Is reusing cooking oil safe?


I am trying to convince my good friend not to save the grease she cooks with for re-use. Instead of draining grease out of a pan after frying it, she saves and stores it to use again. It seems to me that grease that is cooked once is bad enough, twice must be horrible. Am I unjustly picking on her, or am I right that there is an even greater health risk when you cook with pre-cooked oil/grease?

Dear Reader, 

You’re not just picking! Although re-using cooking oil or grease is a somewhat common practice, it can pose some serious health hazards. The most common risk when reusing cooking oil is that it becomes rancid or spoiled. In addition to having strange flavors and odors, rancid oil may contain byproducts that may have negative health consequences. These pesky byproducts are then absorbed into the fried food and ingested by the consumer. 

First, it may be beneficial to understand what’s happening on a molecular level when oil is fried. In order to fry food, the oil being used needs to be heated to temperatures of 347 to 377.6 degrees Fahrenheit (175 to 192 degrees Celsius). However, frying foods at or above 375 degrees Fahrenheit (190 degrees Celsius) can lead to an alteration of the chemical composition of fats in the oil used, resulting in new substances in the body called lipid degradation products. These products can have a number of effects, ranging from an increased risk of stroke, atherosclerosis, elevated levels of low-density lipoprotein cholesterol (the kind that collects in the arteries), Alzheimer's, Parkinson's, and various liver diseases. Other types of these lipid degradation products can promote oxidative stress in the body. This is process results in free radicals, which can potentially contribute to conditions such as cancer, diabetes, and neurodegenerative diseases.  

Generally, harmful byproducts are typically in low amounts when the appropriate oil is used to fry food. Oils that are high in polyunsaturated fats (such as linoleic acid) have been shown to generate more lipid degradation products when compared to oils with low amounts polyunsaturated fats and high amounts of monounsaturated fats. For this reason, choosing options such as olive oil or canola oil rather than safflower oil, grapeseed oil, sunflower oil, and corn oil may help reduce some of that byproduct. The issue becomes more prevalent when oil is reused. It only takes one use to alter the composition of the oil, and reusing oil, especially at too high of a heat, can cause a build-up of harmful byproducts. Each use creates more and more build-up, increasing the risk of harm. Such a connection was illustrated in one study that found a positive relationship between breast cancer development and reusing oil.  

The research into this subject presents a case for using fresh oil every time you cook. However, this may not be achievable due to convenience or cost. If oil must be re-used, then here are some helpful (and healthful) tips to consider when cooking: 

  • Consider avoiding iron or copper pots or pans for frying oil that is to be reused since these metals can shorten the length of time it takes for oils to go rancid.  
  • Shaking food before frying it can remove any excess batter. This can reduce residue in the oil that may remain when used for future use. 
  • Using a good thermometer to fry foods allows you to keep an eye on the temperature and refrain from going above 375 degrees Fahrenheit (190 degrees Celsius). 
  • Turning off the heat after you’re done cooking prevents exposing oil to prolonged heat, which can make the oil become rancid more quickly. 
  • Straining through a few layers of cheesecloth after use can catch any food particles before storing. Giving the oil a few hours to cool can prevent burns from handling hot oil. 
  • Oil is best stored in a cool, dark place. Try keeping it in a secure container in a cabinet.  

So, there’s a good amount of evidence that can help guide your conversations with your friend. Sharing what you’ve learned in this may be a helpful place to start. She may decide to continue reusing oil, and the tips already mentioned can help you provide some additional guidance if she decides that’s the right fit for her. 

Last updated May 21, 2021
Originally published Nov 08, 2002