Testing for Foodborne Pathogens

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Food Product Design

Testing for Foodborne Pathogens

July 1999 -- Focus On: QA/QC

By: Bruce M. Floyd
Contributing Editor

  Microbiological crises in the food industry inflict pain and suffering on the consuming public, and wreak havoc within the affected areas of the industry. In the 1960s, when Salmonella was discovered in an instant nonfat dry milk, the Starlac brand disappeared from the market, (the dry-milk industry has never been the same), and the USDA began offering a voluntary Salmonella surveillance program. Today, the USDA reports that it continues to detect very low levels of Salmonella in the environment of USDA-inspected dairy plants.

  Another famous outbreak, involving under-processed vichyssoise (a cold potato soup), resulted in several deaths from botulism. The FDA then adopted 21 CFR, Part 108, "Emergency Permit Control" and 21 CFR, Part 113, "GMPs for Thermally Processed Low-Acid Canned Foods Packaged in Hermetically Sealed Containers." There have been many other instances since these - the current problems with Listeria in cheese and ready-to-serve processed meats are just the latest in a long line of pathogenic-bacteria outbreaks traced to food processors.

  What can food processors do? This article addresses pathogen-testing procedures for products that are minimally cooked by the consumer, including all ready-to-eat (RTE) products, as well as microwaveable products that may not receive sufficient heating to kill the bacteria in question. (Products that are terminally sterilized are excluded - the Emergency Permit Control process and required better-process-control training and documentation address these problems; products that the consumer fully cooks are also excluded.)

Setting the stakes
  For food-safety evaluations, consider the likelihood of an "at-risk" individual consuming the product if you ship contaminated output. This will take some knowledge of the bacterial quality (BQ) of the products, along with an understanding of who eats or drinks your product.

  For instance, in the case of a company that processes apple juice, you may not know what the BQ is, but you do feel that if product was contaminated, there's a very good chance that an at-risk individual would consume the product. At-risk people can be fatally infected by a pathogenic organism, not simply made ill. Such individuals include those who are very young, very old, immuno-compromised or chronically ill. Pregnant women also fall into this group, which would be expected to consume apple juice on a regular basis. Even though the percentage of the population in the at-risk group is small, it's likely they would be adversely affected if they consumed contaminated product.

  Now ask yourself, how often are you willing to have this happen - once a year, once a decade, after you retire? Also consider how many companies have survived a crisis such as this. What about the poor consumer? Unfortunately, there is no way to reduce risk to zero. The goal is to reduce risk to reasonable levels, fulfilling as much as possible the expectation on the part of consumers that all RTE foods will be safe.

  According to a March 11, 1999, Department of Health and Human Services news release entitled "Salmonella and Campylobacter Illnesses on the Decline," Salmonella declined 13% between 1996 and 1998, and S. enteritidis declined 44% in the same period. There was a decline of 15% in Campylobacter cases between 1997 and 1998, while the incidence of Escherichia coli 0157:H7 did not change between 1996 and 1998. (This report is available online at: www.cdc.gov/od/oc/media/pressrel/r990311.htm).

  Although these declines are attributed to increased regulatory activity and the introduction of HACCP procedures to the meat and seafood industries, the exact cause is not known. Many factors affect the population that the industry serves, including: an increased number of at-risk individuals in the general population; more convenience foods; fewer people who know how to cook; a general population that may not have the resistance to common diseases that it once had; and the ability to track organisms by DNA testing. There's also a heightened awareness of foodborne disease on the part of the public and the medical community, particularly when it comes to distinguishing ailments caused by viruses from those caused by pathogens such as Staphylococcus. Industry changes over the last 30 years have improved the safety of the overall food supply, but the only real protection consumers and producers have is constant vigilance.

