A large proportion of foods can only be consumed after cooking. Cooking, however, is not a one-size-fits-all process. On the contrary, it can be carried out in a variety of ways, and some of them can lead to a significant change in the nutritional characteristics of the food. Take frying, for example: cooking a food by the frying method will greatly increase its fat content. In other circumstances, proteins may be altered, vitamins may be lost, etc. It is therefore good to be aware of how the cooking of food may affect the physiology of the organism so as to give preference to methods with nutritional advantages or, where this is impossible, at least minimise disadvantages and nutrient losses.

What cooking is

Cooking is a process to which food is subjected in order to change some of its chemical and physical characteristics, based on exposing it to heat transmitted by conduction, convection or radiation. Cooking has been practised since time immemorial in various ways for various reasons, most of which can be summarised in the sole objective of making the food more digestible and palatable. More specifically, cooking is practised for:

1. to sanitise the food, destroying thermolabile pathogenic microorganisms (e.g. salmonella, escherichia coli, shigella, campylobacter jejuni) that may be present in it, as well as various toxic substances, such as solanine (present in potatoes);
2. impede the activity of certain enzymes that perform degradative transformations of food (especially if of plant origin);
3. inactivate certain substances in the food that, if left unaltered until introduced into the body, can decrease the bioavailability of certain nutrients (such as so-called anti-vitamins, which hinder the absorption of vitamins);
4. change the appearance of the food (in terms of colour, taste and aroma);
5. produce certain chemical reactions responsible for increasing the digestibility of the food (particularly with regard to starches and proteins), for example the release of flavones present in certain fruits and vegetables (such as lycopene in tomatoes or indole-3-carbinol in cruciferous vegetables) or inositol hexaphosphate (IHP) present in the fibres of legumes and cereals;
6. slightly increase the temperature of the brain so that you feel more satisfied with the meal;
7. stimulate the activity of the immune system.

Below is a summary table of the various toxic substances that are or could be present in food and the ways in which they can be destroyed before the food is consumed:

On the consuption of raw food

Cooking, in many circumstances, is a necessary procedure, but one that involves considerable problems and disadvantages:

1. also destroys microorganisms and yeasts that are beneficial to the functions of the human body;
2. certain types of cooking, especially frying, which will be discussed in a moment, encourages the production of toxic substances in the food, such as acrolein;
3. alters the atomic structure of the minerals in the food, rendering them unusable;
4. destroys most of the nutrients in the food, especially vitamins (especially water-soluble vitamins and vitamin A) and food enzymes;
5. due to the evaporation of water, can often cause the concentration of any toxic substances in the food, such as residues of drugs used in the cultivation of plants.

Now, there are some foods that lend themselves well to being eaten raw, such as fruit and certain vegetables. Ultimately, therefore, where the consumption of a raw food does not presuppose problems with digestion or possible infection or poisoning, the ideal solution would be to avoid any cooking process. Cooking food also has certain advantages.

Cooking methods and techniques

Generally speaking, cooking techniques can be classified into:

1. expansion cooking: during this type of cooking, liquids (such as water) are used to cook the food. This may result, at the end of the process, in less colour than it was when raw. It always involves the loss of nutrients that solubilise within the cooking liquid;
2. cooking by concentration: cooking by concentration involves the use of fat and causes the food to acquire a more intense colour than it did when it was raw;
3. mixed cooking: both liquids and seasoning fats are used;
4. hermetically sealed cooking: assumes that the food is cooked after being wrapped in special wrappings that preserve its aroma and taste;
5. technological cooking: this involves the use of special technological equipment. The most emblematic technological cooking is perhaps that which makes use of the microwave oven, but the increasing development of kitchen robotics is generating new cooking products with interesting features.

Cooking methods can also be classified into two broad strands: stewing and dry cooking. In the first case, the heat produced by the cooking medium is transferred to the food by means of a liquid with which it comes into contact, either directly or indirectly. In contrast, in the case of dry cooking, the heat is transferred directly to the food. Stewing methods include (without any claim to exhaustiveness):

1. boiling or boiling;
2. bleaching or blanching;
3. drowning;
4. the bain-marie;
5. the use of pressure cookers;
6. stewing;
7. brazing;
8. icing;
9. sautéed cooking;
10. frying in oils or fats;
11. cooking in the ashes.

