Nor do national food authorities appear particularly bothered as long as the oil isn't actively harmful, which is rare.

Meanwhile, the chemical tests that should by law be performed by exporters of extra virgin oil before it can be labelled and sold as such can often fail to detect adulterated oil, particularly when it has been mixed with products such as deodorised, lower-grade olive oil in a sophisticated modern refinery. Almost any brand can, in theory, be susceptible: major names such as Bertolli (then owned by Unilever) have found themselves in court having to argue, successfully in this instance, that they had themselves been defrauded by their supplier.

The resulting blend is then chemically coloured, flavoured and deodorised, and sold as extra-virgin to a producer. Sometimes, another vegetable oil such as colza or canola is used. Most commonly, it seems, extra virgin oil is mixed with a lower grade olive oil, often not from the same country. The problem, he says, is that good olive oil is difficult, time-consuming and expensive to make, but easy, quick and cheap to doctor. In Extra Virginity: the Sublime and Scandalous World of Olive Oil, Mueller, an American who lives in Italy, lays bare the workings of an industry prey, he argues, to hi-tech, industrial-scale fraud. The answer, according to Tom Mueller in a book out this month, is very often not. So with a litre of supermarket extra virgin costing up to £4, and connoisseurs willing to pay 10 times that sum for a far smaller bottle of seasonal, first cold stone pressed, single estate, artisan-milled oil from Italy or Greece, can we be sure of getting what we're paying for? We're now, in fact, the world's 10th biggest olive oil-consuming nation. Today, thanks partly to the health claims made on its behalf and partly to the fact it tastes good, the oil Homer called 'liquid gold' is in half of all UK homes and we get through 30m litres of olive oil every year – more than double than we did decade ago. In Britain, of course, it wasn't so very long ago that the most likely place to find olive oil was the chemist. Most alarmingly, a study last year by researchers at the University of California, Davis and the Australian Oils Research Laboratory concluded that as much as 69% of imported European olive oil (and a far smaller proportion of native Californian) sold as extra virgin in the delicatessens and grocery stores on the US west coast wasn't what it claimed to be.

Extra virginity book