Low termperature cooking of meats

Summary

​Experimentally determined D values, and therefore, so-called, 6D processes, established under one set of conditions, frequently are compared with those determined by other laboratories under different conditions. However, several authors have commented that there can be limitations when making such direct comparisons and the reasons for this include the following:thermal characteristics (D and z values) of L. monocytogenes are affected by the medium in which the microorganisms are heated (ICMSF, 1996; US Food and Drug Administration, 2001)D value of L. monocytogenes is affected by ‘strain variation, previous growth conditions, exposure to heat shock, acid, and other stresses, and composition of the heating menstruum' (Doyle et al., 2001)Gaze et al (1989) noted that ‘the techniques used by different groups of workers', including the container in which the microorganisms were heated, can affect the heat resistance of L. monocytogenes.Vanderlinde and Duffy (1999) observed that ‘It is possible to extrapolate the time/temperature combinations .... (required by different countries) .... using these (different) z values, although extrapolations can result in errors especially at temperatures well away from the data (the 60 to 76 oC range) presented.' This means, for example, that the 6D hold time of 673.8 min at 55°C, may not be accurately extrapolated to a 6D hold time of 0.01 min (0.6 s) at 83°C.Different interpretations of data can lead to extrapolation of different conditions required to deliver 6 log reductions in L. monocytogenes counts. For example, the time-temperature combinations derived from the paper by Vanderlinde and Duffy (1999) are not only conservative but also they highlight the inconsistencies that can occur when interpreting primary data. 
It is because of the individual and/or cumulative influence that the factors described above may have on the accuracy of determining minimum heating conditions for L. monocytogenes that caution should be exercised when extrapolating 6D values to the second decimal place.
For example, whilst the data presented by Gaze et al (1989) showed that the exposure times at 70 oC required to deliver 6D processes for L. monocytogenes ranged from 0.84 to 1.62 min when heating in homogenates containing steak, chicken or carrot, these authors nevertheless recommended ‘that the slowest heating point in a product should be held at 70 oC for 2 min to ensure that it is effectively decontaminated of L.if contamination is up to a level of 106 per container.
In this instance the monocytogenes recommendation has been conservative and incorporates a 23% increase above the maximum of the experimentally determined heating times in order to accommodate L. monocytogenes strains that might exhibit atypical D values under some test conditions.