Frequently Asked Questions

I. General Issues

A. Copying and Distributing the PMP

1. May I make copies of the PMP and give them to other individuals?

ANS: Yes, we encourage you to distribute the PMP to other interested persons and organizations. Prior permission is not required from the USDA-ARS for distributing or copying the PMP. We do request that you credit the producers of the PMP and/or the authors of a specific PMP model when including PMP information in an article, journal or other type of publication.

The citation for the PMP can include the following information:

"US Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, Wyndmoor, Pennsylvania, USA. [The year of the PMP version, e.g. 2002 for version 6.1]. www.arserrc.gov/mfs/pathogen.htm."

When citing a specific model in the PMP, we request that you use the reference(s) that appears in the model window. For example, for the aerobic, A. hydrophila model in culture media, there are two citations:

S.A. Palumbo, D.R. Morgan and R.L. Buchanan, Influence of Temperature, NaCl, and pH on the Growth of Aeromonas hydrophila, Journal of Food Science (1985) 50:1417-1421 - http://www.arserrc.gov/MFS/HTML/ERRCPubs/5012.pdf

S.A. Palumbo, A.C. Williams, R.L. Buchanan and J.G. Phillips, Model for the Aerobic Growth of Aeromonas hydrophila K144: Journal of Food Protection (1991) 54:429-435 - http://www.arserrc.gov/MFS/HTML/ERRCPubs/5611.pdf

2. Do I need permission to download the PMP?

ANS: No, you do not need permission to download the PMP.

3. Do I need permission to make photocopies of PMP printouts?

ANS: No, you do not need permission to make copies.

4. Is there a cost to download the PMP?

ANS: No, there is no cost to download or use the PMP.

II. Technical Issues

A. Operating the PMP Program
1. How can the Startup Message be viewed?

ANS: On the Menubar, click on "View", then select "Show Message at Start-up".

2. Why does the model not let me input a certain temperature?

ANS: The models were developed over specific ranges of environmental variables. For example, the Aerobic A. hydrophila in Culture Media model was developed over a temperature range of 41°F (5°C) to 107.6F (42°C). The accuracy of predictions made inside (interpolation) this range is known. However, the accuracy of predictions made outside (extrapolation) of this range (e.g., 150°F) is not known, and the software does not permit such values to be entered.

3. Will the PMP run on a Macintosh computer?

ANS: No. The PMP was developed specifically for Microsoft operating systems.

B. Downloading the PMP
2. What versions of Windows do I need to download and operate the PMP?

ANS:Version 7.0 requires Microsoft .NET Framework 1.1 which must be installed before the PMP. These are compatible with the Windows 98, NT4(SP5), ME, 2000 and XP operating systems. Version 6.1 is compatible with the Windows 95, 98, NT4(SP5), ME, 2000 and XP operating systems.

3. Is there also a CD version of the PMP?

ANS: No. We prefer that you download the PMP from our website. In this manner, you can be assured to receive the most up-to-date version. If you wish to distribute a CD of the PMP, then you can download the PMP installation program from http://www.ars.usda.gov/Main/docs.htm?docid=6786, then copy the file to a CD.

C. Printing
1. How can I print the form, the table or the chart?

ANS: In version 7.0, to print the chart (graph), place your mouse over the graph, right-click, and select “Print”.

To print the table, place you mouse over the table, right-click, select “Select All”, then right-click again and select “Print Selected”. To print only specific portions of the table, highlight the cells that you want to print, then right-click and select “Print Selected”.

To print the entire PMP form, in the Menubar select “File” then “Print” then “Form”. Note - this will not print portions of the table or publication windows that are not visible. To print all of the publication information, highlight the text with your mouse, then hold the CTRL key and press the C key. You can then paste this text into any other document which accepts pasted text, such as Microsoft Word®, NotePad, Correl WordPerfect®, etc.

