Salt and Frying Oil
The theory is that ionic substances, such as table salt, can initiate the formation of small amounts of free radicals when heated in oil to high temperatures. These free radicals spur reactions that create impurities in the oil and, as a result, lower the oil’s smoke point, which means it can’t be reused for frying. The free radicals can also speed the development of rancid aromas and flavors.
To find out how much of an issue it can be, we added 1/8 teaspoon of table salt (an amount you would use to season potatoes before frying) to 2 quarts of peanut oil, heated the oil to 325 degrees, and fried French fries. After letting the oil sit overnight, we repeated the process two more times. We also prepared a control batch of peanut oil following the same steps but without the salt. After each round, we smelled both samples blind to see if we could detect any off or rancid aromas. Next, we fried cubes of white bread in the two oils (since they are a blank canvas and soak up oil so well) and tasted them for traces of rancidity. Finally, we heated both samples and noted at what temperature they each began to smoke.
None of our tasters were able to detect rancid aromas or flavors, and the salted oil started to smoke within a couple of degrees of the unsalted sample, both right around 415 degrees.
THE BOTTOM LINE: Problems caused by salt in frying oil are more likely an issue for restaurants, where the oil is held at high temperatures for an extended time and is used heavily. For home cooks it shouldn’t present a problem, especially since we recommend using a batch of frying oil at most three times before discarding it.