  If you do not know a product's potential to be a vector for a pathogenic organism or toxin, there are many web sites that give information on pathogenic organisms. A good place to start is with the Bad Bug Book, available at http://vm.cfsan.fda.gov/~mow/intro.html. This site lists foodborne pathogenic microorganisms, along with links to further information on each of them. Other web sites for particular pathogens can be accessed through the International HACCP Alliance's web site or by going directly to USDA/FSIS, FDA or CDC web sites.

Laying the groundwork
  Pathogen testing is one of the measures that companies can take to minimize the risk of selling microbially contaminated RTE products. The first step in pathogen testing is to make sure that the process is under control. Start with an in-depth analysis of each food plant and the products made there. (This could be part of the introduction of a HACCP plan for the facility.)

  If your company already has a HACCP plan, have that plan reviewed by an outside auditor familiar with pathogen control in the processing environment. Don't use only people familiar with a particular industry, as this will not introduce a new perspective. For example, the dairy industry is very familiar with post-processing contamination, but this concept is not strong in other industries. Also, separating raw materials and finished products has a different meaning depending on the industry official contacted - common practices in a given industry tend to be invisible to someone on the inside.

  Pathogen testing involves several different areas - environmental testing, equipment swabbing and product testing. Product testing encompasses both raw materials and finished product. Since pathogens either already exist in the raw materials, or enter the product from the environment or equipment, it goes without saying that all three areas should be involved in testing. The quantity and type of testing depends upon the product and process.

  Since testing is of little benefit without the underlying quality programs, let's first look at some good manufacturing practices (GMPs) before discussing pathogen testing. (If a company does not have all of the prerequisite GMP programs in place, all of the pathogen testing in the world will not prevent the shipping of contaminated product.)

  This is a good time to take a slow walk through the plant. Many people take a quick tour of their facilities every day, but miss simple problems that are causing environmental and equipment contamination. Take an "official" copy of the plant's process with you, and consider these questions:

  • Is it possible to follow the written process in the plant? If you are using a schematic of the process, have it put onto an actual floor plan complete with walls, doors, etc.
  • Is there a copy of the written process on the floor, and has it been used lately? Is this process the "official" copy, or a summary prepared by a supervisor or employee?
  • Are there any circular portions to the process, such as rework or divert valves?
  • Are the prescribed times and temperatures being observed?
  • Is anything added to the product after it is cooked?
  • Is the product cooked in the package? If not, how is the packaging sanitized?
  • Is the official written process being followed?
  • Are there marginal notes that change the process or indicate that the equipment has been changed?
  • Is the equipment clean? Is it in repair?
  • Are there odors associated with spoilage present?
  If proper procedures are not being followed or equipment is in disrepair, it will be difficult to design a testing program to address any random sources of system failure.

  Many times, problems that have existed for a long time become invisible to the people who work with them daily. Just because there's a written formula and process-control system doesn't mean it's being followed. It's important to have written procedures, but a piece of paper will not prevent product contamination. Only written procedures that have been validated and are being followed every day will minimize the risk of process failure.

Traffic patterns
  That was the easy part. Now look at traffic flow - including parts of the operation that are not in the process description. I call this the pollinating bee scenario. Are there activities, people or product that move from one area to another on a random basis? The "bees" could be forklifts; maintenance, QC, office or supervisory personnel; or product, such as rework.

  Consider traffic-pattern questions such as: Are there areas where raw and finished product cross paths? Where is dirty equipment washed? Where does the dirty equipment sit while waiting to be washed? Where is the clean equipment placed before reassembly? The answers to these questions might surprise you.

  Next, draw the people- and machine-flow on a process-flow diagram. This can help determine locations for environmental testing that may have previously escaped notice. For example, a plant might have clothing and shoe policies for employees, supervisors and guests, but fail to implement the program for outside workers coming in and out of the plant. Environmental control is important if any of the products you are making could be contaminated before or after processing.

