Olive oil is one of the healthiest fats, but age and other factors can adversely affect it. It will turn rancid over time and many consumers have become habituated to the flavor of rancid olive oil, thinking it was fresh and healthy.
Factors such as storage temperature, exposure to air and light, and the amount of natural antioxidants or polyphenols in the oil at the time of harvest can affect how long it takes for olive oil to turn rancid. If the oil starts to smell like crayons or rancid nuts, chances are its rancid.
Another consideration is whether olive oil is ‘fusty,” which gives it an odor like sweaty socks and is caused by the olives being allowed to sit and ferment in the absence of oxygen before being milled. Olive oils can have an odour similar to that of wine or vinegar, which is caused by fermentation with oxygen, or they can have a musty scent due to mold in the olives. The best way to avoid these defects is to check for quality and freshness.
Examine expiration dates on bottles, and learn as much as possible about producers, growers, and their practices.
Source: oliveoiltirnes.com, “Good Oils Gone Bad Recognizing Rancidity and Other Defects.”
“Olive Oil”, Well Being Journal, November/December 2013, p 35