Dry firing methods include (again, not necessarily exhaustively):

1. the use of the oven;
2. roasting;
3. the use of microwave ovens;
4. steaming;
5. the grill (grilling).

A final particular type of cooking that is only practised industrially is vacuum cooking.

Boiling

  Boiling or boiling involves immersing the food in a liquid, usually water or previously prepared stock, while it is boiling. It can be used for any food, whether of vegetable or animal origin. It is one of the few methods that allows the food to take on palatability characteristics even without the use of cooking with seasoning fats, and therefore has the great advantage that it does not increase the fat load of the food, but may even decrease it, since many of the fats of which it is composed are dispersed within the cooking liquid. Furthermore, sometimes this type of cooking can increase the bioavailability of minerals such as iron and zinc. The use of boiling, however, does not exclusively have advantages; indeed, careful analysis shows that perhaps the disadvantages outweigh the advantages: all water-soluble substances, first and foremost some vitamins (thiamine, pyridoxine, niacin, folate, ascorbic acid and pantothenic acid) are lost in the cooking liquid. This loss is directly proportional to the surface area of the food and the amount of liquid in which it is immersed. Of course, some of these substances can be recovered by taking the liquid in which it was cooked with the food. The amount of potassium in the food is largely lost, and while proteins, by being denatured, increase their digestibility, they become less bioavailable. Depending on the case, an increase in the bioavailability of starch caused by boiling can be considered an advantage or a disadvantage. If the food is dropped into the liquid when it is already boiling, the heat shock caused will limit the dispersion of starch in the liquid. It is cooking by expansion.

Blanching

  It is a variant of boiling that involves a much shorter cooking time, but does not allow the food to be fully cooked (which will then have to undergo other cooking processes). It has roughly the same advantages and disadvantages as boiling, with the difference that the proportion of nutrients that are lost during the process will be far less. This is because the cooking time is reduced. It is cooking by expansion.

Drowning

  This is also a variant of boiling: it involves the cooking liquid never reaching boiling point and is particularly suitable for rather delicate foods. Drowning has the same advantages and disadvantages as boiling. It is cooking by expansion.

The bain-marie

  This method of cooking involves placing the food inside a container that is in turn lowered into another. The second container is filled with water, which, however, remains isolated from the food during the cooking process. The container filled with water is exposed to heat, but without the liquid ever reaching boiling point (the temperature must never rise above 90-95°C). This method can only be used for foods with a liquid or dense, gelatinous consistency, which can be stirred during the bain-marie. If this were not the case, homogenous cooking would be impossible. The foods for which bain-marie is most commonly used are creams, since this method prevents the formation of lumps (caused by the coagulation of egg proteins at high temperatures), as well as fat-based emulsions, since the lipids are not damaged by this delicate cooking process. It is an excellent method of cooking food because the loss of nutrients is extremely low. It is cooking by expansion.

The use of pressure cookers

  This method is a variant of boiling that involves the use of special ‘pressure’ cookers. The food is cooked in the same way as in the boiling process, the difference being that the use of pressure cookers ensures that less water can be used, limiting the loss of nutrients, but at a higher temperature. Cooking times are appreciably reduced, thus also decreasing the time during which the thermolabile compounds in the food remain exposed to heat, and the food remains constantly shielded from oxygen and light, which can alter its nutritional characteristics. Furthermore, studies would seem to show that the loss of vitamin C, riboflavin and thiamin would be far less with this cooking method than with other methods. It is cooking by expansion.

Stewing

  Stewing involves immersing the food directly, as in the case of boiling, into the cooking liquid, which in this case may be wine, broth or other, enriched with seasonings, and covering the vessel so that the heat is evenlydistributed throughout the food. The main characteristic of this method is that it is cooked very slowly. It therefore takes a long time for the process to finish. The disadvantages are the same as for boiling, with the difference that nutrient losses, due to the long cooking process, can be much more pronounced. Another disadvantage concerns the seasoning fats that may be added: they definitely undergo degradation and oxidation, as do fat-soluble vitamins. Proteins usually undergo denaturation during the process. It is a mixed cooking technique.

Brazing

  It is a variant of stewing (some people incorrectly consider these two terms to be synonyms) that requires less cooking liquid (some people refer to braising as cooking with a lot of liquid and stewing as cooking with little liquid): whereas, in fact, in the case of stewing the food is totally immersed in the cookingliquid, braising requires that it not be covered more than halfway by the liquid. In the latter case, it is also more common for it to be enriched with vegetables, spices and herbs. The disadvantages are the same as for stewing, but the nutrient losses are less, since there is less cooking liquid. Proteins are also denatured in this case. It is a mixed cooking technique.

