Thursday, 17 January 2013

But How Do They Know? | Courses of Horsies

EMBERS glow, flames flicker and on go the burgers: patties of prime, premium (and lesser) cow bits squished together in a juicy, fatty disc to be wedged, burnt bits and all, between two pieces of near-stale 'bread', lashings of tomato ketchup and accompanied by a token splodge of potato salad. Ah, the joy of the summer barbecue.

Hold on. You did say 'beef' burger, right?
Retired
Source: me

This week the Food Safety Authority of Ireland (FSAI) published results of tests on beef products on sale in a number of supermarkets in the UK and Republic of Ireland, discovering traces of pork and horse meat in beef burgers and pork in other beef products. In one case, in Tesco Everyday Value Beef Burgers, the product was found to be 29.1% horse meat. The case has proved an embarrassment for the food manufacturers involved and for Tesco, Aldi, Lidl, Iceland and Dunnes Stores, and has ignited a number of debates. Is it ethical to eat horse, ask some? Why did the UK's Food Standards Agency not spot these discrepancies? What failings occurred in quality and supply chain control that allowed this to happen?

I can't answer any of those. But I can answer this: how do they know those burgers contain horse? How can you take a burger, pulverize it and identify the species it is composed of? Welcome to the world of Food Forensics.


Officially, the FSAI discovered traces of horse DNA in the burgers, a statement that, from the off, puts us on the back foot. What does that mean? DNA is a chemical, made of the same atoms wherever it is found, be that a horse, a cow or an orchid. DNA - actual, physical DNA - does not differ by species. Horse DNA is the same stuff as cow DNA, which is the same as that found in wheat, locusts and every other animal, even the ones that hee-haw. DNA is DNA. That being the case, how can the scientists know that the DNA in the burgers came from cows, pigs and horses?

The key is that while the DNA is the same in substance, the order in which its subunits, the 'nucleotide' letter links in the long DNA chain, assemble differ. The letters come together to form genes; the genes string together to form large structures called chromosomes; and multiple chromosomes form a gang called the genome and loiter in the nucleus of every cell of the body. Consider the genome to be the sum total of all of the DNA letters that make up that particular animal, the entire instruction manual to make a horse, a cow or a periwinkle. Genomes, unlike DNA, do differ between species.

It is possible to amplify parts of the genome by a method called polymerase chain reaction (PCR). First, two short 'primers' are made. These short synthetic strands of DNA match and bind to the genome at the start and finish points of the required fragment; they are the starting blocks for the ensuing polymerase reaction - that is, the genome is copied from these points. Loose DNA letters, destined to chain together as a direct copy of the genome fragment, can only join existing chains, not spontaneously pair up with the genome on their own. Thus, the primers kick start the copying process by providing the first links in the chain. Furthermore, the chemical structure of DNA means that the region between the two primers, and only this region, is copied. Why does this matter? Well if you can find primers that bind to parts of the genome that are unique to one species, they will amplify DNA only if that species is present in the sample inspected. Throw short bits of DNA at a beef burger and, if larger bits of DNA bounce back, there's a horse inside.

Nonetheless, cows, horses and pigs are all mammals, and as such are very similar, despite outward appearances. Take a specific gene from one and, chances are, the others will have it too. So we need to look much closer for a suitable primer target. One commonly used gene is cytochrome B, which makes a protein required in all cells to extract energy from sugar. Cows, pigs, chickens, horses, sheep and goats all have cytochrome B, as do we, but the DNA sequences that encode the instructions to make it are not identical. If you line the sequences up alongside each other they match pretty closely, but crucially there are regions that do not match between all of the species, and each species has a different region that is not common to the others.

I fear I may have lost you, so here's a diagram:


Here's the sequence of the cytochrome B gene from each animal, lined up on top of each other. Those open black boxes? That's where the primers go. Now all you need is to mash up some burgers, throw in some primers and enzymes and cross your fingers. But how to know how much of the DNA comes from which animal?

One way to do this is to use a more sophisticated form of PCR called quantitative real-time PCR, using fluorescent fragments of DNA that are specific to the amplified region. For example, you might make a primer for a horse-specific gene sequence that glows green as soon as it is incorporated in the PCR process. The more horse meat present, the stronger the green colour will be as more and more primers bind to the target DNA. But whereas ordinary PCR leads to DNA detection only once the procedure has finished - which would lead to a blanket green signal that could not be differentiated from other meat compositions - real-time PCR is, well, real-time, allowing the user to compare how readily primers glow green throughout the process. This can then be compared to a set of standard meat concentrations, pre-prepared, pre-analysed and pre-labelled the 'reference sausage'. The amount of glowing is proportional to the amount of that meat.

So there we have it. Glowing meat and periwinkles: this is how it is possible not only to determine what animals are found in the food on our shelves but also how much of it each is present. These same techniques are used throughout the world not just for testing the contents of our food, but for a range of species identification purposes. In 2010, these techniques uncovered whale meat on sale in restaurants in Los Angeles and Seoul, derived from a Japanese 'scientific' whaling expedition, despite a worldwide ban on whale hunting and trade in their products forbidden by the Convention on International Trade in Endangered Species. This is food forensics: annoying Tesco, but saving our seas.





References

Matsunaga, T. et al. A quick and simple method for the identification of meat species and meat products by PCR assay. Meat Sci. 51 143-148 (1999) doi:10.1016/S0309-1740(98)00112-0

Koppel, R., Ruf, J., Zimmerli, F. & Breitenmoser, A. Multiplex real-time PCR for the detection and quantication of DNA from beef, pork, chicken and turkey. Eur. Food Res. Technol. 227 1199-1203 (2008) doi:10.1007/s00217-008-0837-7

Koppel, R., Zimmerli, F. & Breitenmoser, A. Heptaplex real-time PCR for the identification and quantication of DNA from beef, pork, chicken, turkey, horse meat, sheep (mutton) and goat. Eur. Food Res. Technol. 230 125-133 (2009) doi:10.1007/s00217-009-1154-5

2 comments:

  1. Fortunately, the Soylent Green was perfectly free of horsemeet.

    ReplyDelete
  2. I really like this post. It's cool to understand the science behind the news story.

    ReplyDelete