Getting the Best of Time & Temperature
By An interview with Jenny Scott, Senior Director, Office of Food Safety Programs, National Food Processors Association
Jenny Scott is Senior Director in the Office of Food Safety Programs at the National Food Processors Association in Washington, DC, where she has held a variety of positions since 1980. NFPA is a not-for-profit trade association that represents the food processing industry on scientific and public policy issues involving food safety, nutrition, technical and regulatory matters and consumer affairs. Scott is responsible for providing expertise and guidance on issues and policies related to microbial food safety and HACCP as well as technical assistance and crisis management related to NFPA member problems. She received an A.B. degree in biology from Wellesley College, an M.S. in bacteriology from the University of Wisconsin, and an M.S. in food science from the University of Maryland. She has published numerous research papers and book chapters in the areas of microbial food safety and food processing. She has been active in professional associations such as the American Society for Microbiology, the Institute of Food Technologists and the International Association for Food Protection, where she was president in 2000-2001.
Food Safety Magazine: What are the drivers that make time/temperature considerations throughout the food supply chain such a critical factor in today’s food safety and quality assurance/quality control (QA/QC) programs?
Jenny Scott: In general, the drivers for the food industry with regard to time/temperature measurement and monitoring include regulatory compliance, Hazard Analysis & Critical Control Points (HACCP) or prerequisite program implementation, and quality assurance to maintain the cold chain or to ensure proper heating of foods. Ultimately, quality considerations drive the need for time/temperature measuring and monitoring activities more so than safety issues.
Of these, regulatory compliance generally is not the most significant driver, because temperature regulations are limited. Most of the regulatory compliance comes into play in segments of the food industry where HACCP is mandatory, such as seafood, meat and poultry. This would apply to cases in which the processor has put a temperature-related critical control point (CCP) into the HACCP plan. Temperatures in HACCP plans generally tend to relate to instances in which you are cooking product or cooling product, or in the case of seafood and the scombroid species, where the hold time also is critical. Also, the Pasteurized Milk Ordinance (PMO) has some temperature requirements in it, and since the PMO has been adopted by all the states there are some temperature requirements for Grade A dairy products. In the meat industry, there have been some policies, rather than straightforward regulations, related to time/temperature. For example, there is a U.S. Department of Agriculture (USDA) policy that poultry should be shipped at 40F, which has become a de facto regulation, because everyone in that industry adheres to it.
Time/temperature is a factor that for certain companies, certain products and certain operations will be considered critical and will they deal with it as a CCP in a HACCP plan. However, many companies consider time/temperature measurement and control as part of a prerequisite program that contributes to overall safety but where the primary reason for these controls is quality. For example, achieving the shelf life that is required to market refrigerated products means that time/temperature must be controlled from the get-go. If not, we won’t get the desired shelf life and thus we won’t maintain the quality. Indeed, the product will go bad long before it becomes a safety issue. There are some exceptions to that; for example, with seafood, we know that there are certain species of fish in which microorganisms can grow and generate histamine if the temperature isn’t properly controlled. In these types of products, there will be a lot more emphasis on time and temperature control in order to prevent a safety issue from arising.
There certainly have been issues of time/temperature control with respect to Listeria monocytogenes. But when a company is dealing with the type of product for which the regulations don’t allow any Listeria monocytogenes, the company typically already has put the controls in place to ensure to the extent possible that the organism is not present. In this case, time/temperature to control growth becomes fairly secondary, as an added safety measure. Clearly, if you have a product for which you cannot prevent Listeria monocytogenes contamination, then keeping the temperature down for the period of time until it gets to the consumer is important to keep the numbers of Listeria down, which is the key to preventing illness.
In fact, time/temperature controls really focus on controlling the growth of microorganisms, including pathogens that grow in or on the product. Thus, it really comes back to the hazard analysis that a manufacturer will conduct for a specific product to decide whether there are specific pathogens of concern that need to be controlled through time and temperature and how to go about implementing control when and where it is appropriate.