  Many of the recent product recalls are related to post-processing contamination, which is why contaminates introduced into the process from the total plant environment must be addressed. Look for less obvious problems that may not happen frequently, or be easy to detect - GMP lapses in frequent hand washing within an area, for example. Also consider where forklift trucks go, and what they drive through to get there. If the plant is divided into zones that separate raw materials from finished product, do the forklift trucks violate these separations - ever? What other random factors might be present? Processing failures are easier to detect than random contamination from an environmental source, but understanding the traffic patterns will help prevent problems.

Pathogen patrol
  Testing should occur in areas that have the potential to contaminate the processing/packaging areas or spaces adjacent to them - not just floor areas. This includes air filters and overhead lines, especially those that sweat. Use the flow chart you created to determine where the microbiological test swabs will be taken. Areas with standing water can be problems, for example.

  The particular organisms tested for will depend on the pathogens that are a problem in the given industry or area of the country in question. Jerry Welbourn, Ph.D., writes in Homewood, IL-based Silliker Laboratories' Scope publication (vol.14, issue 1) : "To verify the effectiveness of cleaning and sanitizing activities in the processing environment and determine if Listeria (monocytogenes) is present, environmental testing is invaluable. Forewarned is forearmed. Drains and floors should be tested initially. If Listeria is discovered in these sites, testing should be expanded to ascertain the extent of the problem." There is a lot of common sense to the process.

  It's necessary to integrate environmental testing with equipment testing - equipment testing validates the cleaning process. It's also important to monitor the effectiveness of the clean-up and sanitizing procedures, as well as validate any changes to cleaning procedures. Many people only run standard plate counts (SPC) and coliform tests on the equipment, but it is a good idea to check for the specific pathogens that are a problem in your industry. This is also a good time to work out a system of corrective action to be taken if the environmental and/or equipment swabs indicate a problem.

  If an area is being washed, it should be completely clean and sanitized after washing. Organic material should be totally absent. Test equipment is available that will rapidly indicate the absence of organic material; one such system is sold by IDEXX, Westbrook, ME, under the name of Lightning®. This device lets the tester know if a surface is adequately cleaned, saving time by avoiding swabbing dirty equipment.

  Many operations are dry-cleaned. Such areas will be free of accumulated product, but they do have a residue of organic material. It's possible to swab these areas to determine if there are pathogens present, as well as to determine the SPC load in the area. Equipment swabbing close to the start of production is recommended. If a problem occurs, swab after clean-up and just before production to see if the results are similar. If the equipment is clean after clean-up, but dirty the next day, a problem in or around the equipment exists and must be located.

  The easiest way to set up a swabbing program is to purchase test kits from recognized laboratories that supply them. For example, Silliker Laboratories sends out kits, and helps plants set up a swabbing program; many times the cost of the kit is part of the total package.

  Rapid test kits for various pathogens are coming onto the market. Their effectiveness needs to be determined in each application - i.e., what level of contamination is being checked for, and what level the particular kit will detect. Also consider these questions: If you have a positive swab, will you be able to identify the type of organism to determine if it is a new strain, or part of a continuing problem? Is the test specific for an organism, or does it find every organism in its class, i.e. all Listeria instead of L. monocytogenes?

  If a product is not cooked in its packaging material, ensure that the packaging is not a potential source of contamination. The UHT (ultra high temperature) industry did not exist until a method to sterilize the film with H2O2 was developed. This does not mean that the film is contaminated, but the potential exists, especially if the storage area is dusty and the rolls of film are not protected. This applies to the packaging room as well. Fortunately, the packaging material can easily be tested.

  Particular attention should be given to the frequency and location of environmental and equipment swabs in the packaging area. It's necessary to be able to prove that the equipment is clean and free of pathogens after a clean-up. This is especially true when cleaning up after a known problem has been discovered.

Product testing
  Product testing involves both raw materials and finished product. Processes without a cook step, and products that the consumer minimally processes, have a much greater need for testing on the raw-material side. Since, in this case, processing does not minimize problems, it's necessary to avoid using contaminated raw materials. This starts with careful raw material selection and by knowing the supplier well. No amount of testing will eliminate the risks from doing business with a poor supplier. This means inspecting the supplier's plant and reviewing each supplier's safety program.

