From a physical-chemical point of view, frying is a drying-and-cooking operation. When a piece of food is plunged into an olive oil bath at a frying temperature, water is immediately evaporated from the surface of the product. Intense bubling of water vapour in the olive oil and a particular sizzling nois occur. Three effects take place in sequence:
- Very rapid drying at the surface with formation of a dried skin. The role of this skin is essential because it gives crispiness to the fried food.
- The dried skin partially isolates the inside of the product from the oil, thus reducing the heat flow toward the inside and the loss of water vapour from the inside. In conclusion, a fried product is a product that has been cooked inside a very thin container made of it's own skin, formed when it was plunged into the oil bath. Only the skin reaches a high temperature, close to the oil temperature, while the inside of the product rearely exceeds the water boiling temperature.
- In the conditions described above, two very different types of transformation take place.
- At the surface, due to the high temperature and the low water content, nonenzymatic reactions take place with formation of a brown colour and a typical flavour. Under these conditions, hydrophobic groups are formed at the surface and therefore more intense interactions take place with the oil components;
- In the inside, cooking takes place at a relatively low temperature (in the range of 85-95°C) with changes in texture, starch gelification, protein denaturation and so forth. The decrease in water content on the inside is very limited and therefore the consistency of the inside remains soft. The original taste of the food on the inside is maintained, but perhaps even accurated by the loss of water. Hence, it may be concluded that head damage (loss of vitamins and essential amino acids, loss or lower digestibility of nutritients, and so forth) is much lower in fried than in boiled or roasted foods. The softness of the inside and the crispiness of the outside are an essential sensory feature of fried foods.
Frying with olive oil is healthy!
In conclusion, it may be said that extra virgin olive oils are perfectly suited for frying and that fried foods are as healthy as they are tasty, provided that:
- real extra virgin olive oil is used, with a low free acidity and a good content of phenolic antioxidants (you won't find this oil in supermarkets or in HORECA distribution channel!!)
- the oil is used or reused for a limited length of time in order to avoid chemical changes due to triglyceride transformations and to interaction of triglycerides with other food components under conditions of high temperature in the frying bath (value of total polar materials).
- the oil is well drained and dried from the surface of the product after frying in order to avoid an excessive amount of triglycerides in the diet.