A.10 BOILING AND SMOKE POINT
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Frying in extra-virgin olive oil The ideal temperature for frying products with a high water content such as vegetables, potatoes and fruit, is 130–145 ∘ C (266–293 ∘ F), while for small, quickly fried products, the frying temperature should be 175–190 ∘ C (347–374 ∘ F). As can be seen from the above values, the ideal frying temperature is much lower than the smoke point of olive oil, therefore it can be concluded that good-quality extra-virgin olive oil is perfectly suited for frying. Furthermore, extra-virgin olive oil contributes to flavouring the fried products and preventing thermo-oxidation due to its high content of antioxidants.
A physical-chemical description of frying From a physical-chemical point of view, frying is a drying-and-cooking operation. When a piece of food is plunged into an oil bath at a frying temperature, water is immediately evaporated from the surface of the product. Intense bubbling of water vapour in the oil and a particular sizzling noise occur. Three effects take place in sequence: 1. Very rapid drying at the surface with formation of a dried skin. The role of this skin is essential because it is gives crispiness to the fried food. 2. 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 its 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 rarely exceeds the water boiling temperature. 3. In the conditions described above, two very different types of transformation take place. 1. 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; 2. 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 accentuated by the loss of water. Hence, it may be