Although I’m speaking primarily about time/temperature on the cooling side, the same issues apply on the heating side. In most operations, a heating step is a CCP, because that is the kill-step for pathogens such as Salmonella, E. coli O157:H7 and the vegetative cells of sporeforming pathogens. On the cold side of the chain, Listeria monocytogenes is frequently a target pathogen. When cooling down a meat item, Clostridium perfringens usually is the primary organism of concern. With seafood products, processors must consider Clostridium botulinum, particularly since strains of this pathogen can grow at refrigeration temperatures. Again, it comes right back to the hazard analysis, where it is very important for companies to identify the pathogens of concern for the various segments of their operations and consider the effect of any time/temperature processes with respect to food safety.
The food processing industry spends a lot of effort in measuring and monitoring cold chain temperatures, be it during storage of raw materials, storage of finished product prior to shipping or storage in the distribution facilities. When a manufacturer has time and temperature under control, quality and safety issues don’t figure in until after the product has left the manufacturer’s control. The issue of time/temperature control during transportation has been discussed often during the past few years. Some of the thinking has been centered on considering this a critical factor, and in the mid-1990s, FDA and USDA issued an advanced notice of proposed rulemaking related to time/temperature control during transportation.
However, although people often talk about cold chain time/temperature in terms of safety concerns, if you look at where the problems have arisen, very rarely has there been an instance that’s safety related with respect to temperature control during transportation. In those rare instances, problems have arisen due to gross temperature abuse of product. And while you’d be hard-pressed to think of a safety problem in the last 20 years that is related to temperature control during transportation, except perhaps for scombrotoxin in seafoods, the food industry would tell you that there have been a number of quality issues. Companies do turn back product simply because of temperature abuse during transportation, which is a problem, just not necessarily a safety one.
Food Safety Magazine: What are some of the challenges facing the food industry in terms of measuring and monitoring time/temperature factors?
Scott: One challenge is ensuring that you are paying attention to the monitoring devices that you have in place. Many companies use circular recorders or thermometers in their cold rooms, but thermometers by themselves don’t do anything—somebody has to be paying attention to the instrument and the readings and be able to make adjustments, if necessary. Just because you’ve walked into the refrigerated unit and it seems cold does not mean that it is operating at the appropriate temperature for quality control.
There also are some challenges with respect to measuring time and temperature during transportation. In talking with a couple of major manufacturers, I noted that one doesn’t do any kind of recording during transportation and the other sends recorders with all of its product shipments. This reflects the nature of each company’s products. One product is more sensitive with respect to quality, and perhaps even safety, issues during transportation, and as such, the company has the need for that kind of routine time/temperature information, while the other company feels they have control over the operation given their specific product. Many of the major food companies have their own trucks, so they do have very good control during transportation, whereas others are using shipping companies and may feel that they need a little more control than the shipper may offer.
If a company is shipping product with a time/temperature recorder, the communication of the collected data can be a challenge. The information has to come back to you in some way: Either the recorder has to be sent back so you can download the information, or the recipient has to take that information, download it and send that information to you. With advances in information technology capabilities, it is becoming very simple for companies to collect and share this type of information. We’re seeing all sorts of arrangements where this is being done, but it does require some agreement and planning between the buyer and seller as to how such a system will work.
Time and temperature data certainly are simple to read; essentially, you’ve got numbers from your measurements and you’ve got some idea of the temperatures that you don’t want to exceed. Since microbial growth is time/temperature related, one of the challenges to the industry occurs when you exceed your guidelines, potentially allowing the growth of spoilage organisms or other bacteria. Let’s say that you want to ship at 40F and you want to receive at 40F. Further, let’s assume that if monitoring indicates that 40F is exceeded, the product may become unsafe. If the product arrives at its destination and the temperature is measured at 41F or 42F, someone is going to have to do some interpretation related to the amount of time that product was at temperatures above 40F and make a decision as to whether this is still acceptable product. The challenge is that this has to be done fairly quickly and accurately. Sometimes it is just easier for companies to say they will not accept any product over 40F, regardless. It should be noted that sending product back that arrives at 41F would not be justified scientifically, particularly if it’s been, say, only a few hours that product has been at that temperature.
