by Zoë Martin, Society of Biology
In light of the horsemeat scandal I was interested in understanding a bit more about how we determine what species are actually populating the products on supermarket shelves.
Testing facility websites reveal that the food industry mainly relies on two techniques to detect horse meat in beef: the protein-based ELISA (enzyme-linked immunosorbent assay) method or the DNA-based PCR (polymerase chain reaction) method. In addition to testing for horsemeat, laboratories are also testing for the veterinary drug Phenylbutazone or bute.
Two of the testing facilities, Reading Scientific Services Ltd and Eurofins based in Germany, explain that the ELISA technique may not be appropriate for detecting contaminated meat as it has a detection limit of 1-2%. The detection limit for PCR techniques is verifiable at 1% but it is possible DNA can be detected at much lower levels.
PCR was discovered by Kary Mullis in 1983 and two forms of the technique can be used to test meat:
- Real-time PCR involves the use of short lengths of DNA that are only found in a horse. These lengths of DNA can pair up with the complementary horse DNA in the sample. The DNA is then amplified and the copied DNA can be counted in real-time.
- Restriction fragment length polymorphism (RFLP) PCR involves replicating the specific animal genes within the sample multiple times. Then restriction enzymes can cut the copied DNA at particular points that will produce fragments that are specific to horse DNA. These fragments are then separated out by gel electrophoresis and the pattern that is produced can be compared to a control sample of the horse DNA.
Although these techniques highlight how genetic techniques have developed since the structure of DNA was discovered 60 years ago, a press release from Reading Scientific Services Ltd explained that there is no definitive way to determine the quantity of horse DNA in each sample. This is because the methods rely on replicating mitochondrial DNA and there is no way to verify how much mitochondrial DNA is found in any given cell (it can vary from 1 – 10,000 per cell). This is certainly something to consider when reading any figures reported in the media.
The horsemeat scandal is a stark reminder of the increasing complexity of our food supply and, until we all own a DNA analysis toolkit, vegetarianism is looking more and more appealing.