The olive oil in your kitchen cupboard is most likely rancid. Now, before you get all up-in-arms either defending the quality of your oil or horrified at the possibility of having used rancid oil, just relax. Most people don't even know what rancidity tastes like (or don't realize that what they think olive oil should taste like is actually the taste of rancid oil).

I have a way for you to learn the taste of rancidity. I like experiments and direct tasting of ingredients. For example, in the course of researching a future blog post, I came across several conflicting references to iodized salt being bitter or not being bitter. I happen to have in my kitchen both iodized salt (to clean my cast-iron skillet) and grey sea salt collected from the French coast near Guerande (my favorite all-purpose sea salt).

Tasting them straight and hoping to really identify differences is just silly, the taste is too strong. So, I grabbed a kitchen scale, measured out 10 grams of each, dissolved them in two separate 1/4 cups of water and tasted, and tasted, and tasted. [For the record, it was obvious that the two had different tastes, but I would never have described either as "bitter."]

Before I delve into this experiment, let me first say that rancid oil is not a health risk; it's not the equivalent of botulism or moldy spoilage. Rancidity is a defect in taste which grows stronger over time and is an instant disqualification from an oil being considered "Extra Virgin" during taste testing by the International Olive Oil Council's taste panels [in no way related to so-called "death panels"]. It is a synonym for oxidation in fats.

The experiment is simple. Take a shallow saucer, fill it with olive oil from what you may think is a non-rancid bottle. For the purposes of this experiment, it really doesn't matter if the bottle you have is already slightly rancid. Take your saucer, place it on a windowsill that receives a little sunlight. After a week, fill a second saucer and place it next to saucer A. After a second week, you now have three samples of olive oil: the original closed bottle, saucer B (out for one week), and saucer A (out for two). Taste them in that order. That taste which grows in intensity as you taste samples exposed for more time to the open air and light is rancidity. Not only is it an unpleasant taste as it gets stronger, but the presence of rancidity means that more complex and nuanced flavors that may have existed in the early life of that oil have long ago been destroyed.

The sad fact is that from the first crush, the first breaking of the protective skin of an olive, olive oil starts on its path towards noticeable rancidity. It will never get better. On average, you have 18-24 months of life for an olive oil from its creation, with serious variances in both directions.

If you've seen metal rust, then you've seen oxidation in action. The bulk of the molecules in olive oil are triglycerides, E-shaped molecules with a backbone called glycerol and three legs, most of which are called oleic acid. When oxidation occurs, these little legs break off of the E-shaped molecule and take on the new designation, "Free Fatty Acid." The percentage of free fatty acids in olive oil is one measure of the oxidation (or rancidity) of olive oil. It is one of the two chemical tests that the International Olive Oil Council performs on olive oil, in those countries who are members of the I.O.O.C. (a U.N. body of which the U.S. is not a member but most olive oil producing nations are).

There are two types of rancidity in olive oil, related in their effect upon the oil, but not in their cause. One type is called hydrolytic rancidity, and can only occur in the presence of water. The only time in the lifecycle of olive oil that hydrolytic rancidity can occur is while the olive is on the tree (i.e. birds and bugs breaking the olive skin), while the olive is being removed from the tree (i.e. violent, olive skin breaking removal methods), while the olives are being stored to be crushed (i.e. long storage times), and while the olives are being crushed and mashed (i.e. improperly long crushing time). Once the olive water and olive oil are separated, hydrolytic rancidity isn't a factor. Free fatty acids are a measure of hydrolytic rancidity.

The other type of rancidity, oxidative rancidity, occurs in the presence of oxygen, and is further stimulated by heat, light, and time. The I.O.O.C. tests for this also by testing the level of peroxides, but the true evolution of this type of rancidity will occur after the I.O.O.C. has already tested it (i.e. improper storage, pre and post bottling, by the manufacturer, distributor, retailer, and buyer).

So, if you have an olive oil that was crushed and extracted in, say, November of 2007, then you might have a few more months before it's noticeably rancid, on average. If your oil is made from Arbequiña or Taggiasca olives, they probably went rancid a few months ago. If your oil's mainly Picual, you may have one or two more years.

Are you ready for the big rub, though, the caveat to these acceptable year-long storage dates?

There's a chemical defense that olive oil (well, okay, lots of fats) has against oxidation/rancidity, they are called antioxidants (clever name, huh? anti-oxidation). Not only are antioxidants one of the supposedly healthy reasons to use olive oil, the antioxidants are also some of the chemicals responsible for the more interesting, complex, nuanced flavors of great olive oil. The longer a bottle has been in this tweetle-beetle battle against oxidation, the more of its antioxidants have been sacrificed to postpone eventual, inevitable, unavoidable rancidity. Even if not rancid yet, the older your olive oil is from its creation, the less nuanced, the less complex, the less full-flavored it happens to be.

Lessons to be learned: Drink olive oil (or just use it) as young in its life as possible; keep it away from heat, light, and air (no storage in a windowsill, above a stove, or in an air-filled container); and use it liberally before it dies on you.