Glazing

  It is a variation of stewing and braising that involves the use of a cookingliquid with a syrupy consistency consisting of fats, liquids and sugars. Same advantages and disadvantages as stewing or braising depending on the amount of cooking liquid. It is a mixed cooking technique.

Sautéed cooking

  Sautéing involves cooking the food in a small amount of oil or fat in a pan over low heat. This cooking method requires very short cooking times (no more than a few minutes). Sauté cooking assumes an increase in the fat content of the food. It is cooking by concentration.

Frying

  This is the cooking method that is performed by total immersion of the food in boilingmelted oil or fat. The only advantage of this method of cooking is the low loss of nutrients to which it leads (vitamin A, due to its high degree of unsaturation, can be destroyed during this type of cooking), while the disadvantages are various: firstly, this method leads to a large increase in the fat load, with all the health and fitness disadvantages that this entails. Furthermore, while all the thermolabile vitamins in the food are totally destroyed, when the cooking fat reaches a certain temperature, called the smoke point and different from product to product, the formation of acrolein, a hepatotoxic substance, takes place. This is concomitant with the release of volatile substances in the form of whitish smoke. A handbook with the various smoke points of different fats (to be considered as indicative) is made available here.

Moriondo (2010) recommends the following precautions to take when preparing fried food:

1. Change frying oils often: if the fryer works hard, a change may be necessary after two or three days;
2. Keep the fryer clean: filter the oil daily, clean the filter and oil pan thoroughly. Carbonised crusts, viscous oily residues or the remains of old oil accelerate the deterioration of new oil. It is therefore a good idea to clean thoroughly as often as possible;
3. do not overheat: temperatures above 180°C clearly accelerate oil degradation. Use a controlled thermostat if possible;
4. do not leave the fryer on hot for longer than necessary: the habit of keeping the fryer on hot, for convenience’s sake, beyond the indispensable time, significantly worsens the quality of the oil;
5. Monitor the quality of the oil: a heavily used oil is recognisable by its increased heat, viscosity and tendency to produce smoke during frying;
6. do not ‘renew’ used oil by adding fresh oil: in contact with used oil, fresh oil spoils more quickly;
7. choose suitable types of oil: only use edible oils or fats that resist heating. The addition of stabilising additives cannot prevent or appreciably retard the deterioration of an oil;
8. Treat the food to be fried properly: salt, water, spices, meat juice, etc. shorten the life of an oil used for frying. Therefore, it is necessary to put as dry and untreated food as possible into the fryer.

It is cooking by concentration.

Baking in the ashes

  Cooking in ash requires the food, cleaned and possibly seasoned and flavoured, to be wrapped in foil and placed in the embers of an oven or barbecue. This type of cooking is nutritionally advisable. It is an airtight cooking process.

Baking

  Baking is one of the most recommendable: during baking, the heat produced by the oven is transmitted to the food through the air. Baking causes the water in the surface layers that make up the food to evaporate, leading to the formation of a crust. It has the ability to retain nutrients that would otherwise be lost during the cooking process. Baking therefore has the great advantage of limiting the loss of nutrients. Care must be taken, however, not to keep the food in the oven for too long and/or at too high a temperature, since in this case the food could partially char, causing the thermal degradation of the nutrients it contains and thus giving rise to the formation of substances which, under certain conditions, could even be toxic. There are particular types of baking which I will list and illustrate below:

1. cooking au gratin: involves sprinkling the surface of the food with a special preparation, called gratin, which will help make the food, after cooking, more golden and crispy. It is a cooking by expansion;
2. foil cooking: this involves placing the food, after being seasoned and flavoured (and, in some cases, pre-cooked), inside a sealed foil or baking paper wrapper and then baking it in the oven at a fairly high temperature (200-220°C). This is hermetic cooking;
3. cooking in a salt crust: this requires the food to be completely covered with coarse salt, moistened and possibly also sprinkled with a little fine salt, egg white or flour. It is then baked in the oven at temperatures between 200°C and 220°C. It is an airtight bake;
4. baking in a pastry crust: this involves the food, almost always pre-cooked and with the addition of particular ingredients (usually valuable), being wrapped in a special pastry, such as puff pastry, brisée pastry or bread dough, and then baked in the oven. It is hermetically cooked.

