Certified extra-virgin olive oils from the European Union are required to have a “best by” date on the label. But labels cannot always be trusted. When it comes to shopping for olive oil, an extra layer of caution is drizzled on.

Tom Mueller, the author of the soon-to-be-published “Extra Virginity: The Sublime and Scandalous World of Olive Oil,” writes that since the time of the Phoenicians, growers around the Mediterranean have dealt with — and in — counterfeit olive oil. And in modern times, cheaper oils are often disguised as extra-virgin olive oil.

“It’s not legal anywhere, but it happens everywhere,” Mr. Mueller said in an interview.

Four years ago, Mr. Mueller, who lives in Liguria, Italy, published a hair-raising article on black market olive oil in the New Yorker that changed the mind-set of every cook who read it.

It described a network of rogue tanker captains and unscrupulous bottlers that reaches from the Mediterranean to Mumbai, Rotterdam and Galveston, Tex., dedicated to doctoring old, inferior and fake oils. They are chemically deodorized, spiked with chlorophyll and beta-carotene to turn them greenish-gold, then labeled with meaningless terms like “cold pressed” or “100 percent pure” and sold as the finest extra virgin.

Such oil is not dangerous to eat. But it lacks the flavor and health-giving properties of true extra virgin, and is certainly not worth the high prices it commands at markets. “I could buy that oil at the Port of Oakland any day of the week for $10 a gallon,” Mr. Englehardt said.

Bulk prices for true extra-virgin oils are around $20 a gallon.

Loopholes in European Union regulations generate another category of deception, according to Mr. Mueller. For example, much of the oil produced in Spain is trucked to Italy, where it is repackaged legally as “Italian” oil.