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The joy of olive oil lies in its many delight­ful aro­mas and fla­vors — from volup­tuous ripe olive to bright green grassy notes and from a soft sub­tle fin­ish to a zippy pep­pery kick — there is a world of sen­sory explo­ration await­ing the adven­turer. But like any great explorer, you will be faced with risks — croc­o­diles in those placid waters. This is an intro­duc­tion to the most com­mon defects you will find in olive oil: what they are called, what causes them, and how to rec­og­nize their pres­ence.

Any dis­cus­sion of defects must start with ran­cid­ity.

The sad truth is that most peo­ple in the US, for exam­ple, are accus­tomed to the fla­vor of ran­cid olive oil. Olive oil is no longer an occa­sional pres­ence in the kitchen so it is time to change that.

Olive oil is a per­ish­able prod­uct.

Olive oil tastes best when it is fresh. Think of olive oil on a fresh­ness con­tin­uum that goes from just-made, har­vest-fresh at one end to com­pletely ran­cid at the other. How long it takes an olive oil to go from one end of this fresh­ness con­tin­uum to the other depends on many fac­tors: stor­age tem­per­a­ture, expo­sure to air and light, and the amount of nat­ural antiox­i­dants in the olive oil in the first place. All olive oils, even the finest ones, will get ran­cid even­tu­ally.

This is why you must never hoard olive oil: use it and enjoy it. Waiting for a spe­cial occa­sion to use your good olive oil? How about din­ner?

Do you have a clear sense of what ran­cid oil smells and tastes like?

A good image for many peo­ple is the smell of crayons. Another help­ful item — some­thing that almost every­one has tasted — is ran­cid nuts. Rancid is fat gone bad, some­thing we have encoun­tered at some time. On a ran­cid scale of 0 to 10, almost every­one will notice a 9 or a 10. The trick is to develop the con­fi­dence to pick out ran­cid­ity when it is a 5, or a 3, or lower. The fla­vor of ran­cid­ity in olive oil is usu­ally accom­pa­nied by a greasy mouth­feel; the greasi­ness is often notice­able first.

Go to your cup­board and pull out the olive oil. How old is it? Is there a ​“Best By” date? Generally, that date is two years from when it was bot­tled. Unfortunately, that doesn’t tell you when it was har­vested and milled. Harvest date is the most reli­able indi­ca­tor since it tells you when the olive oil was actu­ally made. Sniff it.

Taste it. Crayons? Putty? Old peanuts? Don’t worry about throw­ing out old olive oil; feel good about it! Don’t be sur­prised if the purge of your pantry includes not just old olive oil but things like old whole wheat flour (which gets ran­cid because of the oils in the wheat germ), crack­ers and cere­als.

Olive oil is best con­sumed within a year of har­vest.

Most oils, if unopened and stored in a cool dark place, will still be good for up to two years, but they steadily lose the fresh fruiti­ness you want in olive oil. Greener har­vest, robust olive oils will keep bet­ter than del­i­cate ripe ones because of the higher con­tent of com­pounds called polyphe­nols in greener oils. You can rec­og­nize the pres­ence of these polyphe­nols because they con­tribute pep­per­i­ness and bit­ter­ness to the fla­vor of an oil. If an oil is del­i­cate and soft, made from ripe olives, then you will want to use it quickly, within six months or a year at the most.

The sec­ond most com­mon defect of olive oil is called ​“fusty.”

Fustiness is caused by fer­men­ta­tion in the absence of oxy­gen; this occurs within the olives before milling. This is why it is so impor­tant for olives to be processed into oil within as short a time as pos­si­ble after har­vest. Olives left to sit in bags or piles for even a few days will pro­duce fusty olive oil.

And what does fusty smell and taste like? Unfortunately, the answer for a lot of peo­ple is ​“olive oil.” For many peo­ple in the US and abroad, fusty fla­vors in olive oil are the norm. When I was train­ing for an olive oil taste panel, I remem­ber vividly the day I poured my usual sup­pos­edly extra vir­gin olive oil into a warm skil­let and was enveloped by the smell of fusti­ness. I threw out that bot­tle and never looked back.

Now I know that good olive oil smells like fresh green or ripe olives and that the smell I always asso­ci­ated with olive oil was the smell of fer­mented olives. It’s dif­fi­cult to come up with a sin­gle descrip­tor for the fusty smell, but some things might help: sweaty socks, swampy veg­e­ta­tion, or too-wet com­post heap. A good way to taste an exam­ple of the fusty defect involves table olives. Look through a batch of Kalamata-style olives and see if you can find any that are not pur­ple or maroon-black and firm but instead are brown and mushy. Eat one. That is the fla­vor of fusty.

Rancid and fusty are by far the most com­mon defects of olive oil. Occasionally you may run into a winey-vine­gary defect. That is caused by fer­men­ta­tion with oxy­gen and can be rem­i­nis­cent of vine­gar or nail pol­ish. Another defect that crops up once in a while is musty. Caused by moldy olives, it tastes of dusty, musty old clothes or the base­ment floor.

How does a shop­per use their knowl­edge of this cham­ber of hor­rors?

Start with fresh­ness. Look for dates on olive oil bot­tles. Try local pro­duc­ers if you are lucky enough to live in an area where olive oil is made. Learn as much as you can about the grower.

Whenever pos­si­ble, taste before you buy. And if you open a bot­tle and find it ran­cid, return it. An eth­i­cal pro­ducer will do every­thing they can to get a qual­ity prod­uct to you, but they lose con­trol once the bot­tle is out there in the dis­tri­b­u­tion chain. Buy from peo­ple you trust.

By pay­ing more atten­tion to the fla­vors of olive oil and exper­i­ment­ing in your kitchen and at the table, you will dis­cover the amaz­ing diver­sity of this won­der­ful food. Let knowl­edge and expe­ri­ence embolden you; damn the croc­o­diles — full speed ahead!