  Set up a program for testing raw materials that reflects the risk associated with your process and end users. Be sure the sampling is valid for the product. If you have doubts about the number and quantity of samples needed for a particular pathogen, the FDA's Bacteriological Analytical Manual can provide some assistance. This is a good time to seek advice from a qualified specialist. Be sure the person taking the samples is trained in the proper sampling techniques. This includes the proper preparation of the samples in the lab; proper technique eliminates the possibility that a contaminated ingredient will cross-contaminate other products, which would make troubleshooting impossible.

  Be sure to carefully check out the lab doing the testing, including its references and its reputation with other customers and within the regulatory community. Determine that the lab is capable of finding pathogenic organisms when they are there - it's nice to have negative results, but not when problems go undetected. Also, make sure the lab runs positive and negative controls, and that it uses recognized, valid test methods. The lab's sample-preparation area must be isolated from culture areas, and provided with separate ventilation to eliminate the possibility of cross-contamination from other clients' samples.

  Many rapid-test methods screen for families of organisms, which are valid as long as none of the family is present. A lab needs to be able to perform the long, and sometimes tedious, verification procedures to make sure that good product is not rejected - you don't want false positives anymore than you want false negatives.

  It's often best not to have a pathogen-testing lab at a plant location. It should be located away from any processing facility, so there's no possibility of a control organism entering the plant. Unless a plant generates a very large number of samples, it doesn't pay to set up a pathogen-testing lab - commercial labs can assure rapid turnaround of samples and provide volume discounts. The lab's quality is the most important thing: you must be able to trust them when they report a positive finding.

  If a product sample tests positive for a pathogen, the entire lot is positive. It's not possible to segregate product within a lot. The manufacturer chooses the lot size based on the potential loss the company is willing to assume. And it gets worse - product made after the last verified complete equipment cleaning to the present time needs retesting at a higher sample rate.

  A lot with a positive result cannot be retested; this is strictly forbidden by the FDA. The sample can be retested if the original test is believed faulty, but that's the reason for all the caution in selecting the lab in the first place. Once a pathogenic organism has been found in a lot, it may not be legally sold or shipped as-is. Disposition into animal food may be restricted, depending on the organism and animal being fed.

  There's always the possibility of a positive pathogen test on finished product, but this is where expanding pathogen testing to include the environment, raw materials and equipment will help. Also, an acceptable reprocessing method should be determined when the testing program is initiated, so that any financial loss can be minimized. The important thing is knowing there's a problem before the product is shipped, eliminating the need for a recall, or worse.

  The plan of action for responding to the discovery of pathogens in the plant depends on the location and frequency of the positive results. A positive L. monocytogenes sample from a location next to the packaging machine is much more serious than a positive warehouse swab, unless that particular part of the warehouse is used to store packaging material. This illustrates why it's important for the person that reviews the test results to be familiar with the entire operation.

  Environmental control is an integral part of GMP compliance. Under such a program, testing detects breaches in the system before they reach crisis proportions. It also confirms that the system is working to reduce the possibility of pathogenic contamination to a reasonable level. Remember - products cannot be tested into compliance. Testing is only one part of a total quality system that is based on sound processes that are fully integrated with all of the prerequisite GMP programs.


Bruce Floyd established Process Systems Consulting, Iowa City, IA, after working more than 30 years in the food processing industry. He has had extensive experience in sanitation, quality control, regulatory relations, and product and process development (both domestic and international), and specializes in integrating ingredient and manufacturing specifications into total process systems. A graduate of Georgia State University, he has successfully completed all areas of the Better Process Control School at the University of Minnesota, and has been qualified by the International HACCP Alliance as an instructor. He can be reached via e-mail at bfloyd7192@aol.com.


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