Food Safety Testing
Allergen Testing
Most of the rapid allergen test kits are enzyme-linked immunosorbent assay or ELISA-based tests that detect and measure a specific allergen in food samples. For example, this test could be used to screen for the presence of cross-contaminants in food products due to allergen residues. The test kits compare samples against known levels of allergen by a colorimetric reaction. Results are obtained within 2 to 3 hours and they can be quantified using optical instrumentation. Also, allergen strip tests screen samples for food allergens at specific concentrations in a few minutes (10-20 min). Both the ELISA test kits and the strip tests require a minimal amount of training and equipment and are commonly used.
Commercial testing for egg (ovalbumin, ovomucoid), peanuts (Ara h1, Ara h2), wheat (gliadin), hazelnut, milk (casein, b-lactoglobulin), crustacean (tropomyosin), and soy residues are available through several manufacturers (ex: Biotrace International, Neogen Corporation, R-Biopharm, Elisa Systems, etc).
Rapid Methods for Microbial Testing
Microbial testing provides the detection and quantification of pathogens and spoilage microorganisms in a variety of products. Not only do these analyses allow processors to address food safety issues if pathogenic bacteria are detected, but they also predict the shelf life of a product when evaluating spoilage microorganisms.
Culture techniques
Culture techniques, which are commonly used in microbiology, rely on the growth of microorganisms under specific conditions (ex: time, temperature, oxygen content, pH and pressure) in liquid or on solid media containing nutrients. Culture methods using standard media require between 1 to 7 days for detecting the presence of microorganisms. Often, an enrichment step and/or a confirmation procedure is required before and after microbial detection, respectively.
Biochemical and enzymatic methods
Rapid methods for microbial testing assess the microbial population through the metabolic activity of the cells. Such techniques include chromogenic media, colorimetry, ATP (adenosine triphosphate) determination, protein detection and biochemical kits.
- Chromogenic media for culture techniques. This method relies on the detection of enzymatic activities of target microorganisms by the addition of dyes or organic compounds. Microorganisms growing in the presence of these compounds may fluoresce under UV light or produce distinctive colours. Compared to a standard media analysis, this method enables more sensitivity and rapid analysis (1) (within 24 hours). Chromogenic media to identify Listeria monocytogenes and E. coli are commercially available (ex: Bio-Rad, Weber Scientific, etc).
- Colorimetry or optical systems measure microbial growth by monitoring pH and other biochemical reactions that generate a color change as microorganisms grow and metabolize in a culture broth. Test for the detection of coliforms, E. coli, yeast, molds, lactic acid bacteria, Listeria spp., and Enterobacteriaceae are commercially available (ex: Neogen Corporation).
- ATP Bioluminescence. ATP, which is the energy source in all living cells, produces light in the presence of luciferin-luciferase, oxygen and magnesium. It is believed that the amount of light generated is related to the amount of ATP, and therefore biomass. However, ATP technique measures all organic matter on the surface, instead of only microbial contaminants (2). In other word, ATP systems could detect ATP on a food contact surface but does not identify the source. This is why this method is more used as a hygienic indicator than bacteria quantification system. ATP swaps are commercially available (ex: BioTrace Bev-Trace; Cogent MLS; Millipore Milliflex; Celsis CellScan).
- Protein detection is used to evaluate in real time (within minutes) if the cleaning operation in a food plant has been carried out to a satisfactory standard. These hygiene indicators can be used as a tool to help improve the sanitation performance in your plant. Results are evident with a change in color. Minimal training is required. (ex: BioTrace provides several kits for protein detection).
- Biochemical and enzymatic identification kits. These kits are capable of identifying microorganisms by combining a series of biochemical tests and a database developed by the kit's manufacturer. Each media included in the test kit system is inoculated and the color reactions of each media provide data for the organism detection. A minimum of 24 hours is required for reliable results. Test kits such as API system, Enterotube II, etc are commercially available. Enterotube II (Roche Diagnostics) was developed to identify species of the Enterobacteriaceae. Advantages of this system include speed and ease of inoculation. On the other hand, API system (bioMérieux) is available for Enterobacteriaceae, as well as, gram negative non- Enterobacteriaceae, Campylobacter spp., Lactobacillus, Staphylococci, yeast, etc. This system has an excellent data base and is easy to inoculate, however, readings and interpretation of reactions require training.
- Electrical methods, such as the use of impedance, conductance and capacitance determine the bacterial growth. This method relies on the fact that metabolites produced by microbial metabolism during growth, increase the conductivity and decrease the resistance of the media. Results could be obtained within hours. The detection time varies depending on a few variables: the temperature at which the assay is conducted, the medium, the generation time of the bacteria and the inhibition properties of the media. Systems that measure the impedance are commercially available (ex: Bactometer, RABIT for Windows", BacTrac, etc).
Antibody-based methods
Enzyme immunoassay (EIA), also known as enzyme-linked immunosorbent assay (ELISA), is a common technique for antibody-based detection of microorganisms. This method ensures sensitivity and specificity for pathogen detection. Results can be interpreted visually (qualitatively) or using an instrumental read-out (quantitatively). Test kits for the detection of Salmonella, Listeria, Campylobacter, E. coli 0157:H7, Pseudomonas, Staphylococcus aureus are commercially available (ex: BioControl Systems, Bio Trace, etc).
Nucleic acid-based methods
Compared to antibody-based methods, there are relatively few commercialized versions of nucleic acid-based methods.
- Hybridization. This method is based on the detection of target microorganisms by the identification and binding of specific DNA sequences unique to a particular microbial group. After the binding reaction occurs, detection is accomplished colorimetrically (2). Commercial test kits (GENE-TRACK®, AccuPROBE®) for the detection of Salmonella, Listeria, E. coli, Staphylococcus, Campylobacter and Yersinia are available (ex: Neogen Corporation, Gen-Probe).
- Polymerase Chain Reaction (PCR) is based on the identification of a very specific DNA sequence from a target microorganism, followed by amplification of the sequence for the detection of the microorganism (2). This technology offers a specific and reliable detection of pathogenic bacteria within 24 hours. Test kits for the detection of Listeria monocytogenes, E. coli 0157:H7, and Campylobacter jejuni are commercially available (ex: Applied BioSystems, BioControl).
Microbial Testing Methods
Official Methods for the Microbiological Analysis of Foods (Health Canada)
Toxin Testing
Mycotoxins (toxins produced by certain species of mold) such as aflatoxin, deoxynivalenol (DON), fumonisin, ochratoxin, T-2 toxin, zearalenone and histamine; as well as bacterial toxins (ex: bacillus, staphylococcus enterotoxins) can be detected by ELISA based test kits. These kits are commercially available. (e.x. R-Biopharm, Biotrace International, Neogen Corporation, Elisa Technologies).
References
- Downes, F.P. Compendium of methods for the microbiological examination of foods. 4th ed.. American Public Health Association, Washington, DC, 2001.
- Illsley, R.A., Jackson, E.D., McRae K.B., and Feirtag, J.M. 2000. A comparison of commercial ATP bioluminescence hygiene monitoring systems with standard surface monitoring techniques in a baking facility. Dairy Food Environmental Sanitation 20:522-526.
- McIntyre, D.A. 2004 Comparison of total cost, method efficiency, and laboratory productivity of selected microbiological test kits. Dairy Food Environmental Sanitation 24:398-407.
For more information, email the Food Safety and Inspection Branch or call 204-795-8418 in Winnipeg.