Exercises

Exercise 1

Calculate the D value of the organism under these conditions and compare with the values given in Table 2.3.

Note: The initial minute values are ignored in the best fit determination as these are probably due to sublethal injury (Chapter 5, Section 5.2.2) causing a 'shouldering' effect on the viable count.

Exercise 2

Calculate the Z value of the organism and compare with the values given in Table 2.3.

Exercise 3

Using the Pathogen Modeling Program choose the following settings:

Note the time to reach the level of concern is 32.2 hours.

(1) Alter the pH to values between 4.5 and 6.0 and see the effect on the predicted growth curve. Why does the program not allow you to go below pH 4.5? (See Table 2.8).

(2) Return the pH to 6.5 and alter the temperature to 8^oC (recommended refrigeration temperature) and 10^oC (poor refrigeration).

Note the affect on time to reach the level of concern. Also note that this organism, unlike most foodborne pathogens, is able to grow under refrigeration temperatures.

(3) Repeat the steps above Salmonella as the chosen organism as a comparison.

Compare the predicted results with those for these organisms in Table 2.8.

(4) Reduce the inoculum level to 100 (log2) organisms and note the time to reach level of concern. Further reduce to 10 (log₁₀1) and 1 (log₁₀0) organism and compare the time taken to reach the level of concern.

Comment

Although the model is very useful for illustrating principles of microbial growth it is not perfect. An example is that if Salmonella is selected then the program does not predict any growth for the organism under anaerobic conditions. However Salmonella is a facultative anaerobe.

Note: A 100-fold decrease in initial inoculum size does not cause a 100-fold delay in the time to reach level of concern. This is because micro-organisms grow exponentially. Hence an increase from 1 - 2 - 4 - 8 - 16 - 32 - 64 - 128 (ie approx. 100-fold increase)is equal to 7 doubling times. If the doubling time is 30 minutes then 7 doublings would take 7 x 20 minutes = 210 minutes (3.5 hours). See Table 2.8 for examples of doubling times.

Exercise 4

This examples focuses on the data given in Section 10.1.5

(a) Using published data on the incidence of Salmonella spp. in raw chicken the figure 10.6 is obtained.

(b) The D value at 60^oC is 0.4 minutes.

Hence a cooking period of 3 minutes at 60^oC is equal to:

3.0/0.4 = 7.5 decimal reductions, or approx. 7D kill

A 7D kill means:

10⁸ Salmonella/g being reduced to 10¹ Salmonella/g

and also:

10⁵ Salmonella/g being reduced to 0.01 Salmonella/g, which is 0.1 Salmonella/10g, or 1 Salmonella/100g

By calculating the decimal reduction for the Salmonella population spread the incidence of surviving Salmonella is determined. This is not an easy calculation and is why programs such as @RISK (Palisaide-Europe) are invaluable. Subsequently the surviving Salmonella are able to multiply in the time period during storage (use the Pathogen Modeling Program to investigate this and also refer to Tables 2.3 and 2.8).

In summary this predicts that 1% of the chicken samples had sufficient numbers of Salmonella that survived the cooking process to give a probability of infection (P_i) of 4.1 x 10^-8/g food consumed. See Section 9.1.4 for an explanation of P_i.

Note: There are relatively few studies giving the relative spread of microbial counts in food products. Most studies give the range only.