As readers of Food Quality & Safety well know, there has been an ever-increasing interest in recent years regarding testing for common food pathogens such as E. coli, Listeria, and Salmonella in commercial foods. Publicity around food disease outbreaks has increased in the media, which in turn has sparked an increased interest in governmental regulation around food safety issues, including the passage of the Food Safety Modernization Act in 2011. More tests are now being required than in the past and testing for food pathogens has become a global concern.
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Explore this issueApril/May 2018
Traditional Pathogen Testing
Testing for pathogens in a food testing lab is a multi-step process and preparing the enrichment media constitutes much of the activity. Figure 1 illustrates the typical process flow in the test cycle. After a food sample is received for testing, it is documented for recordkeeping purposes, and weighed. Prior to use, enrichment media is prepared and autoclaved for sterilization purposes, and must pass QC checks, which can take several days. The test sample is added to the QC approved enrichment media, and incubated for a period of time, depending on the test method. Finally, the test sample is analyzed, and the test results are recorded and released to the customer. The test method used determines how the sample is analyzed. For example, testing for common food pathogens is typically not a quantitative (or numeric) test, but rather a simple qualitative (i.e., pass/fail) test.
Testing for food pathogens using this traditional method is highly manual and time-consuming, and fraught with problems in the testing process. First, there are numerous opportunities for human error that can affect test results. For instance, incorrect data may be recorded about the test sample, both pre- and post-test. There may also be inconsistencies in the volumes of media prepared for testing, which can have an impact on test results. Evaporation during the sterilization (autoclave) step is a very common problem and can cause measurement uncertainties in the test results. Next, there are a number of safety concerns in the testing process, particularly around enrichment media preparation as it is traditionally done. Autoclaves are used repeatedly, both to heat the enrichment media and to sterilize test containers, leaving various opportunities for contact burns from the autoclave itself or from glassware/containers. Large volumes of heated enrichment media, and the transport of same, also brings the potential for burns caused by contact with hot fluids. Finally, the post-enrichment incubation times may be long due to the time required to bring samples up to test temperature. With many standard testing methods, the enrichment media needs to be at the target test temperature, and using standard media preparation practices, each media container of approximately 3375 milliliter (mL) will need to be pre-warmed prior to use. These media containers are typically placed in an incubator or other heating source in order to do this. Heating large volumes of enrichment media takes time and failing to have the enrichment media at the proper test temperature will obviously influence the test results.
Increasing Efficiency of Media Preparation
Automating the media preparation process in foods pathogen testing can alleviate many of the problems described above. Most notably, the throughput of test volume may be dramatically increased if QC-approved concentrated sterile enrichment media is added to test containers holding pre-heated and sterilized water prepared by an automated media preparator, which brings the enrichment media up to the final test volume. Tables 1 and 2 show an example of how the use of concentrated enrichment media in this manner can allow for up to an 85 percent reduction in the amount of enrichment media that would need to be autoclaved, allowing for far greater throughputs. Dramatically decreased labor and energy costs result from processes that are more efficient, and which require significantly less autoclave time.