D. References for Models
1. Where can I find references for the PMP models?

ANS: The reference(s) or source for each model, is shown in the “Source and/or Relevant Publications” box. For references in which the senior author(s) is a USDA-ARS employee, we provide the PDF file. For articles with other types of authorship, the reference is not provided as a PDF file, due to journal copyright restrictions. Note - there may be multiple publications for a model. Typically this is because different environmental parameters were modeled at different periods of time.

III. MODEL USE AND INTERPRETATION ISSUES

A. Choosing a Model
1. How do I determine the correct model to use for a cooling deviation?

ANS: a. Models are developed for specific environmental conditions. For example, the model may have been developed from data generated in a microbiological broth, in a specific food, or in synthetic food. In each case, the accuracy of model predictions is known only for the matrix for which it was developed. To learn more about the specific conditions under which a model was produced, read the associated references shown in the model window.

b. In certain instances, there may be multiple forms of a model, such as aerobic and anaerobic versions. In this case, choose the model that is closest to your product. For example, choose an anaerobic model if your product is vacuum packaged. Choose an aerobic model if you wish to understand how the bacteria will react when the package is opened and exposed to oxygen.

c. In many cases, you will not find a model that exactly matches your food product’s formulation. In this case, it is advisable to choose a model that will provide more liberal estimations of growth or inactivation. For example, culture media (broth) models typically predict shorter Generation Times (or higher growth rates) than those observed in food. This is true as long as you set the environmental parameters (temperature, pH, water activity/NaCl) to the values that match your food. A similar example is that a sterile raw food model will normally predict shorter Generation Times (or higher growth rates) than a non-sterile raw food or one that contains inhibitory additives

B. Applying a Model
1. Can I use the models for situations where the product temperature changes over time?

ANS: Currently, there are dynamic temperature models for C. botulinum and C. perfringens, and static temperature models for other pathogens. We are in the process of producing dynamic temperature models for a variety of other pathogens, and expect these to be available in future versions of the PMP.

To use a static model for situations where temperature changes over time, you will need to calculate the growth at specific temperatures, and then add these individual growth calculations to determine the total predicted growth over the entire time-temperature range. For example, suppose you want to predict the growth of L. monocytogenes in a product that has the following time-temperature profile:

0.0 hours 98.6°F
0.5 hours 71.2°F
1.0 hours 63.4°F
2.0 hours 50.1°F

For fail-safe predictions, we will assume that the product was at 98.6°F for 0.5 hours. At 71.2°F for 1 hour, and at 63°F for 2 hours. More conservative estimations can be made if you collect time-temperature data in shorter time intervals.

First, set the environmental conditions to match your product. In this example we will use the aerobic broth culture L. monocytogenes model for NaCl. Set the conditions to: pH=6.5, NaCl=1.0%, 0 ppm nitite.

Set the “Initial Level” to 3.0.

Next, set the temperature to 98.6°F (37°C), then click the box “Calculate Growth Data”. Next, click the “No Lag” box (for more fail-safe predictions). Since we are assuming that the product was at 98.6°F for 0.5 hours, we subtract log 3.0 (from the Initial Level) from the “log(CFU/ml)” value at 0.5 hours. However, you’ll notice that there is no value given at 0.5 hours. Therefore, average the count for 0.4 (3.64 log[CFU/ml]) and 0.6 hours (3.76 log[CFU/ml]). This would equal (3.64 + 3.76)/2 = 3.70.

Subtract 3.0 (starting level) from 3.7 and this equals 0.70 log(CFU/ml).

Record this number.

Next do the same calculation at 71.2°F for 1 hour, keeping the initial level at 3.0. The prediction at 1.0 hour is 3.63 log(CFU/ml). Therefore, subtract 3.0 from 3.63, and this equals 0.63 log(CFU/ml).

Record this number.

Finally, repeat this procedure at 63°F. At 2 hours the prediction is 3.67 log (CFU/ml). Subtract 3.0 again from 3.67, and this equals 0.67 log(CFU/ml).

Record this number.

Now add all three numbers: 0.70+0.63+0.67 = 2.0. Therefore, the prediction is for 2.0 logs of growth with this cooling profile.

