|MadSci Network: Biochemistry|
The short, quick answer is that your oil has gone bad. How do you store it and how old is the oil? I tested out the oils I have around the house: grape seed oil, olive oil, and vegetable oil. None of these produced a fishy smell when heated. These oils are less then a month old and are stored in a cool and dark cabinet. Olive oil can last about 20 days at room temperature exposed to light. If kept out of the sunlight then somewhere around 3-4 months. If you have you oil near a heat source, like a stove or a furnace vent then it well degrade even faster. The same would apply to your oil. The taste or smell for rancidity can vary widely from person to person. So a chemical test is used in industry to determine when oil has gone rancid. The action of air and enzymes will age your oil even faster. Yes, by applying heat you speed up the process, so your cold oil may be somewhat rancid but by heating it you push it over the threshold and have fishy oil. For info on olive oil http://www.oliveoilsource.com/olivechemistry.htm Some other reading Chemistry, Flavor, and Texture of Lipid-Containing Foods by Fereidoon Shahidi and Hugo Weenan Lipids in Foods: Chemistry, Biochemistry, and Technology (Progress in Food and Nutrition Science) by F. D. Gunstone Canola and Rapeseed: Production, Chemistry, Nutrition, and Processing Technology edited by Fereidoon Shahidi
Try the links in the MadSci Library for more information on Biochemistry.