Product recalls, illness outbreaks, regulatory requirements to validate and verify process control measures and customer requirements have placed the question of how to best deploy microbial testing strategies to improve product safety. However, microbial testing is only one tool in our arsenal and before undertaking a testing program of any type, it is important to understand your objectives and exactly what microbial testing can and cannot do. To address how best to use microbial testing, PMA hosted the webinar “What Microbial Testing Can and Can’t Do for You”.
The webinar was moderated by PMA Chief Science & Technology Officer Dr. Bob Whitaker with presentations from PMA Vice President of Food Safety & Technology Dr. Jim Gorny and University of California, Davis, Extension Research Specialist Dr. Trevor Suslow.
The key learnings revealed during the webinar were:
- Microbial testing is currently employed to verify environmental and equipment sanitation practices, determine microbial quality of inputs like irrigation water or compost, monitor wash water quality, screen raw products prior to harvest and test finished products prior to shipment. This last use of testing finished products for pathogens has emerged in light of recent outbreaks as a condition of sale to protect against product liability for some customers. This permits them to message to consumers that they have tested the product so as to assure its safety. Microbial testing has evolved over the last several years from “traditional” microbial testing that is slow and laborious to newer immune-assay techniques, DNA-based methods using polymerase chain reaction (PCR) technology and ultimately whole genome sequencing. At each step in the evolution, costs for doing the tests have decreased and the speed to get a result has increased.
- It is important to understand the selectivity and sensitivity of any microbial test that might be employed, i.e. how effective is the test in identifying only the specific target organism and not close genetic relatives and at what concentration or level will it find that target? This knowledge is critical to avoid testing samples and getting a positive sample and making a decision to discard the product or re-sanitize a facility only to find that the result was a “false”-positive and the extra expense was not necessary. Similarly, it is also important to avoid the opposite scenario where a sample tests “negative” but might be later found to have really been “positive” and affected products are in the market and might impact public health. When selecting test methods, be sure that the test has been validated to find the target organism in the commodity you produce. Fruits and vegetables naturally contain a numerous chemical constituents and diverse microbial populations that can interfere with some tests and both false positives and false negatives are possible.
- There are service laboratories available that can assist producers in setting up various types of testing programs. However, be sure that the lab is certified to perform the tests they use and that the tests have been validated for whatever you are about to analyze for and the food or material you are about to test. Specifically, ask questions about whether the test has been validated for your commodity and examine the data. Cost is an important consideration, so work to match costs with need and objectives. In-house testing programs are not common in the produce industry owing to the specific skills required, capital expense of setting up a lab and concerns over proximity of testing activities and food production.
- One aspect of detection of pathogens in environmental, input or product samples is “enrichment”. In most instances, the level of pathogen in a sample is very low and the organisms might exist in a metabolically slowed state due to the hostile environments often found in produce production. These low level contaminations make it difficult to isolate DNA so most tests have an enrichment step where the sample is placed in a liquid solution high in nutrients and incubated at ideal temperatures to help it recover and grow. After several hours the organism grows out and can be harvested and enough DNA extracted to perform the test. If the enrichment step is not long enough to permit recovery and growth, then the presence of a human pathogen could be missed and a false negative result obtained. While tempting to shorten enrichment steps to meet demands for quick results so that decisions can be made, failure to employ sufficient enrichment can result in loss of sensitivity. The webinar provided an example where 18 hour enrichment permits 100 percent detection of Salmonella spp in lettuce under the conditions used.
- Equally as important as the test is the sampling program; how many samples do you take and where do you take them from? Sampling is really the limiting factor in most product testing programs and must be carefully evaluated in facility and equipment sanitation verification applications. For product testing, sampling really comes down to how many samples must be taken to find a contaminate that is believed to be both infrequent and if present, only there in very low concentrations. An example was given in the webinar for a contaminate present at the level of 0.5 percent or more visually, 1 red ball in a bucket with 199 white balls. If you blindly withdrew 30 balls and “tested” them to find the red ball, you would have an 86 percent chance of not finding the red ball. That is why product testing is often described as finding a needle in a haystack. Alternatively, if a catastrophic contamination were to occur so that the distribution of contamination was more uniform, product testing could be an effective tool. An example was given describing the recent Blue Bell ice cream Lm foodborne illness outbreak where had product testing been employed, the contamination may have been detected as over 90 percent of the samples tested by FDA, tested positive for Lm.
- Defining “lots” becomes important when embarking on setting up product testing programs. Producers need to determine up front how to define lots so that is a positive samples within a lot, actions can be taken to remove that lot from distribution. One must also ask the question of what to do with adjacent lots if they might have been exposed to a similar production environment in a field or if a positive lot might have been processed across the same line as other products. In the process or packing environment, the industry is adopting processing “breaks” to provide time to sanitize the equipment when production lots are changed during the day to better manage cross contamination issues.
Compositing can be used to control testing costs. With compositing, samples are pooled together and tested. The bulk of the cost for testing is in the test kit so if multiple samples can be tested with one kit, cost of the kit can be spread out over those samples. This might be a useful strategy for environmental samples or input samples. Of course, if a positive is uncovered, then a second round of testing is generally needed to pinpoint where the in the end, microbial testing can be a valuable tool but it is not a food safety program. You cannot test your way to food safety and testing cannot replace a risk and science-based food safety program. Each testing opportunity must be evaluated, objectives set, methods selected and ramifications to operations and the costs that add weighed against benefits to product safety. There are still significant limitations to sampling programs that reduce the value of raw and finished product testing so difficult decisions need to be made as to where the best “spend” needs to be made to improve safety, e.g. training, wash water control, sanitation, etc.
Watch the full webinar below:
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