Chef Giuseppe Da Prato
Frying is one of oldest ways of cooking; in Ancient Rome fats such as olive oil or lard were mostly used. One of seasonal culinary preparations consumed during the Saturnali period were frictilia, cooked using a basis of eggs, flour and honey (sugar was not contemplated). This could very likely be the ancestor of what we are talking about today.
Technically frying consists in immerging food into a fat that has reached a high temperature. It consists of cooking with heat convection and at the same time achieving texture and outside colour. Convection is one of the most efficient ways of heat transmission, both for this reason and taking into consideration the high temperature of the fat, the cooking time is limited. A few precautions should be born in mind:
- The temperature of the fat must be high enough to allow a rapid thickening of the surface in contact with it ; but not too high to burn the outer layer before also the inside is cooked.
- The liquids inside the food need to evaporate, migrating to the surface and producing the bubbles that can easily be observed during the frying process. The use of batter allows a higher pressure to be reached compared to that of a fat, preventing the same fat from permeating inside. At the same time the barrier created by the batter favors the retention of a humidity level that makes the fried food crunchy on the outside and soft and moist inside.
Although there are a variety of different batters available, good fried food is the result of a sequence of chemical reactions which occur during the interaction between flour, breadcrumbs or batter and the hot fat. In fact thanks to the Reactions of Maillard, it is possible to see the origin of a bond between aromatic substances, the beginning of caramelization and the perfusion of the fat. The exact equilibrium among this feature is not simple to understand: you need to look for the harmony between the browning on the outside of the food, the cooking of the internal part and the correct absorption of the fat. Other than the cooking temperature, the factor that binds all these requirements is the dipping time. The longer the dipping, the darker the crust will be with burnt flavours, the inner part will lose its moistness and the amount of fat that permeates will increase. Contrary to this, the surface will not be crunchy, the food will not be correctly cooked inside and the steam produced will not have had the opportunity to supply the necessary pressure to block the fat from permeating the surface.
The presence of steam causes the softening of the outer layer of fried foods: once the pressure from the cooking heat ends, the steam moves to the surface and tends to attach itself to the batter, softening it. This is one of reasons you avoid adding salt when preparing the batter: the hydroscopic power speeds up this migration and the undesired softening. Thus, the custom to salt fried foods only once they are cooked and after the excessive fats have been absorbed, preventing too much salt from being assimilated. This also explains why, in order to taste dry, inviting and crunchy fried food, you should eat it in a very short time. The fundamental parameter to determine whether a fat can be used to fry is its smoking point, which indicates the temperature at which it begins to smoke. Passing this threshold, the triglycerides break up into the components: glycerol and fatty acids. The last ones, with high temperature, are subject to thermoxidation which results in peroxides, aldehydes, polymers and ketones. The glycerol dries out creating acrolein, hepatotoxin with a strong and unpleasant smell.
Fats used for frying
The fat used during cooking, in addition to being the system for heat transmission, flavours the immersed food, it keeps the surface moistened preventing it from attaching to the container and, given the high temperature, it favours the caramelization of the crust. For subjective and general reasons the aromatic component will not be considered below, since this would suggest only the use of olive oil. Therefore the key factor in choosing a fat is its capacity to resist at high frying temperatures.
The fatty saturated acids that exist in animal fats decompose at low temperatures, and therefore are not advisable for frying. Yet some preparations (for example cotoletta alla milanese) require the use of butter. Once this has been brought to a high temperature, it produces acrolein which is a toxic and unpleasant substance, so the use of ghee is generally preferred: if butter is melted in a bain-marie, without boiling, for around 15 minutes the water evaporates and the casein agglomerates. Once filtered, you obtained a butter without much of its water, which facilities the hydrolysis of the fatty acids, with a higher smoking point in relation to that of butter (respectively 200o and 130o).
Once you have understood the process of how margarine has been created, you can easily understand why it is not advised for frying, independent to the type of oil from which it has been obtained.
Therefore, it is preferable to use vegetable fats for frying, since they have a higher smoking point compared to animal fats and therefore better resistance while cooking.
The Institute of Shortening and Edible Oils supplies the smoking point of the most frequently used cooking oils:
- Corn oil 200-210° C
- Peanut oil 210-220° C
- Sunflower oil 220-240° C
- Olive oil 190-230° C
These values refer to oils that have not already been used, since reheating lowers the smoking point and allows polymerization to take place, which is the molecular aggregation that gives a darker colour and viscous texture. The range of reported values depends essentially on the degree of oil refining, even if it is necessary to point out what lowers such values. The presence of free fatty acids, of mono and glycerides lowers the smoking point; correct storage of the oil (temperature, darkness, oxygen) helps the oil to maintain its original value; using the oil more than once and /or heating it for a prolonged period of time lowers the value; the presence of salt or bacteria in the food being cooked, the mixture of different oils and the presence of water all result in a lowering of the smoking point; the cooking container (shape, size and exposed surface) affects the variation of the smoking point, since a greater surface for thermal exchange means a higher variation in the critical temperature.
Good Fried Food
Fundamentally, good fried food is not so difficult to obtain although a few precautions have to be born in mind. The previous points raised show how the smoking point of the oils mostly used in the kitchen is higher than the necessary temperatures needed for good frying: about 160°C for longer cooking of larger pieces, about 175°C for frying with a batter or crust, and about 180°C for quick frying of small pieces.
Some advice for correct frying:
- Add only a few pieces at a time to the frying pan so that the frying temperature remains constant, there is enough space, and the internal humidity can vaporize.
- Raise the heat after the food has been introduced for a few minutes to allow the temperature to return to its original value, and then lower it again.
- Check that the food is dry or completely covered (batter, bread crumbs etc).
- Prefer the use of deep-frying.
- Use a metal strainer to drain quickly and efficiently.
- Leave to dry on absorbent kitchen paper, and avoid placing layers on top of each other with subsequent dripping of the oil and condensing of the humidity.
- Salt only when completely drained.
- Do not reuse oil.
- Gain experience regarding the equilibrium between the cooking time and the temperature so that the result is golden outside while correctly cooked inside.