At one level the relationship between temperature, time and the death rate of pathogenic bacteria is well known. At a given (lethal) temperature a plot of the log of the surviving bacteria against time is a straight line with the time taken to decrease the population by one log10 unit known as the D (for decimal reduction) time, usually expressed given the temperature for which the D value applies, e.g. D70 = 1.2 minutes for the D value at 70°C for a given organism of 1.2 minutes. If the log of the D time is plotted against temperature, the temperature required to reduce the D time ten-fold is the Z value, expressed in °C. Again, this is another nice log-linear curve at lethal temperatures.
However, this is all quite theoretical and if you were to plot the D times at a given temperature taken from food studies in the literature on a graph it quickly becomes apparent that the variation at any one temperature is scarily widely distributed around the mean, typically ten-fold either side. So, for the example given above, the D70 time could range from 0.12 to 12 minutes depending on the study. The reasons for this are many and varied but I was reminded of this very recently when I saw a paper on the effect on the D time which occur at low water activities.
The paper is “Exponentially Increased Thermal Resistance of Salmonella spp. and Enterococcus faecium at Reduced Water Activity” by Shuxiang Liua, Juming Tanga, Ravi Kiran Tadapanenia, Ren Yanga and Mei-Jun Zhub, which was published in Applied and Environmental Microbiology (doi:10.1128/AEM.02742-17). The authors looked at thermal resistance of the two organisms on an inert matrix, but then confirmed their observations using what flour. They commented that “temperature-dependent water activity (aw, treatment temperature) plays an important role in the sharply increased thermal resistance of Salmonella enterica serovar Enteritidis PT 30 and its potential surrogate Enterococcus faecium NRRL B-2354.” So as the water activity decreases the thermal resistance of these organisms increases; the drier the food the longer it needs to be heated to achieve a given kill.
In some ways this is not new information, after all there have been problems killing Salmonella in low water activity foods such as nuts requiring whole new assessments of the times and temperatures required to achieve control. However, it does serve as a reminder that a D time is not universally applicable to all foods under all conditions.
I was involved in an exercise looking at the effects on thermal inactivation of various parameters such as water activity, and the document can be found here: www.mpi.govt.nz/dmsdocument/11578-background-document-on-factors-influencing-the-heat-inactivation-of-bacteria-in-foods.
The report comments: “There are numerous factors which influence the D-value of pathogens and many of them can have a considerable influence. As an example, changing the Aw from approximately 0.95 to 0.83 has been shown to increase the D-value for Salmonella by 140 fold. The pH, fat content (which may be related to aw), incubation atmosphere and conditions experienced prior to heating all have an influence on the D-value measured.”
This information goes some way to explaining the variability of D values at a given temperature in that different foods have different physico-chemical characteristics that may affect the D value. It also means that for a food process the D vale will need to be obtained for the food in question (or one very like it) so that parameters can be set prior to validation. It also means that generic guidelines to cook food to X°C for Y minutes are difficult or will need to be produced in a very conservative manner. More on that another time.