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A general rule of thumb is that 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 that 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.” It is caused by fermentation in the absence of oxygen; this occurs within the olives before they are milled. 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, both 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 in fact the smell of fermented olives. It’s difficult to come up with a single descriptor for the fusty smell, but here are some things that 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 put their knowledge of this chamber of horrors to use? Olive oil shopping is a topic for another article, but here are a few things to do right away. 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 that it’s 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!