When you place warm food in the fridge compartment, complex physical processes occur with them that can be described by the laws of thermodynamics. The main change concerns internal energy substance, which directly depends on the temperature and state of aggregation of the molecules. The moment it enters the chamber, an active process of heat exchange begins between the body of the product and the environment.
Internal energy consists of the kinetic energy of the movement of molecules and the potential energy of their interaction. Since temperature is a measure of average kinetic energy, its decrease inevitably leads to a decrease in this indicator. The products give off heat to the walls of the evaporator, and their molecules begin to move more slowly, which we perceive as cooling.
It is important to understand that this process does not occur instantly and depends on the thermal conductivity of the product itself. The change in internal energy is negative (ΔU < 0), since the system releases energy into the environment of the refrigerator. This is the fundamental principle underlying the operation of any household cooler, be it an old one ZIL or modern Smart refrigerator.
Thermodynamic nature of cooling
From the point of view physicists, placing a product in a refrigerator is a classic example of an irreversible heat transfer process. Heat spontaneously transfers from a hotter body (product) to a colder one (air in the chamber). Internal energy of the system decreases exactly as much as the heat was given off, if you do not take into account the work of external forces, which in this case is zero for the product itself.
The rate of this process is described by the law Fourier thermal conductivity. The greater the temperature difference between the product and the air in the chamber, the more intense the heat exchange at the initial moment. However, as it cools, the temperature gradient drops and the process slows down, approaching the state of thermodynamic equilibrium.
⚠️ Attention: Do not put excessively hot foods in the refrigerator. A sharp jump in temperature can disrupt the operating mode compressor and lead to a local increase in temperature around other products, disrupting their storage mode.
Inside the chamber, the refrigerant circulating through the tubes evaporatortakes this thermal energy. It boils at low temperatures, effectively removing heat from the walls and air. Thus, the internal energy of the products is redistributed into the cooling system of the refrigeration unit.
Change in the kinetic energy of molecules
The main factor determining body temperature is the average speed of the chaotic movement of its molecules. When the product cools, kinetic energy the particles decrease. In liquids and gases contained in products (water, juices), molecules lose speed, which makes the substance more viscous or solid.
In solid products, such as meat or vegetables, the atoms of the crystal lattice begin to vibrate with a smaller amplitude. This decrease in traffic intensity directly correlates with the decrease in internal energy. Physicists call this change thermal motion, and it is its attenuation that we record with a thermometer.
- 📉 A decrease in temperature leads to a decrease in the average speed of molecules.
- 🧊 In water, the process of formation of hydrogen bonds begins when approaching 0°C.
- ⚛️ The potential energy of interaction between molecules can also change during phase transitions.
It is worth noting that different substances cool at different rates due to differences in heat capacity. Water, which is abundant in foods, has a high heat capacity, so it releases energy more slowly than, for example, fats or dry matter. This explains why a watermelon takes longer to cool than a piece of cheese.
Why do metals seem colder in the refrigerator?
Metals have high thermal conductivity. When you touch a metal shelf, it very quickly transfers heat away from your hand, creating a feeling of intense cold, although it is the same temperature as a plastic container.
Phase transitions and latent heat
Of particular interest is the process when the temperature of the product reaches the freezing point of water. At this moment, the internal energy continues to decrease, but the temperature stops falling. All released energy is spent on restructuring the crystal lattice - this process is called crystallization.
The energy that is released or absorbed during a phase transition without changing temperature is called latent heat. When food is frozen, water passes from a liquid state to a solid state (ice), and the internal energy of the system drops sharply due to a change in the potential energy of interaction of molecules, although the kinetic (temperature) remains the same.
| Process | Change T (°C) | Change U (Energy) | Transition type |
|---|---|---|---|
| Cooling above 0°C | Decreases | Decreases | No phase transition |
| Water freezing | Does not change (0°C) | Decreases sharply | Crystallization |
| Ice cooling | Decreases | Decreases | No phase transition |
| Moisture evaporation | Does not change | Increases (absorption) | Vaporation |
After complete freezing, a further decrease in temperature again leads to a drop in internal energy due to a decrease in the kinetic energy of ice molecules. Modern refrigerators with the function No Frost can speed up this process due to the active circulation of dry air, which promotes faster heat removal.