Roasting

  This method of cooking involves placing the food directly in contact with a red-hot surface. In this way, the heat gradually spreads from the outside to the inside, which will always remain less hot than the outside. The disadvantages of this cooking method are considerable: a considerable amount of sugars and amino acids are lost due to the Maillard reaction (identifiable by the black crust that forms on the surface of the foodstuff), especially in the surfacelayers of the foodstuff, a large proportion of thermolabile vitamins are destroyed and lipids may decompose due to the heat. In addition, the partial carbonisation of the superficial layers of food results in the formation of toxic compounds, which result from the degradation and thermal decomposition of proteins, sugars and lipids. To minimise this phenomenon, the food should always be kept moist during cooking. A characteristic of roasted food is the partial loss of its lipid potential, since the fats it contains usually tend to diffuse outwards. The advantages of this method include increased protein and carbohydrate digestibility. It is cooking by concentration.

Microwave use

  One can consider the use of the microwave oven as the technological cooking par excellence. It exploits electromagnetic waves. The possibility of using electromagnetic waves to cook food was a chance discovery: in the 1940s, a researcher left his sandwich for a while on a device that emitted waves of this type and, after a few minutes, taking the sandwich back, he realised that it was cooked. The microwave oven consists of a special device, called a magnetron, which is capable of generating electromagnetic fields. They produce a vibration of the water molecules in the food, which are subjected to friction and consequently to heat production, which propagates from the inside to the outside of the food (contrary to the common oven). Commonly used appliances use frequencies between 915mHz and 2450mHz and power between 400W and 900W. The advantages of this method are clearly of an organisational nature, since it greatly reduces food preparation time, while as far as the disadvantages are concerned, firstly, since microwave cooking is based on a movement of water molecules and since this substance is not uniformly distributed within the food, it is possible for the cooking process to present a lack of uniformity, with some areas being cooked and others half-cooked or raw. Furthermore, if the food is left inside the microwave oven for too long and/or at too high a temperature, all the water in the food will eventually evaporate. Temperatures will therefore rise and nutrients will undergo degradation processes. Finally, it is not possible to roast or brown food using this appliance.

Steaming

  Steaming consists of superimposing on a vessel filled with boilingwater a second vessel, the bottom of which is made of steam-permeable material or a thin grid. Inside this second vessel is placed the food, which is insulated from the environment with a lid. The cooking of the food is caused by the hot steam that spreads from the bottom container to the top one. As it is a very gentle method of cooking, it has a few disadvantages: the first, of an organisational nature, is that the cooking time is relatively long, since the temperature of the steam is lower than that of the cooking media covered by alternative methods; secondly, this method is not suitable for cooking meat, which needs to be exposed to rather high temperatures from the start in order to be well cooked. The advantages of steaming certainly include the fact that the loss of nutrients, especially vitamins, is far less than with other cooking methods. It is cooking by expansion.

Grilling

  Grilling involves placing the food on a grill structure and placing it on top of a heat source such as burning wood or a barbecue. The most important advantage of this type of cooking lies in the fact that it causes the loss of lipids from foods that are rich in them, especially meat: when cooking meat foods, the blood contained therein coagulates, releasing water, the bonds are broken and the fat melts, dripping from the grill. Disadvantages include the loss of nutrients such as vitamins and minerals, as well as the production, as a result of the burning of the fatty parts, of substances with a known carcinogenic effect, such as benzopyrene (if the grill is thoroughly washed after each use, this production is reduced). It is cooking by concentration.

Vacuum cooking

  Vacuum cooking is a special type of cooking used exclusively in industrial catering. It presupposes that the food is placed in a vacuum and cooked, inside the same casing with which it will later be marketed, using various techniques (wet, oven, steam, bain-marie, microwave) at a temperature of between 65°C and 97°C for a time to ensure that the food reaches a core temperature of at least 72°C for no less than 2 minutes. This method guarantees the destruction of many pathogenic microorganisms that may be inside the food (not all of them), although spores survive. At the end of cooking, the food is immediately and very quickly cooled and then marketed. Food produced in this way retains extremely good nutritional and organoleptic characteristics, as there is no contact between the food and the cooking medium, and it has a very long shelf life.