If the recipient’s policy is to return all product that arrives at temperatures above 40F, the shipper likely will transport product at a lower temperature to ensure that it’s never above 40F when it arrives. Deciding at what point to reject a load can be a tricky question, because there often is a negative economic impact involved. Implementing a system that would integrate the time and temperature and give you information about what the effect of the time/temperature combination the product has seen really means with respect to microbial growth would certainly be useful to a recipient who is having to make a decision right on the spot. Some companies, particularly larger companies, have technical people on staff who can evaluate acceptability of product outside the required temperature ranges. It may be that all they have to do with a product that arrives above 40F is to use those lots before those lots that are already in storage. In other words, rather than using the first-in/first-out approach, the product that has the higher temperature exposure would be the first out, which would still give the company the same quality that they require.
Finally, one of the biggest challenges with regard to time and temperature control during shipment involves less-than-full loads, or LTLs, where there can be different products on board that may require different temperatures for quality reasons. Also, products are being dropped off at different locations, which means the truck gets opened and closed more often to unload partial loads and then continue on to the next stop. The essential nature of LTL shipments raises many questions: If you’re carrying a full truck of one company’s product and it is being dropped at multiple locations, where do you monitor temperature in the load? Where is the worst-case temperature? Is it the product closest to the back door because the doors are being opened frequently, or is it the product that is staying on the truck for longer periods of time? How do you monitor to get the data that’s reflective of each of the locations where you’ve dropped product? I don’t think these are insurmountable problems, but they are difficult to address given all the variables involved.
Food Safety Magazine: What advice can you provide about the selection, proper use and calibration of temperature measuring and monitoring systems/tools?
Scott: First, you need to select a thermometer thermocouple, data logger or other time/temperature device that is appropriate for its intended use. Second, it must be accurate for its intended use, which means that the device should be calibrated as frequently as required to ensure the desired control. Most temperature measuring/monitoring devices come with information about calibration. The frequency of calibration is going to depend on the intended use. Depending on a processor’s HACCP and quality assurance programs, hand-held thermometers may be used by quality control personnel to check temperatures, but there also may be temperature monitoring and recording systems installed on the line to monitor processing conditions or a product in process. In those cases, thermocouples or similar devices are installed in the heating and cooling equipment in a manufacturing facility. Quality control personnel can then use a different type of thermometer to take a separate reading as a verification check on the line.
If the company is operating under mandatory HACCP, the company certainly would need to calibrate any kind of temperature monitoring or recording devices related to any critical control points. Quality assurance personnel may be able to calibrate thermometers, thermocouples or other temperature measuring devices in-house using a standardized thermometer that is referenced to the National Institute of Standards and Technology (NIST), against which you can calibrate. But some of these devices do have to be sent back to the manufacturer for recalibration, such as most of the portable data loggers.
The main point here is that whether you are doing your own calibration against a NIST-certified thermometer or sending a device back to the manufacturer for service, it has to be recalibrated periodically. If not, the device or thermometer may or may not be accurate. NIST-calibrated thermometers also should be recalibrated annually. Conducting a check on whether the devices are properly calibrated is another aspect that must be given attention. A processor can use an ice water bath, which should be at 32F, to see if the certified thermometer accurately reads that temperature. In addition, thermometers need to be calibrated near the range at which they’re going to be used. So, for thermometers used for refrigeration, you would want to calibrate at refrigeration temperatures, and not at room temperature, whereas for thermometers used during heating you would calibrate at elevated temperatures. Where thermometers are used for a range of temperatures, two and three point calibrations should be conducted.
Since data loggers and monitoring systems are electronic equipment, they generally need some type of routine servicing from the system manufacturers. The supplier will generally provide guidance as to the frequency needed to maintain equipment, but a company needs to let the manufacturer know the conditions of use in the plant atmosphere for more precise calibration. For example, you may need more frequent servicing of monitoring equipment if you have high humidity in your facility.
