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Issues: Food Safety

Microorganisms and Lab Testing for Produce

If produce contacts fecal waste, harmful microorganisms can contaminate produce and survive for many weeks. Well-known microorganisms such as E. coli O157:H7, Salmonella, Shigella, and Cyclospora are fecal contaminants that have been associated with produce-borne outbreaks.

To prevent bacterial contamination in the first place, the produce industry has developed Good Agricultural Practices and Hazard Analysis Critical Control Point principles. Safe handling is also critical.

Lab Tests for Microorganisms

Aerobic plate counts (APCs) estimate the total number of microorganisms in produce. E. coli counts can indicate the presence of fecal contamination. This bacterium is commonly present in feces and its occurrence in foods suggests that feces may be present.

Coliform and fecal coliform assays were originally developed to indicate the presence of E. coli and consequently fecal contamination. However, for produce, these assays are questionable indicators of fecal contamination because they detect many bacteria other than E. coli that do not originate from feces but are commonly present on produce.

Assays also can be done on specific pathogens such as Salmonella, E. coli O157:H7, and Listeria monocytogenes. These methods generally can detect 1 pathogen per 25 grams of produce.

Tests on Other Foods

The methods used for testing produce are generally the same as those for meat and dairy but the interpretation of results can differ depending on how products are processed. For example, if meat or milk is heated sufficiently to kill pathogens, coliforms should also be killed. Hence, coliform counts of pasteurized products should be very low, whereas they may be high on raw produce.

Tests on Processed Foods

Many harmful bacteria can grow and survive for long periods on exposed interior contents and juices of fruits and vegetables. Hence, special measures must be taken to avoid contamination by harmful microorganisms during fresh-cut produce preparation, and proper refrigeration is essential during storage of such products.

What Tests to Use for Produce

Usually harmful microorganisms are not uniformly distributed on foods such as produce, so the only testing approach to ensure the absence of harmful microorganisms is to test the entire product. This is neither practical nor possible; hence, a statistically designed sampling protocol is needed to provide meaningful test results.

Generally, microbial tests for fresh produce have limited value unless the produce is cut or further processed. APCs can be useful in identifying spoilage. Testing for E. coli can be helpful in identifying potential fecal contamination. Tests for specific pathogens such as Salmonella, Listeria monocytogenes, and E. coli O157:H7 may be beneficial in identifying microbial hazards.

Coliforms, Fecal Coliforms, and E. coli

E. coli is carried in the intestinal tract of most humans and mammals, and is therefore present in feces. Hence, detecting E. coli in foods can be evidence that fecal contamination has occurred.

E. coli is one of many bacteria detected in the coliform test which is easy to perform and requires only one to two days to complete. The coliform assay was developed to serve as a rapid test to detect E. coli and, hence, fecal contamination of food. Unfortunately, many other bacteria besides E. coli are detected by the coliform test, so this assay often yields false-positive results that do not truly detect fecal contamination.

To increase the specificity of the assay, microbiologists developed the fecal coliform test that uses a higher temperature of incubation and a slightly more selective medium to culture bacteria present in the intestinal tract. Although this assay is more selective for E. coli than the coliform test, several bacteria that may not be found in the intestinal tract are also detected by the fecal coliform assay. Hence, the fecal coliform test is not always a true indicator of fecal contamination.

Interpreting Test Results

Results should be interpreted by trained microbiologists who are knowledgeable of microbiological testing. The significance of microbiological tests can be easily misinterpreted by those outside the field of microbiology. For example, neither the coliform assay nor the fecal coliform assay is a true indicator of fecal contamination of fresh fruits and vegetables.

Frequently, bacteria that are commonly present on vegetables and vegetation, such as Klebsiella and Enterobacter, do not originate from feces but are detected by both the coliform and fecal coliform assays.

The only test that is meaningful as an indicator of possible fecal contamination is the E. coli assay. If the fecal coliform assay is used to test produce, then additional tests must be performed to verify that E. coli is present.

High levels of E. coli could be evidence of fecal contamination or gross unsanitary processing practices. Detection of pathogens such as Salmonella, E. coli O157:H7, Shigella, or Campylobacter jejuni in fresh produce that will not be cooked or given a pasteurization treatment before eating is a serious concern.

Detecting a few hundred coliforms or fecal coliforms (not E. coli) per gram on fresh produce is not a significant concern. Detecting thousands of bacteria per gram based on aerobic microbial counts is not evidence of a food safety hazard; however, APCs of 10 million or more microorganisms per gram can be evidence of microbial spoilage.

Taking Action from Test Results

Results by themselves do not indicate what, if any, actions are required. The action one takes depends on the test in question and the significance of the results. This assumes that the test is appropriate for the product being analyzed and the results are interpreted correctly. In many cases, inappropriate tests are conducted and, consequently, results are often of little or no value. A good food microbiologist can be valuable in suggesting appropriate tests and helping to interpret results.

