Did my headline catch your attention? Good. But it is not a click bait because reuse of cooking or frying oil is definitely better to a certain extent— tastewise. Many people used oil only for the first time they use it because of one reason- the possible adverse health effects. Yes, it is true that reused oil increases the cholesterol level and produces acrolein, a carcinogenic chemical.
Well, the words “frying” and “oil” often get the bad rep to begin with. So who am I trying to convince?
But let me explain first. Keep reading.
Frying and Maillard reaction
Ever wondered why foods turn brown when fried at high temperatures? And why they smell more noticeably better? The reaction involved in all of these is called the Maillard reaction. This reaction was first described in 1912 by French chemist Louis-Camille Maillard. The Maillard reaction is a complex series of reactions that occur in food between carbohydrates (sugars) and proteins (amino acids). Complex might not be a good way to describe it, maybe. Even today, food chemists and scientists have not yet fully understand it entirely.
Before the cooking or frying oil reaches a high temperature, the sugars and amino acids in the food gather enough energy to react together. By the time the cooking or frying temperature reaches 284 °F (140 °C), this is where all the magic happens. The food being fried turns attractively brown—also described as nonenzymatic browning. More examples of this are grilled steak, browning french fries, baked goods and roasted coffee. But browning is just the tip of the iceberg.
As the food continues to be cooked at a high temperature, the proteins and carbohydrates in the food continue to react (clashing and fusing). In the process, new flavor compounds are produced. This is actually one way scientists make artificial flavors in their products. Here is a link for a patent from Ajinomoto for producing artificial meat-like flavor via Maillard reaction.
As for the compounds during frying, here are few of them.
Compounds produced during frying
One study in the UK suggests that Maillard reaction-driven aroma compounds in french fries increase as the cooking time increases. The study found out that potato varieties Desiree and Maris Piper are relatively high in sugar-derived aromatic compounds, including 5-methylfurfural and dihydro-2-methyl3[2H]-furanone. While amino acid-rich variety King Edward is relatively high in pyrazines and Strecker aldehydes.
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In pork and chicken, the interaction between Maillard reaction and thermal degradation of lipids leads to volatile compounds. The compounds nonanol, 2-methyl-3-furanthiol, and 2-furfurylthiol, to name a few, are involved in flavor development in chicken.
These compounds are interestingly absent in boiled meat.
When it comes to color in cooking oils, the desirable color development is generally believed to be triggered by Maillard reaction as well. But this part is usually less discussed and is underated. This is because when it comes to it, what usually comes to mind is the gradual darkening of the oil over time.
Reused oil imparts new flavor compounds
With all these flavor compounds produced after frying, you would not want to discard the oil after the first time using it. Because reusing oil imparts a much better taste the next time.
During frying, the frying oil undergoes an oxidation process. Oil oxidation is a series of reactions where the oil reacts with oxygen, which gradually degrade the quality of the oil. Heat and light also affect the quality. Aside from oxidation, hydrolysis and polymerization also occur during frying. But along these chemical reactions are the production of certain flavor compounds. When the oil is reused, its slightly oxidized nature imparts an extra better flavor in fried food. The desirable flavors in fried food like meat are usually produced at optimal oxygen concentration.
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But be careful when reusing the same oil for multiple occasions. Because over time, the oil degrades totally in quality. It is evident when it becomes rancid, and develops off-flavors and odors. Another way to tell if the oil has been overused is its very low smoke point or burning point. The smoke point is the temperature where the oil starts to produce visible smoke, indicating fat breakdown. With this being said, this leads us to the question:
How many times can frying oil reused before discarding?
Slightly oxidized frying oil gives better flavor in fried food. But repeated reuse of cooking oil is detrimental to health, as I have mentioned. This is especially true when you do not use a deep fryer.
The folks at Cooks Illustrated did an experiment to tell how many times frying oils can safely be reused. They used two identical ovens filled with 2 quarts (1.8 L) of canola oil: 1 for frying chicken and one for potato chips. The process was simple: strain the oil after each fry, and use a test kit to determine the presence of compounds that indicate degradation.
After each fry, the fried chicken oil increasingly became darker, and the smell grew stronger. However, the test kit indicated the presence of compounds only after the fifth use, and off-taste and flavor only developed by then.
The potato chip oil, on the other hand, stayed clean-smelling and clear. And the test kit barely showed any signs of presence of compounds even after the 8th batch of frying, at which they stopped testing.
Based on these observations, they recommend the following:
- Reuse cooking oil three to four times for battered and breaded.
- For foods that are cleaner to fry such potato chips, oil can be safely reused at least 8th times. Likely even more if refilled with fresh oil.
So that is about it. Did I miss something about this topic? Or perhaps you might want to add something. I particularly want to hear from someone who once worked at a famous fast food chain. In your experience, how often did you guys change the oil in the deep dryer? Appreciate it if you leave a comment below 🙂
