How To Tell If Deep Fryer Oil Is Bad?

There are many ways to determine the freshness of cooking oil and varying opinions on how often a fryer vat should be switched out with fresh oil. Sometimes, using your senses is the best gauge.  

How does the oil look? The longer oil is used, the more its molecules break down. Spent oil tends to be darker, often with a thick, gummy consistency. Degraded oil can also have a frothy or soapy appearance. 

How does it smell? Oil that has turned, or nearing that point, has an acrid, bitter smell. Oil picks up the smells and flavors of the food cooking in it, just as the food can pick up the smell and flavor of the oil. If too much food has been run thru a batch of oil, it can affect the taste of the food coming out of the fryer. 

How does your food taste? Are your fries still crisp, nicely browned and taste like, well, french fries? Or have they picked up flavors or aromas from other foods nearby? If you notice your food beginning to take on a different taste than it normally does, it could mean your cooking oil is no longer fresh.

Senses Aside, What’s the Science?

  • Know the type of oil you are using. Different oils have different smoke points and shelf lives. Oil that can withstand higher temperatures (peanut, canola and vegetable oils versus the lower temp-restricted olive oil) generally has a longer life, but there’s a flip side. Oil that is too hot will degrade more rapidly.

  • Proper temperature control. Generally, optimal frying temperature for most foods is between 330-350 degrees. If you wish to get the most out of your oil, be careful about not rising to a temperature above 375 degrees. Modern commercial fryers do an excellent job of maintaining the proper temperatures for your fried foods.  It’s up to the restaurant manager to make sure their fryers are turned off and oil is cooled and covered when not in use. 

How Long Should Oil Last?

How long should your oil last? Much of it depends on what you are cooking.  

  • Coated foods will degrade oil more quickly than non-coated

  • Is water being introduced into the oil? This is not only dangerous but it also degrades your oil. Be sure to drain or shake excess water away from your fryer before adding it to your hot oil.

  • Don’t allow oil to sit uncovered when not in use. Like water, air can also quickly degrade oil.

How Can You Extend Oil Life?

  • Maintain reasonable fryer load. Your food will cook more evenly at a stable temperature if it has room to move in the vat. 

  • Lower frying temperature to get the most life from cooking oils. 

  • Keep your fryers clean – Make sure you follow the recommendations of your fryer manufacturer. Don't forget to clear off food particles from the heating elements when you clean your vats. 

  • Filter the oil – This one may be the most important tip. Many experts, as well as fryer manufacturers, recommend filtering oil twice a day -- after lunch and dinner runs.  At a minimum, train your workers to routinely skim food particles from oil throughout the day.  

  • A full change-out to fresh oil is preferable to topping off.  When adding fresh oil to the vat, it is difficult to determine how much old oil is being used to fry your fresh foods and maintaining quality control and costs become harder to manage.


You're the expert in frying. We're the expert in what to do with your waste grease.


What Should a Restaurant Manager Do With Used Cooking Oil?

  • Never discard used cooking oil down a drain (clogs and costly plumber bills, as well as city fines)  

  • Don’t dump it into the trash bin (attracts rodents, smells bad enough to turn away customers and could cause grease spills, possibly incurring fines).  

  • Partner with a reputable grease service provider who will take your waste oil and turn it into something of value. DAR PRO solutions puts 100 percent of the used cooking oil it extracts back into the environment

DAR PRO Solutions is a thought leader in the industry for used cooking oil collection and grease trap services. Our innovative equipment solutions, strong customer support, and service expertise will make handling grease a worry free part of your business. Reach out to a DAR PRO representative today and find out how your ghost kitchen or restaurant can benefit from our partnership. Call us 24/7/365 at 855-DAR-PRO1 (855-327-7761).

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05/20/2019