Routine microbiological tests, such as the APC, are usually done to provide a general picture of overall microbiological quality. However, results often vary widely and normally have little or no relationship to safety. In such cases, two possible actions are usually recommended:

• If the results are typical for a particular product and processing facility, no action is required aside from retaining the results as part of normal record-keeping procedures. Such information can provide valuable historical data for the company.
• If the results differ greatly from what is normally observed, quality control personnel should try to determine what caused the change and, if the change is undesirable, take steps to remedy the problem. For example, an APC of 1 million per gram of product that typically yields 10,000 per gram suggests that something has happened to adversely affect microbiological quality. The source could be product obtained from suppliers, problems in cleaning and sanitation, or even poorly conducted sampling or analytical procedures. The results themselves do not necessarily indicate a problem serious enough to warrant drastic action. They do, however, indicate that a problem exists that could adversely affect shelf life and consumer acceptance if allowed to continue.

In the case of pathogen testing, positive results are usually more serious. Lots of samples that test positive for a particular pathogen should immediately be retained for retesting. If the presence of the pathogen in the product is confirmed, the contaminated lot should be either destroyed so that it does not enter commerce or should be further processed and given a treatment such as pasteurization that will kill harmful microorganisms.

Quality control personnel should attempt to determine the source of the contamination. Although frequently difficult, determining the source may prove invaluable in preventing future problems.

If a contaminated lot of product has already been shipped, an immediate recall should be issued. Depending on the stage of transit, regulatory authorities may also need to be notified.

How Often to Test

Testing is most effective and efficient when it is done as part of a well-designed HACCP or GAP system. Sampling plans should be designed based on volume of product, previous microbiological history, and potential for contamination at various stages of processing.

As a general rule, one should plan on testing produce contact surfaces for proper cleaning and sanitation at least daily and general environmental testing at least weekly. Microbiological sampling of the product itself should be based on a statistically designed sampling plan. Usually this will involve taking representative samples from each production lot.

Because testing can be very expensive, companies should strive to conduct the minimum testing necessary to ensure the highest degree of quality and safety. Unfortunately, no one answer can be given to accommodate all situations.

Standards and Inspections

At present, no microbiological standards exist for fresh produce. Because the microflora of fresh produce varies widely, it is unlikely that a set of standards could be developed. However, companies may wish to develop their own internal standards based on typical microbiological profiles of their particular products.

Some non-produce foods are subject to regulatory standards. For example, pasteurized fluid milk must contain no pathogens, not more than 10 coliforms per milliliter, and total plate counts of not more than 20,000 per milliliter.

The responsibility for inspecting produce during production, processing, and distribution rests on the industry itself. However, both federal and state regulatory agencies often conduct random testing of products. In addition, the U.S. Food and Drug Administration conducts random inspections of imported fresh produce.

Finding a Lab

Testing can be done by government, university, and private laboratories. However, each organization has a different mission, and the testing they do will usually reflect that mission.

Government laboratories usually focus on assuring public safety. Hence, they will most often analyze products for private industry when there is a threat of foodborne illness. It should be kept in mind that information discovered by government agencies is usually in the public domain and may be used against the company requesting the analysis.

The mission of most universities is to produce and disseminate information that will benefit the general public. Many universities have outreach or extension programs for this particular purpose. The most well-known of these programs is the Cooperative Extension Service found in most colleges of agriculture. The Cooperative Extension Service can provide limited information, training, and sometimes laboratory analysis on agricultural products.

Most universities have neither the funds nor the personnel to conduct routine analysis normally required of produce companies. Rather, they cooperate with private industry by helping the company solve unusual problems or in conducting focused, privately funded research. The Center for Food Safety and Quality Enhancement at the University of Georgia is an example of a program in which private industry and university faculty collaborate on research related to food safety and quality.

In most cases, produce companies will need to enlist the services of private consulting laboratories for routine analyses. Private laboratories vary greatly in the type of services they provide, their technical expertise, and ability to interpret results. Produce companies should consider the following in selecting a private laboratory:

• How much experience does the laboratory have in testing produce? With what types of testing and products have they been involved? Ask for, and check, references. A reputable lab should be able to furnish a list of other companies who have used their services.
• How does the lab determine its own quality control? A good lab will usually have standard operating procedures to check the accuracy its procedures and results.
• What is the background and training of its personnel? Lab personnel interpreting results should have training or equivalent experience in food microbiology. For example, training in clinical or medical microbiology will rarely provide the background required to correctly assess results with foods.
• What follow up services do they provide? Laboratories that provide follow up services such as training and HACCP can often be of significant benefit to your company.

For More Information

Good sources of information and expertise include the Cooperative Extension Service in each state, the Agricultural Research Service of the U.S. Department of Agriculture, and the U.S. Food and Drug Administration.