Calibration of other devices dependent on temperature and used to monitor key quality or safety parameters is equally important. For example, pH varies with temperature and any of the guidance on how to take pH will specify the need to calibrate pH meters to the temperature at which you will take the reading. Generally, it is preferable to have food at room temperature to take a more precise pH reading. Similarly, in measuring water activity (Aw) you also get different readings based on temperature. The methods for determining water activity will specify taking the reading at room temperature and calibrating at that temperature. The most important aspect is to calibrate at the same temperature at which you’re taking the pH or Aw reading. This is only an issue if you’re operating very near a critical value; otherwise, the reading may be somewhat inaccurate but should not have a major effect on the outcome. For example, if you’re operating an acidified food process where the goal is to be at pH 4.6 or below, and you’re operating in the 4.5-4.6 range, then you will want to be pretty accurate with the temperature.
The proper use of time and temperature devices also is important. You need to measure temperature at the proper location in the food to ensure an accurate reading. Typically, if we’re heating up a food, we’re looking to measure the coldest part of the food as it heats; if we’re cooling down the food, we want to measure the hottest part as it cools. Depending on the thermometer that you’re using, you have to insert it at a certain depth to get an accurate reading. It may be difficult to use a thermometer that has to be inserted up to two inches in a very thin product.
You also need to consider the conditions of the locations at which you are measuring or monitoring ambient air temperature. If you are measuring the temperature of a cold room, you don’t want your thermocouple placed where it is getting the blast of air directly from the refrigeration unit, because the reading will not reflect the general room environment. You may want to place it next to the cold room door where the door is going to get opened and closed in order to get a good reading of the warmest temperature. However, this could cause problems if there is an alarm device installed that goes off when the temperature rises too much. If you’re going through the door frequently, you could be setting off the alarm needlessly. Yet, you want to ensure that the thermocouple is in a location where it measures the warmest food in a cold storage area. It is sometimes necessary to do a temperature distribution of the room using multiple thermocouples, and over time, bring product in and out of the room to try to decide where the best location is to measure the desired temperatures. You may want to measure at more than one location in a cold room. Temperature distribution is fairly easily done, especially if you have a system in place for monitoring multiple temperatures. If a company does not have the capability to conduct this on its own, the company can have a consultant come in and do it.
Food Safety Magazine: Do you have any advice for our food processing/foodservice readers on how to improve their strategies for managing or measuring time/temp. or where they can get more information?
Scott: The food company must do the proper hazard analysis to make sure that the right target organisms are identified, which will establish what they’re trying to control during heating, cooling and hot or cold holding steps of the process. They must recognize that as food goes through the process, all the way from various manufacturing steps through to the consumer, the organism of concern may change at different stages and therefore the company’s strategies may have to change, as well.
If interested parties want information on temperature measuring and recording devices and data loggers, they ought to obtain information from the various manufacturers, which they may be able to do by searching the Internet, by reading the trade magazines, or by going to a trade show exhibit and locating all the manufacturers of this type of equipment. After gathering this information, the potential user can do a comparison based on what devices will best address the company’s time and temperature measurement or monitoring requirements.
If the reader is looking for advice on microbial growth and time/temperature relationships, I would point them toward the International Commission on Microbiological Specifications for Foods (ICMSF) series of books titled Microorganisms in Foods. There are three fairly new books of interest in the series. First, Volume 5 discusses the characteristics of microorganisms, which is helpful if you want to know what the minimum/maximum pH and temperature for growth of pathogens in particular foods, for example. Volume 6 deals with specific commodities, which is helpful if you want to get more information on a particular type of food product. Volume 7 deals with microbiological testing and food safety management, going well beyond Volume 2, which it replaces, to provide sampling plans, the statistics behind the sampling, and information on risk assessment and Food Safety Objectives (FSOs).
If you’re in foodservice, the FDA 2001 Food Code is an excellent source of information. In addition to publishing the specific temperature requirements, which are fairly restrictive but based on science, the guidance now includes the agency’s rationale as to why it chose these particular temperatures and holding/cooling/heating times.