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What happens when you deep fry with oil?

1 The deep frying process

Deep fat frying is primarily a dehydration process, which means that water and water-soluble substances are extracted from the product being deep fried and transferred to the cooking fat. At the same time, the product being deep fried absorbs surrounding fat. If the product to be deep fried is placed in hot fat, the water on the surface evaporates and water moves from the inside of the product being deep fried to the outer layer, to compensate for the loss of water at the surface. As the water released does not readily move from the hydrophilic surface of the food to the hydrophobic cooking fat, a thin layer of steam forms between the fat and product being deep fried. This stabilises the surface of the food, which means that it protects the surface against the permeation of the fat until the water has evaporated from the food. At the same time, the layer of steam stops the food scorching and burning (see Fig. 8).


Protected by the steam, a crust with a large number of pores and cavities forms on the surface of the product being deep fried.


Once the majority of the water has evaporated, the product being deep fried sucks the fat into the vacated cavities and the inside is cooked. The cooling effect at the surface of the food gradually diminishes. The rising temperature resulting from this causes what is known as the "Maillard reaction". The protein constituents (amino acids) react with the sugar present and cause browning. This gives the food a pleasant aroma. (3)


2 The life cycle of the fat

Due to its composition and the various external influences, the cooking fat is constantly exposed to chemical reactions during a deep frying cycle (from adding fresh fat through to throwing away the aged fat). The condition of the cooking fat can be divided into various phases which are followed through during a cycle (see Fig. 9).

The first phase (a) starts with the unused, fresh cooking oil. The fat has not yet been heated and has also not yet come into contact with the product being deep fried. In its fresh state, therefore, there are no deep frying aromas or polar materials as yet. These are not produced until the ageing of the fat increases. The water only evaporates extremely slowly and remains on the surface of the product being deep fried for a long time. The product is overcooked and becomes slushy, but without hardly colouring.

In phase (b) the proportion of polar materials increases. As a result of the fat coming into contact with the oxygen in the air and being heated, decomposition produces a number of desired bonds which are responsible for the large majority of the typical and pleasant deep fat frying aromas. The flavourings and aromatics typically associated with deep fat frying are responsible for bringing the fat further into the optimum deep fat frying range (c). Here the ideal volume of water is extracted, without too much water escaping. At the same time, the Maillard reaction is set in motion as a result of the improved extraction of water. The fat now has contact for a sufficient length of time to brown the product perfectly and give it the typical, desired taste.

In the course of the life cycle, the curve falls sharply back towards the optimum. Bonds are produced in the fat which result in a deterioration of the condition of the oil (phase [d]). At the same time this means a deterioration of the product being deep fried in the oil.


As the decomposition progresses, the colour of the fat becomes increasingly darker and the taste more rancid and abrasive. The product being deep fried absorbs an increasing volume of fat during this phase, as the water is quickly extracted due to the extremely high proportion of polar materials. French fries, for example, become hollow inside. The more quickly the water leaves the fat, the more prolonged the contact between the fat and the product being deep fried, increasing the volume of fat which permeates the product being deep fried.

In the last phase (e), the cooking fat is no longer fit for consumption and should therefore be replaced or freshened with fresh oil. (5)

The curve profile described is attributable to various reactions triggered, among other things, by the effects of oxygen in the air, light or heat. The unsaturated fatty acids play an important role in these reactions, as the double bonds can react extremely quickly. There are essentially three main reactions which will tell you more about them in the following articles.


(1)    Template for redrawing from: Vorgänge zwischen Frittiergut und Frittierfett während des Frittierens; aid Verbraucherdienst, 42nd year, March 1997, p. 56, Fig. 1.

(2)    Bertrand Matthäus, Which fat and oil for which purpose? Features and specification of oils and fats (Power- Point presentation), German Federal Institute for Cereal, Potato and Fat Research, Münster.

(3)    http://www.margarine-institut.de/ presse2/index.php3?id=88. Last up- dated 08 April 2014

(4)   Template for redrawing from: Qualität des Frittiergutes in Abhängigkeit von Erhitzungsdauer nach Blumenthal (1991); aid Verbraucherdienst, 42nd year, March 1997, p. 57, Fig. 2.

(5)    aid Verbraucherdienst, 42nd year, March 1997, p. 57 – 59.

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