Where did the horse and pig DNA in beef burgers come from?

OK, we've seen the jokes on Twitter (My Lidl Pony is my favourite) but the discovery of DNA from horses and pigs in beef burgers and beef products does raise a serious issue.

And the first question that needs answering is; where did it come from? Let's look at the facts.

• Fact 1:

Most of the horse and pig DNA found in burgers by the Irish Food Safety Authority was present in very small quantities.

Only one burger they tested (from Tesco) contained significant amounts. If horsemeat or pork had been added deliberately, the adulteration would have been at much higher levels -otherwise there is no point in doing it.

• Fact 2:

Contaminated burgers were made at three separate processing plants (and some contained imported ingredients from Spain and Holland). Again, that suggests deliberate adulteration is less likely - would they all be at it?

• Fact 3:

Horsemeat is not processed at the three plants concerned. (Two in Ireland, one in the UK).

So it is difficult to see how contamination (for example, because machines were not cleaned properly between batches of different meat) could have arisen. But that argument might not apply to the two continental suppliers since horsemeat is eaten widely across Europe.

• Click here for the full list of beef burger results.

Both the Irish and UK food watchdogs are investigating the incident and let's hope they get to the bottom of it.

But my best guess is that the horse and pig DNA came from contamination (the pig DNA was much more common) rather than adulteration. We don't know where that might have happened.

The issue involved is this: Why didn't five big supermarket chains spot that they were selling contaminated burgers? And why didn't the UK's own food watchdog detect it either?