Pretty bottles of olive oil make great holiday gifts, but what most of us who delight in drizzles of the golden liquid don’t realize is what’s in your bottle might not be what you expect—or want. And we’re not talkin’ fat.

The United States recently became the third largest olive-oil consuming nation in the world, and just overtook Greece -- a country where the people of Crete alone consume 26 gallons of olive oil per person per year, says Tom Mueller, author of the investigative olive oil missive five years in the making Extra Virginity: The Sublime and Scandalous World of Olive Oil (W.W. Norton, Dec. 2011).

That’s a whole lot of olive oil we’re taking in here, as we all search for the magic elixir to stretch out the years—or, at least, improve the ones we have.

“The chemistry of the Mediterranean diet is just starting to be understood by Harvard’s School of Health. The chemicals found in real extra virgin olive oil are full of natural antioxidants and anti-inflammatories, and a whole range of vitamins that science is just starting to put names to; for thousands of years it’s why people [in certain countries] have lived longer lives,” says Muller.

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    There 700 different kinds of olives in the world, all with radically different taste profiles, from mild and gently fruity versions like an arbequina, to more robust and pungent types like taggiasca.

    But as the old adage goes: you get what you pay for.

    “When producers try to make olive oil at a low cost with a high profit, everybody loses,” says Mueller. “People expect to get healthy stuff, and in fact they might be getting rancid, old oil. It’s not only not good—it’s bad.”

    Two recent studies by the University of California, Davis, part of the Robert Mondavi Institute for Wine and Food Science, put your average, everyday supermarket olive oils to the test:

    “Our laboratory tests found that samples of imported olive oil labeled as ‘extra virgin’ and sold at retail locations in California often did not meet international and U.S. standards,” says the initial July 2010 report. “Sensory tests showed that these failed samples had defective flavors such as rancid, fusty, and musty. Negative sensory results were confirmed by chemical data in 86 percent of the cases.” That’s a whole lot of bad oil.

    And not only did they find much of it past its prime, but also that bottles labeled as extra virgin olive oil were adulterated with cheap, refined non-olive oils.

    “Some producers will deodorize their oil--run it through a chemical and heat refining process that kills bad smells and flavors, and pretty much a lot of its remaining health benefits. Others take an inert liquid fat and juice it up with a little of the good stuff to give it flavor, then sell at a lower cost as extra virgin olive oil.”

    So what can an olive-oil loving consumer do? Quite a few things. To find the best olive oil out there (and keep it in good shape), follow these few, simple rules:

    Savor, Don’t Save! Olives are a fruit, and, if you think about it, olive oil is a kind of fruit juice. You wouldn’t keep orange juice for years on end, right? Treat olive oil the same way. If you’ve been saving a fancy bottle for a special occasion, don’t..  Olive oil doesn’t have a shelf life past the year mark. Consuming it within 6 months of purchase is an even better plan.

    Keep It Dark. Like wine, olive oil can be ruined by sunlight and heat. Store in a dark, cool place—never leave it next to the heat of your stove, no matter how convenient. Make sure your bottle is always well-sealed as too much air can oxidize all that gorgeous golden liquid. And while you’re at it, avoid clear of glass bottles. They might make for nice presentation, but dark glass is a far better protector.

    Dust is a Bust. Shop for your olive oil in a reputable store that moves merchandise quickly. If there’s dust on that bottle, chances are it’s past its prime.

    It’s a Date. The best olive oils will have a perishable date on the bottle, but more important is a harvest date. This tells you when the fruit was picked, and is the best bit of proof for how fresh what’s in your bottle actually is.

    Hunting Season. Olives are grown not just in parts of Spain, France, Portugal, California, and Italy, but also in places like Australia, New Zealand, Chile, Argentina, and South Africa. “Their spring is our fall,” offers Mueller, “So when their oil comes online, it’s just about when olive oils from the northern hemisphere are 6 months old and losing their pizazz.” Keep an eye out for the country of origin and buy from place where the season is ripe.

    Eyes on the Prize. Look for a seal from a credible organization, California’s Olive Oil Council or Italy’s D.O.P., which translates to mean Protected Designation of Origin—meaning the olives used come from the place the bottle says, and nowhere else. And if a producer has a sticker on their bottle boasting about a recent prize won for great oil, that’s not a bad sign, either.

    Sweet and Low. “Generally speaking, good extra virgin oil has a high smoke point where the oil burns,” says Mueller. “Its’ a waste of good first-rate oil to fry. Not only will it start tasting like popcorn, but bad things start to happen chemically." Instead, stick to light sautéing when cooking with extra virgin olive oils, and using for dressings, drizzles, emulsifiers, and just plain flavor enhancing.

    “My favorite thing is to take a plain baked potato, split it, crush it with fork, and pour liberal amounts of good oil on it. I’ve never experienced a potato like it!” Other favorites? Drizzling a big, pungent olive oil on steak and even over vanilla ice cream. “It sounds crazy, but it brings out textures and flavors you never noticed before. It’s like liquid sunshine,” he says.

    And finally, Mueller urges that consumers think less about counting calories and more about healthy perks. “It may be 120 calories per tablespoon, but it packs a huge health wallop. Olive oil is a fabulously good thing to eat. High starch and high sugar diets do such damage to our bodies,” he offers, “But olive oil? It’s a healthy fat. It’s liquid health!”