Top Italian olive oil producers are under investigation for allegedly passing off lower-quality products as “extra virgin”, raising fresh concerns about allegations of consumer fraud in the industry.

Turin police are examining whether seven companies – Carapelli, Bertolli, Santa Sabina, Coricelli, Sasso, Primadonna, and Antica Badia – have been selling virgin olive oil as 100% extra virgin.

According to allegations in Italian press reports, an analysis of samples from all seven brands found that they did not meet EU labelling rules for extra virgin olive oil.

One of the companies under investigation, Coricelli, said the inquiry was based entirely on taste tests by professional tasters that could not reliably discern whether the oil met industry standards.

“The protested batch, before being sold on the market, had been carefully analysed either by the company or independently recognised laboratories and all the analysis confirmed compliance to the quality standards,” Coricelli said.

The company promised to present the prosecutor investigating the case with counter-analysis to confirm that its behaviour was “fair and correct”. The other companies under investigation did not respond to requests for comment.

Extra virgin is the highest quality olive oil, and consequently the most expensive. It comes from the first press of healthy olives, and has a maximum acidity of 0.8%. Virgin olive oil, which is hard to find in a supermarket, is of a slightly lower quality, with a more acidic flavour, while plain olive oil is refined and often treated with chemicals and deodorised to mask the taste of rancid, spoilt oil.

It is not the first time that Italian olive oil has come under scrutiny. In 2012, Italian investigators looked into claims that some of the largest producers had adulterated domestic oil with less expensive imports from Spain, Greece, Morocco and Tunisia. There was also a case in which a major olive oil dealer, Domenico Ribatti, was jailed for trying to pass off Turkish hazelnut oil as olive oil.

The business of trying to pass off adulterated or misbranded oil and calling it “extra virgin” was chronicled in depth by the author Tom Mueller, who in his 2011 book, Extra Virginity, described how the product that is seen as key to healthy living and the Mediterranean diet, has long been a favourite object of fraudsters who take advantage of consumers’ ignorance about what extra virgin olive oil ought to taste like.

“Olives are stone fruits, so extra virgin olive oil is like fresh squeezed fruit juice. It should be fruity, pungent, and bitter. Those are the indications of freshness, quality and health,” Mueller said.

The investigation unveiled this week, he added, reminded him of an old Italian expression: it was as if investigators had discovered hot water. “Anyone who knows anything about the industry knows there is a lot of this going on,” he alleged.

At the centre of the problem lies the immense market power of the sellers and the inaction of the Italian ministry of agriculture, Mueller said, despite allegations of problems in the industry that have made news for a decade or more.

The issue is a difficult one for consumers to navigate. Extra virgin olive oil is a major component of a healthy Mediterranean diet, and Mueller has a few tips for consumers, including looking for bottles with a best by date to ensure freshness.

He also recommends that consumers seek out a precise point of production on the label (not just an Italian flag) when they are buying extra virgin olive oil, and that they try to visit a vendor that performs stringent quality control on the oils it sells.

Also, choosing bottled oil that is sold in a dark glass in a quantity that you will quickly use is a good indicator, since high-quality oil can rapidly go bad after it is opened and stored in a hot or bright area.