The "cooking" of foodClassification of heat treatments Cooking methods Different temperature / time ratios Nutritional aspects of heat treatments
- Classification of heat treatments
- Cooking mode
- Different temperature / time ratios
- Nutritional aspects of heat treatments
We have quoted the word cooking in the title because the systems with which man supplies heat to food are many and different from each other. However, it should be noted that heat, in itself, is not a food preservation technique and indeed makes them even more defenseless against microbial aggressions and, therefore, more perishable. It is rather an excellent "sanitizer" because it inactivates microorganisms, their toxins or any chemical compounds that can make a raw food harmful.
Foods treated with heat are usually kept longer than raw materials because another essential feature is always associated with them: packaging in a more or less hermetically sealed container. In short, it is a combined action:
- the heat destroys the microorganisms present in the raw food and inactivates the enzymes that can make the product go bad;
- the hermetically sealed container separates the food from the rest of the world and prevents altering or pathogenic environmental microorganisms from reaching it again and making it go bad.
Take for example the preserves: they can last unchanged for years (not less than 5, but they can go beyond 10), they remain stable at room temperature, but when you open the package it is advisable to consume the product as soon as possible or alternatively, store it in the refrigerator.
Not all microorganisms and dangerous chemical residues are equally sensitive to heat (we speak of thermolabile and, conversely, thermostable compounds). In order for heat to perform its functions well, the food must be brought to sufficiently high temperatures and remain there for the right time. Therefore the effectiveness of heat treatments applied to food is always measured with a temperature / time ratio. In general, the higher the temperature reached, the shorter the time required to obtain the "sanitizing" effect. Let's take as an example the classic pasteurization of raw milk, which can be done in two ways:
- bringing it to 65 ° C and keeping it at that temperature for 15-25 minutes;
- heating it to not less than 72 ° C and holding it for 15 seconds.
Based on accurate experimental tests, it can be said that from the microbiological point of view the results obtained with the two systems are the same, that is, it is possible to effectively inactivate the main microorganisms acting as food-borne diseases. What changes is the impact of heat on milk macro- and micronutrients. Other experimental data confirm, in fact, that between the two systems, the one that allows to maintain the nutritional characteristics of the milk is the second, defined as high pasteurization.
A fundamental concept emerges: among heat treatments, in general, the nutritional value is less influenced by those in which even very high temperatures are reached, but for very short times. Conversely, a heat treatment that operates at lower temperatures, but for longer times, adversely affects much more on the nutritional factors.
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Classification of heat treatments
The heat is transmitted in four different ways: 1) by direct contact, 2) by radiation, 3) by convention, 4) by microwave. Each of them can influence in its own way the hygiene and sensorial characteristics of the product. In the first three, the heat always diffuses in a centripetal direction from the outside to the inside. In the case of microwaves, however, it is generated simultaneously in all points.
Heating by direct contact It is obtained by putting the food in direct contact with the heat source (flame, metal surface or heated stone). It is the system that allows the best heat transmission and is what is generally used when cooking. Pasteurization and Uterization (UHT method) of milk and other drinks exploit this method: liquid food is in fact made to flow in direct contact with heated metal surfaces.
Heating by radiation Heat is transmitted by infrared rays from a heat source; the food is not in direct contact with this source, but is placed very close to it because infrared rays travel only in a straight line and lose a lot of effectiveness with increasing distances. So in this type of cooking it is necessary to make sure that the food rotates on itself to expose its entire surface to radiation. The classic example is grilling roast chickens or kebabs. After heating by direct contact, it is the most effective method of heat delivery.
Convection heating A fluid, which can consist of air, water or fats, transfers heat from a heated surface to the food; think of baking bread in the oven, boiling meat or frying fish or vegetables in oil.
Microwave heating It is a cooking mode that differs greatly from the previous ones in how heat is generated in the food. Microwaves are very high frequency electromagnetic waves; when they hit a food, they put (let's say) the molecules in vibration, in particular the smaller ones, that is, those of water, if this is not linked to salt or other components. The more intense and powerful the microwaves are, the more the water molecules vibrate; in doing so, part of their energy is transformed into heat. Consequently, in this type of cooking, the heat does not penetrate from the outside towards the inside of the food, but is instantly generated in all points. The problem is that in solid foods the water is not evenly distributed, so the heat is not generated with the same intensity everywhere: this is why it is said that it forms in the food in an uneven way, but "in leopard spots" "And that, therefore, microwaves cannot always effectively inactivate dangerous or altering microorganisms. This statement is only partially true: if, after the microwave treatment, the heat is left to spread evenly in the food, the differences in temperature that occurred at first will balance out within a few minutes. The diffusion of heat in the food heated by microwave will be even faster by keeping the food moving during cooking and immediately afterwards; that's why all microwave ovens are equipped with a support to which a rotation movement is impressed.
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