Caitlin Smith has a B.A. in biology from Reed College, a Ph.D. in neuroscience from Yale University, and completed postdoctoral work at the Vollum Institute.
Whether prepared in a restaurant or sold in grocery stores, consumers want to know their food is safe. That’s the job of food-testing laboratories, which test food generally for both pathogens, such as bacteria, and chemicals and small molecules, such as antibiotics and pesticides.
Michael Clark, market development manager in the food-science division at Bio-Rad Laboratories, says his company’s food-testing customers are split evenly between contract labs, food manufacturers and government and university labs. The latter include labs operated by the U.S. Food and Drug Administration, and universities with food-science programs.
Generally, customers decide what to test for based on the “at risk” compounds, or those most likely for a given food type. Often this coincides with compounds that are regulated for a particular food. For example, says Lauryn Bailey, global marketing manager for food and environmental markets at AB SCIEX, “with fruits and vegetables, people would test for pesticides but not antibiotics.” On the other hand, meat are tested for antibiotics, or bacteria. Clark says that poultry, for example, often are screened for contamination with Salmonella or Campylobacter.
Testing for chemicals and small molecules
Mass spectrometry (MS) typically is used to identify chemical contaminants such as pesticides, antibiotics, hormones, dyes, illegal compounds and chemical adulterants (prohibited chemicals added to foods, as well as bisphenol A, or BPA).
Bailey says two types of MS generally are used for food testing. Targeted screening, or “nominal” testing, scans samples for one or a panel of known compounds, such as a list of the most likely contaminants for that particular food type. Nontargeted screening uses MS to search for unknown contaminants in a food sample, which is useful when you don’t know what you’re looking for.
Targeted testing can be accomplished using relatively simple, low-mass accuracy and low-resolution mass spectrometers. Nontargeted screening requires higher resolution (and usually more expensive) hardware, which is essential for detecting low levels of contamination.
“Food is complicated,” says Bailey. “Because of the complexity involved, data processing and software are key.” The software picks out the expected components in the food sample, thereby enabling users to focus on the unexpected—and possibly contaminating—components. “First you identify the normal or expected molecules and remove them from the analysis,” she says. “Then you can focus on the abnormal molecules.”
Usually, higher-resolution MS instruments are impractical for routine testing or surveillance, says Bailey, and most food-testing labs are only just starting to use them. Nevertheless, although higher-res MS instruments are pricier, AB SCIEX is seeing the biggest sales uptick in higher-end models such as its QTRAP® 6500. “Its greater sensitivity saves lab workers time and labor with reduced sample preparation,” Bailey says.
Other mass spectrometers available for food-testing include Thermo Scientific’s Q Exactive™ Focus, a hybrid quadrupole-Orbitrap system that can scan at speeds high enough for high-throughput screening. Waters also offers a specific Pesticide Screening Application Solution (consisting of high-resolution liquid chromatography, followed by mass spectrometry on its Xevo® G2-S QTof system) with UNIFI™, a scientific information system. Bruker’s PesticideScreener™ is a turnkey, MS-based system coupled with Bruker’s TargetAnalysis screening software.
Testing for pathogens
Pathogen testing in food mainly involves looking for likely or suspected forms of bacterial contamination by pathogens such as E. coli, Salmonella, Listeria, Campylobacter and Staphylococcus. These tests fall into two categories. One is plating food samples on agar plates and watching for the growth of bacterial colonies, which may take several days. This process can be accelerated using chromogenic agar and proper enrichment procedures—tools offered in kits such as Bio-Rad’s rapid chromogenic-agar assays.
Pathogen tests using real-time, quantitative PCR (qPCR) provide faster food analyses (next-day test results) and can be adapted for automation. Examples include Bio-Rad’s iQ-Check® Real-Time PCR Kits as well as qPCR-based tests for pathogen detection from Thermo Scientific.
Choosing whether to use a plate or PCR test depends on several factors, according to Clark. If speed is paramount, qPCR is the obvious choice. But sample amounts are important to consider, too. “If you have a relatively clean product and a lot of it, you might want to plate that, because it could be cheaper,” he says. But sometimes it’s advantageous to use a combination of both tests, starting with the plate assays. “Then, instead of running a hundred PCR tests, you can just pick the one questionable colony off the agar plate that you want to confirm and run that with the PCR test.” The type of food also can influence which test to choose (see below).
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Whichever assay type you use, however, sample preparation is largely the same: The food sample is homogenized with buffers included in the assay and then incubated as long as overnight at an appropriate temperature for the particular food type. During the enrichment incubation, the target pathogen grows. However, “different pathogens usually need different enrichment media, incubation times, nutrients and temperatures to grow,” says Clark. So it isn’t usually possible to run multiple tests on one prepared sample.
Main challenges
A major challenge in food testing is preparing the sample such that “matrix interference” is minimized. Matrix refers to molecules in the sample that are not targets and are present in such abundance that they drown out (or suppress) the weaker signals users are interested in (such as low-level contaminants).
Higher-resolution MS systems can help with this by removing some matrix material during electrospray, in which sample molecules are ionized prior to separation. Bailey says AB SCIEX’s QTRAP 4500 MS system is popular for food-testing applications because, even though it is less sensitive than the QTRAP 6500, it is “rugged enough to handle repeated injections and less susceptible to matrix interference.”
The intrinsic properties of the food create another challenge. For example, when testing dairy products for contamination, the plentiful casein and whey proteins can interfere with the test by “swamping” the target signal. In addition, “spices are naturally inhibitory for pathogen proliferation,” says Clark. Some substances, such as cinnamon, are inhibitory to PCR.
Background bacteria also can be a problem. “The natural flora of food can get in the way of pathogen detection, because all of the bacteria are competing for nutrients,” says Clark. “They can make the target bacteria die out, or overgrow and mask them.” One solution is to add an additional prep step in which a fraction of the enriched sample is regrown into another sample, which knocks down the background bacteria, says Clark. Using PCR rather than the plating assay can lessen this complication.
Be advised, though, that food testing for contamination isn’t necessarily quantitative, and in the case of plate and qPCR tests, it is not. “Usually people don’t care how much is in there,” says Clark. “When it comes to food contamination, there is zero tolerance.”
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