As with all PMP models, you would need to validate the predictions for food types and formulations that are different from the model.

2. How can I get a more Fail-Safe prediction?

ANS: For growth models, chose the “No Lag” prediction option. This option does not add a Lag Time to the growth scenario. Also, choose models that were developed in a sterile broth system. Typically, the Generation Times/growth rates observed in broth media under optimum conditions for NaCl/water activity and pH are equal to or greater than that observed in food.

For inactivation models, more fail-safe predictions can be attained by setting the environmental parameters to values that predict a greater inactivation rate.

C. Interpreting Model Output
1. How do I interpret the PMP model predictions if the model was developed in a different food matrix than the one I'm interested in?

ANS: Without experience in the use of models, it is difficult to know if the model you use is over- or under-predicting bacterial growth or inactivation when applied to another food matrix. As such, it is best to use models to understand potential trends in bacterial behavior as the environmental conditions change. Only through validation studies (e.g. inoculated pack studies) would you be able to have confidence in model interpretation for your food of interest.

2. How can you interpret a model that was developed in a sterile system to a situation where the food contains spoilage flora?

ANS: In general, this situation is most relevant to growth models. Depending on the pathogen, spoilage flora (e.g., bacteria, fungi) can markedly inhibit the growth of pathogenic bacteria. This is especially apparent at refrigerated temperatures where the growth rates of psychrotrophic (cold-loving) organisms may be greater than that of the pathogen. Therefore, in these situations, the maximum density (level at stationary phase) of a pathogen may be 3 to 5 log10 levels less that observed in a pure culture. Also, the growth rate may be inhibited. Therefore, in general, Generation Time will be shorter and growth rates and maximum population densities will be higher in sterile culture systems compared to systems containing spoilage flora.

D. Lag Phase
1. What is the significance of the Lag Phase Duration (LPD)?

ANS: The Lag Phase is the time required for the cell population to adjust to the food environment and begin growth. The Lag Phase is the most unpredictable aspect for a growth model, compared to the Generation Time. This is because the Lag Phase will vary depending on the previous “history” of the organism. For example, the Lag Phase Duration (LPD) of bacteria grown at 98°F in culture medium and then transferred to ground beef at 50°F will be different than the LPD of bacteria grown at 70°F. This is because the previous environment of the bacteria will result in different cellular changes that are necessary before it can again grow in a different environment.

The LPD represents a distribution of lag times for individual cells in the matrix. As you notice on most growth curves, this produces a concave-shaped curve [See growth curve at right] between the lag phase and the growth phase. Consequently, a portion of this curvature is included in the calculated lag phase and a portion is included in the growth phase.

2. How do I use the Lag and No Lag options in the Growth Models?

ANS: Selecting the “Lag” option will result in a prediction of the Lag Phase Duration based on the experimental data for the model.

Selecting “No Lag” will remove the period of time for the calculated Lag Phase, and will begin predictions with growth of the bacteria. Note - a portion of the curvature between the LPD and the Growth Phase is included in the Growth Phase. Therefore, the starting level that appears in the chart and table will not be the same as the value that you set as the Initial Level. The time calculated to reach the Level of Concern will use the Initial Level, although it will not appear in the table or chart.

E. Confidence Limits
1. How do I interpret the Upper and Lower Confidence Limits?

ANS: The Upper (UCL) and Lower Confidence Limits (LCL) indicate the variation in the predictions at a confidence level of 95%. If you are looking for more fail-safe predictions, then use the UCL. Use the LCL if you want more conservative estimates.

F. Initial Level and Level of Concern
1. What is meant by “Initial Level”?

ANS: This is an arbitrary value that you can set to indicate the initial level of bacteria in the sample at the beginning of the growth scenario. The lowest and highest values that you can select are restricted based on the levels used in the experimental data that form the model.

2. What is meant by "Level of Concern"?

ANS: Level of Concern is an arbitrary level that you select for a target level of growth. The calculated time to reach the Level of Concern is shown in the box labeled “Time to Increase [Level of Concern minus Initial Level] logs”.