The role of humidity and heat transfer
The internal energy of products changes not only due to thermal conductivity, but also through the processes of evaporation and convection. The surface of the product, especially if it is liquid or juicy fruit, actively evaporates moisture. A significant part of the internal energy is spent on evaporation, which leads to additional cooling of the surface.
In old-type refrigerators with a drip system, the humidity is higher and evaporation occurs more slowly. In systems No Frost the air is dry, which can lead to moisture freezing out of products (sublimation) if they are not packaged. This also reduces the internal energy of the product, but due to the loss of the mass of the most energetic water molecules.
- 💧 Evaporation of moisture from the surface requires energy, cooling the product.
- 🌬️ Air convection accelerates heat exchange between the product and the walls of the chamber.
- 📦 Packaging slows down the loss moisture, maintaining the structure and weight of the product.
⚠️ Attention: Storing unpackaged products in the blowing area of the freezer will lead to “frost burn”. The surface will dry out, and the internal energy will change unevenly, which will spoil the taste.
Heat transfer also depends on the surface area. A sliced product cools faster than a whole product, since the area of contact with cold air is larger. This is important to consider when planning inventories: it is better to divide large volumes into portions before cooling.
The influence of packaging on the rate of energy change
Packaging plays the role of a thermal barrier that slows down the change in the internal energy of the product. Low thermal conductivitymaterials, such as foam or thick plastic, create resistance to heat flow. This allows products to cool more slowly, but also more evenly, preventing sudden changes in temperature.
On the other hand, metal containers or thin cling film practically do not interfere with heat transfer. In such cases, internal energy changes quickly, which can be useful for rapid cooling, but risky for long-term storage due to the risk of drying out or freezing.
☑️ Packaging rules for storage
Vacuum packaging is a special case. The absence of air (convective environment) sharply reduces the rate of heat transfer, but at the same time prevents oxidation and moisture loss. The internal energy in such products changes mainly due to thermal conductivity through the walls of the package and contact with the shelves.
Practical implications for storage
Understanding how internal energy changes helps to properly organize storage. A rapid decrease in temperature (shock freezing) allows you to create small ice crystals that do not damage the cellular structure of the product. Slow cooling leads to the formation of large crystals that tear the cells, which, when defrosted, results in loss of juice and taste.
Modern refrigerators are equipped with zones of different temperatures precisely to control these processes. In the freshness zone (Zero Zone), the temperature is maintained at about 0°C, which makes it possible to slow down the change in the internal energy of bacteria without freezing the product itself. This extends shelf life without loss of texture.
Incorrect loading of the refrigerator, when food is placed close to each other, disrupts air circulation. As a result, heat transfer slows and the internal energy in the center of the food stack decreases very slowly, creating a risk of spoilage. Always leave gaps for cool air to circulate.
Frequently asked questions (FAQ)
Why can’t a hot pan be placed in the refrigerator?
In addition to the load on the compressor, the hot product will create a powerful upward flow of warm air. This will cause condensation of moisture on other products and walls, and will also locally increase the temperature in the chamber, which can trigger the growth of bacteria in neighboring supplies.
Does water freeze instantly at 0°C?
No, at 0°C a phase transition begins, requiring the removal of a large amount of energy (latent heat of fusion). Until all the water turns into ice, the temperature of the mixture will remain at 0°C, even if the internal energy continues to decrease.
Does the material of the shelf affect cooling?
Yes, glass shelves have greater heat capacity and thermal conductivity than plastic racks. Contacting the bottom of the pan with the glass shelf will accelerate the removal of internal energy through the bottom surface of the product.
Can food be stored at positive temperatures for a long time?
Only if the temperature is close to 0°C (freshness zone). At +4...+6°C (standard shelf), the internal energy of bacterial molecules is still sufficient for active reproduction, so shelf life is limited to several days.