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How the Oil Changes During Frying?

We knew that old or bad oils could only produce soft, greasy and rancid food, but at the same time the food is causing some changes in the oil. First of all, let’s have a look at the oil.

Common Frying Oil

From the nutritionally less-favored animal fat e.g. Lard.

  • High cholesterol

  • High palmitic fatty acid

  • Absence of natural antioxidants

To the origin of artificial trans fats e.g. Partially hydrogenated oil (PHOs), we had changed the oil we use when there is a healthier choice.


And for now, common oils being used are:

1. Refined Plant Seed Oil – particularly high oleic (mono-unsaturated) varieties that have stronger resistance to oxidation than those high poly-unsaturated oils.

e.g. High oleic sunflower oil, High oleic canola oil


2. Extra Virgin Oils – keeping the natural flavour and antioxidants content.

e.g. Extra virgin olive oil, Extra virgin coconut oil


Among these oils, refined high oleic plant seed oils are the major choices of frying oils due to their high smoke points and stability, neutral flavor and light body. Knowing what oils we are using, let’s talk about the condition of deep frying?


Condition of Deep Frying

Enough volume of oil is heated up to 160-180°C, and the food is fully immersed in the hot oil. Basically you can fry any food from meat and poultry to seafood and vegetables (particularly potato). Lots of different reactions take place between the oil and the carbohydrate, protein, lipid and moisture in food.

French Fries, also known as fries in North America region or chips in European region, is the most widely purified deep fried product in the world.

Breakdown of Fat Molecules by Steam

Steam, generated from the moisture of food, breaks down the fat molecules of the oil i.e. hydrolysis of triacylglycerol (triglycerides) into diacylglycerols, monoacylglycerols, free fatty acids and glycerols. The free glycerols are immediately converted into acrolein, the irritating gas that hurt your eyes, noses, lungs, etc.

The picture below shows major reactions during deep frying:

Transfer of Fat Between the Oil & the Food

The fats in food are exchanged with those in the oil. The transfer is depending on the amount of fat in the food. Food with lower fat tends to absorb more from the oil and vice versa.

Fried food with low fat:

  • Mainly plant origin e.g. potatoes

  • Lean muscle tissue e.g. chicken breasts or thighs

  • Lean fish products e.g. cod fillet


Fried food with high fat:

  • Animal products e.g. pork, beef, lamb, poultry

  • Fatty fish e.g. herring, sardines 

Notice that polyunsaturated fatty acids e.g. EPA or DHA are released from fatty fish to the frying oil, which impart fishy off-flavour to the oil, and decrease oil’s stability against rancidification.

How to Reduce the Exchanges?

Most developed countries have overweight population due to excessive dietary energy intake and sedentary lifestyle. Thus low fat foods are preferred and methods had been developed to reduce amount of oil absorbed during frying.

  • Pre-drying

  • Pre-heat meat slices in very hot oil (220-300°C)

  • Batter and Breading

These methods facilitate the formation of crust over the food surface that acts as a barrier to fat.


Oxidation of the Oil

Dissolved oxygen in the oil reacts very slowly in room temperature, but they are quickly reacted with the poly-unsaturated fatty acids at the frying temperature.


New oxygen can only slowly enters and dissolves in the hot oil. Unless there is foaming, further oxidation is less likely to take place, though there is polymerization. Adding anti-foaming agent such as silicone can inhibit foaming.


Heavy metal ions (and some complexes) in food such as copper and iron from the meat, are released into the oil which speeds up oxidation.


Most fried substance contains proteins, which are reactive to free radicals. Proteins react with first batch of oxidation products (hydroperoxides or aldehydes) and inhibit further oxidation. However under substantially high temperature there is also the formation of harmful products (aromatic heterocycles) that are precursors of carcinogens. Thus, it is recommended the frying temperature should not exceed 180°C.


Oxidation products and the food are both more polar (stay with water but not oil) than the oil hence they will stay on food surface that deteriorate the nutritional value of the food. No doubt we should avoid oxidation by any means.


How to Inhibit Oil Oxidation?

Free radicals are generated when oil reacts with oxygen and sunlight. They are the most important intermediates in lipid oxidation that trigger subsequent reactions. Old oil and oil stored improperly will be a pool of free radicals and oxidation products. The subsequent polymerizations of the oxidation products generate even more harmful carcinogenic substances.


Antioxidants are substances that can react with free radicals to inhibit lipid oxidation.


Natural antioxidants in frying oil:

  • Tocopherols

  • Some sterols and Phospholipids

  • Tyrosol derivatives in olive oil


Some antioxidants are transferred from fried food to frying oil:

  • Tocopherols

  • Carotene and related carotenoids


Some antioxidants are not soluble in frying oil and remained in fried food:

  • Phenolic derivatives e.g. chlorogenic acids, tannins, flavonoids and anthocyanins, and ascorbic acid (vitamin C)

Enjoi Suggestions:
  1. Use stable healthy Enjoi oils

  2. Store the oil properly. Close the container and avoid sunlight, high temperature and moisture.

  3. Use correct frying temperature

  4. Use the oil wisely. Try to use fresh oil to fry food that damages the oil less.

  5. Dry the food before frying, but season the food after frying.

  6. Keep the fryer and oil clean.


Read also:

The Cost of Frying Oil



Sukumar Debnatha, K.K Bhatb, N.K Rastogic "Effect of pre-drying on kinetics of moisture loss and oil uptake during deep fat frying of chickpea flour-based snack food", LWT - Food Science and Technology, Volume 36, Issue 1, February 2003, Pages 91–98.


Karacabey E, Turan MS, Özçelik ŞG, Baltacıoğlu C, Küçüköner E, "Optimisation of pre-drying and deep-fat-frying conditions for production of low-fat fried carrot slices." J Sci Food Agric. 2016 Oct;96(13):4603-12


Jan Pokorný* and Zuzana Réblová "Effect of Food Components on Changes in Frying Oil." Institute of Chemical Technology, Department of Food Chemistry and Analysis, Technická 5, CZ-166 28 Prague 6, Czech Republic

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