3. Is there a recommended Level of Concern?

ANS: There is no recommended Level of Concern. You must select the level.

4. Is there a generally accepted level of a pathogen in food at which point it becomes a food safety issue?

ANS: The minimum level of a pathogen that poses a health risk depends on many factors, such as the susceptibility of the affected population and the quantity of food that is consumed. We do not indicate any specific level for a pathogen

G. Other Issues
1. What would be expected to happen to a pathogen if the product was frozen?

ANS: In general, bacterial numbers decline when the product is frozen. The extent of decline is based on various factors, such as the freezing temperature and the rate of temperature change to reach a frozen state.

H. Water Activity
1. What are the consequences of calculating water activity based on a known level of NaCl that is added to the product?

ANS: Water activity of food is a measure of the level of water that is not tightly bound to the food matrix, and that is available for bacterial growth. This value varies from 0 to 1, with most hazardous foods being in the range of 0.85 to 0.99. This value is based on all of the salts, not just NaCl, that may be in the food matrix. Consequently, if you add say 1% NaCl to a food, you can enter this value in the PMP input box. However, the calculated water activity level is a reflection of this level of NaCl and not that of the total level of salts that are in a food. For this reason, we advise that you measure the water activity of your product and then change the value in the input box to match this value.

Water Activity (aW) may be calculated from the NaCl concentration. It is presented for convenience and is not used in any calculations based on NaCl level. At high water activities (> 0.974 = 4.5% NaCl), the effect of different humectants on microorganisms is approximately equal. If the water activity of a food is known, the equivalent NaCl level can be entered into the model.

I. Generation Time and Growth Rate
1. What is meant by the Generation Time?

ANS: The Generation Time is the time (usually in hours or days) that it takes for bacteria to divide. To convert this to Growth Rate, simply divide 0.301 by the Generation Time. [The value 0.301 is the log10 of 2.]

2. What is meant by the Growth Rate?

ANS: The Growth Rate is the change in bacterial numbers over time, typically expressed as log10 per hour or day. To convert this value to Generation Time, divide 0.301 by the Growth Rate.

J. Sources of Information and Data
1. Can you recommend other information resources to learn more about Predictive Microbiology?

ANS: Click on the Reference tab on the File menu and select the article "Model Development". This article provides an abbreviated overview about model development. Also, in the same Reference tab, you can select Publications List and look for other publications that we offer in PDF format or as a citation. Another source, although more technical in nature, is a book titled Predictive Microbiology: theory and application by McMeekin et al. (1993). A good book on general Food Microbiology is Modern Food Microbiology by James Jay (2000).

2. Can I get predictions for my food when there are no models for it in the PMP?

ANS: The published literature is a good source of information about bacterial behavior in food. However, the behavior of bacteria has not been described for the majority of formulated foods. Another source for data is a free on-line database called ComBase, located at www.combase.cc This database contains thousands of records for various pathogenic and spoilage bacteria in many different environmental conditions.

K. Model Production
1. How are Predictive Models produced?

ANS: Models are typically produced from laboratory data where a food or microbiological broth is inoculated with a single bacterial species or a mixture of bacteria. If the researcher would like to model the effects of intrinsic factors of a food/broth, then a range of these factors are added to the sample, such as by adding various levels of salt, organic acid, or preservative. Next, the samples are incubated at various temperatures and the bacterial levels are measured at predetermined time intervals. After the experiments, the data are organized and the a curve is fitted to the data. This is termed Primary Modeling. In the case of growth, parameters for the lag time (if present), growth rate (or generation time) and the maximum population density are calculated. In the case of bacterial inactivation, the lag time (if present) and inactivation rate are calculated. In Secondary Modeling, the change in a specific parameter is modeled as a function of a change in the environment (e.g., temperature, salt/water activity, pH) is modeled. The variation in predictions (upper and lower confidence levels) are also calculated. Finally, all of these data are developed into a model interface, like the PMP, so that users can easily make predictions of bacterial behavior. For more information on model development, go to the Toolbar, select References and click on “Model Development.”

