizing value for a particular product. The custom value is one that has been optimized by applying a sterilizing value that has been shown to be safe by appropriately designed and executed thermal death time studies. These optimized processes are specific for a particular product formula and take advantage of various food factors that generally reduce the heat resistance of the target organism from the benchmark heat resistance values determined in the laboratory in narrowly defined media. Many food factors can reduce the sterilizing values (F0) required to produce a commercially sterile product by as much as 50% or more.

Reduced thermal processing can be beneficial to many product quality attributes such as vitamin and color retention, and flavor profiles. Other benefits from reduced processing requirements are the obvious savings on energy inputs and the possible economies of higher productivity rates that might result.

Thermal death time studies are inoculated packs done in miniature. Spores of the target organism are inoculated into homogenates of the product and are heated in the product environment and then later evaluated for the number of survivors. Heating of inoculated samples is done using a range of heating times at one temperature and repeated using a number of heating times at as many as five different temperatures. Results from this matrix of heating variables help to characterize the heat resistance values at one temperature and a decimal reduction time, or D-value, can be established for that individual temperature. Plotting D-values as a function of process temperatures and measuring the slope of that curve gives a Z-value. Knowledge of the D and Z of a target organism in a particular product allows the process authority to accurately predict the response of the organism to a wide range of temperatures, which allows for implementation of a highly optimized but safe process. The information gained from thermal death time studies can also be useful in allowing for more accurate evaluation of process deviations. Where “book” or more conservative assumptions are made in evaluating a deviation, the result of the evaluation may dictate that a lot of affected product be destroyed, where the more accurate custom data might be used to salvage it.