For the past few decades, fried foods have become amongst the most popular items consumed around the globe. From Southern fried chicken to Mexican chimichangas to Japanese shrimp tempura, there is no escaping the omnipresence of deep-fried delicacies on menus worldwide. And it’s easy to see why—no other cooking method produces such a distinctive crispy crunch whilst retaining juicy tenderness.

In order to prevent fried foods from becoming tough and rubbery as they fry, they are usually coated with a protective insulating layer of batter or breading. This gives us the best of both worlds: tender, steamed food in the middle with a crisp, crunchy layer on the outside. But that flour coating will slough off, causing issues down the line for subsequent frying sessions, which can number in the dozens each day for a busy restaurant. A basic rule of thumb: the more particulate matter you introduce to oil and the finer those particles, the faster your oil will break down.

As much as fresh oil helps restaurants in the preparation of lip smacking dishes, there are significant downsides of using leftover oil. Stretching oil beyond its useful life has several negative impacts that restaurants must manage. 

Why Cooking Oil Breaks Down

Before we explore these negative effects, let’s briefly explain why oil breaks down. Three common chemical reactions contribute to the breakdown of cooking oil: hydrolysis, oxidation and polymerization. 

Hydrolysis is caused primarily by hydrogen molecules in water reacting with frying oil, giving fried foods an acidic or tainted flavor. Excess moisture, high temperatures, foreign matter, emulsifiers, and the free fatty acids caused by oxidation also contribute to hydrolysis.

Oxidation occurs when oxygen molecules react with long fatty acid chains in the oil and break them up. Heat catalyzes the reaction and accelerates oxidation. Additionally, trace metals, foreign matter, and UV light can cause further oxidation.

As frying oil breaks down, non-volatile products of oxidation and hydrolysis begin bonding together (polymerizing) at high oil temperatures, forming clumps that accumulate on the oil’s surface. If these particles are large enough, foaming will result, further accelerating the rate of oil breakdown.

It’s All about the Taste

Now that we’ve explained how cooking oil breaks down, let’s explore the negative impacts during its life cycle. As the chart below demonstrates, oil begins to break down after as little as one day of regular use. 


Cooking oil’s sweet spot is generally attained on day two. By day three, however, the oil will need to be replaced or filtered. Some telltale signs of old oil are foam on the top surface, an inability to reach frying temperatures without smoking, and a dark, dirty look and musty, fishy aroma—thus making the food less appealing to consumers.

Moreover, using old oil can have adverse health effects. Old oil has high TPM and FFA levels that are proven to be carcinogenic. In some countries, a TPM reading exceeding 25 percent or more is considered unhealthy. Operators who are frying food in oil with high TPM levels can be held liable by local governments or agencies.

When rotating or “cascading” oil with multiple fry vats, operators often run into bad practices by using old oil to top off fry vats containing new, clean oil. This will cause the new oil to turn dark and break down faster. Essentially, operators are spoiling new oil with old oil. To avoid this, it’s essential to have best cooking oil management procedures in place.

Can restaurants extend the sweet spot for oil? The answer is yes. To extend oil life (and reduce the associated costs of replacing oil on a daily basis), restaurants should implement routine filtering after each meal period, As the chart below demonstrates, effective oil filtering can extend the sweet spot up to eight days.


Some other best practices to keep cooking oil in this zone: use a stainless steel skimmer to skim off any floaters and large pieces of debris that might be lurking; calibrate fryer temperatures to not exceed 350 degrees F; and minimize the amount of ice crystals or water that enters the fry zone. With these best practices, you’ll ensure the maximum life of your oil, and both your budget and your customers will thank you for it.

Corby Stow is the Director of Business Development at Oil Solutions Group. Oil Solutions Group was founded to provide extended oil life solutions to the foodservice industry. OSG designs and sells only best-in-class products that improve oil quality to more than double oil life, provide greater sustainability by reducing waste, improve flavor profile, and reduce top-line oil cost.

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