2. How can I find out more about how a specific PMP model was produced?

ANS: While the model window is open, the reference(s) or source of the information can be found in the “Source and/or Related Publications” window.

L. Using PMP Models for HACCP Plan Development
1. Can I use the PMP to validate HACCP plans?

ANS: The PMP models are only valid for the conditions used to produce the model. The reference (or references) found in the “Source and/or Related Publications” window presents an explanation for the methodologies used to produce the model. Therefore, if the conditions (e.g., food formulation) used to produce the PMP model do not match your food system, then you must validate the model for your specific application. Validation normally involves laboratory studies where your product is inoculated with a specific bacteria and then you record the levels of growth or inactivation. These data can then be compared to the PMP model predictions to see if they are within the predicted 95% confidence intervals. If they do not match, then the PMP model is not valid for your application. In this case, assuming sufficient experimental data have been collected, your data may be used to develop a new model that it would be valid for your food product.

M. Types of Models
1. Are there models for Campylobacter bacteria?

ANS: Currently, the PMP does not contain models for this species.

2. Are there spoilage models?

ANS: Currently, the PMP only contains models about the effects of irradiation on spoilage bacteria.

N. Survival (Non-Thermal Inactivation) Models
1. What acidulant (acid) was used in the broth models to adjust pH?

ANS: You should read the publication for the model to determine the acid that was used. In general, Survival (Non-Thermal Inactivation) models were developed using an organic acid (lactic acid) as the acidulant.

O. Heat Inactivation Models
1. Can I use the PMP inactivation models to measure process lethality?

ANS: In their current form, PMP models are not suitable for determining process lethality calculation. To make these calculations, it is necessary to know the Z value and Tref temperature, and to be able to calculate F values over a range of changing temperature. We are currently developing a new tool to perform these calculations in future versions of the PMP inactivation models. If you know the valid Z value and Tref for your food product, then you can use these with a spreadsheet tool that can be downloaded from the American Meat Institute’s website at www.meatami.org.

P. Cooling Models
1. Why won’t my complete time-temperature profile show in the cooling input table?

ANS: You can only enter up to 50 combinations of time and temperature in the cooling profile box.

2. How do I input cooling profile data into the Cooling model?

ANS: You can directly type in your cooling profile data in the table on the left of the screen or you can import the data from a file. To learn how to input the data, click on the “Show me how” button at the bottom of the time-temperature window. After following these directions, you can import the time-temperature data by clicking on the “Import Cooling Profile” button at the bottom of the time-temperature window.

3. Can you use these models if you do not know the product temperature but you know the room temperature where the product is maintained?

ANS: No. You must measure the temperature of the product, specifically the part of the product that has the highest temperature.

4. What does it mean when my cooling profile is similar to the times in the USDA Food Safety & Inspection Service Appendix B, yet the PMP C. perfringens or C. botulinum model shows more than 1.0 log10 of growth?

ANS: The minimum recommended cooling times listed in Appendix B (http://www.fsis.usda.gov/OA/fr/95033F-b.htm) are a guide for safely cooling meat products. Depending on the food product, these times may result in a prediction of more than 1 log10 of growth. As stated in Appendix B “If the product remains between 120° F and 80° F more than one hour, compliance with the performance standard is less certain.” However, you would need to validate the model in your food product before knowing that the model makes accurate predictions.

Q. Clostridium botulinum Models
1. How do I interpret the C. botulinum models?

ANS: The USDA Food Safety & Inspection Service (9CFR-Docket 95-033F) states that the cooling profile of a product can not result in growth of C. botulinum. In some instances, the PMP C. botulinum cooling model may predict a value of growth that is less than 0.3 log10. A predicted value lower than 0.3 log10 does not represent growth, because the log10 for one multiplication of C. botulinum is 0.3 log10 or greater.