When placing food in the refrigerator, we rarely think about complex physical processes, unfolding inside the package. However, it is these processes that determine how long food will remain fresh and safe. The basis of storage is a change in the thermodynamic state of a substance, which directly affects biochemical reactions.
Internal energy is the sum of the kinetic energy of the movement of molecules and the potential energy of their interaction. As the temperature decreases, the speed of particle movement decreases, which is what we record as cooling. In this article we will analyze in detail how and through what mechanisms energy transformation occurs in various types of products.
The physical essence of internal energy during cooling
Any food product is a complex thermodynamic system. When you place a warm watermelon or a freshly prepared dish on the refrigerator shelf, the heat exchange process begins. The internal energy of the system U consists of the energy of the chaotic movement of molecules and the energy of their interaction with each other. As the temperature decreases, the average kinetic energy of molecules decreases.
According to the first law of thermodynamics, the change in internal energy is equal to the sum of the work of external forces and the amount of heat transferred. Under refrigerator conditions, external work is usually not performed on the product (the volume of solids and liquids changes slightly), so the change in energy occurs solely due to heat transfer. The product gives off heat to the colder air of the refrigeration chamber.
It is important to understand that the decrease in internal energy does not occur instantly. The speed of this process depends on the thermal conductivity of the product itself and the temperature difference between the product and the environment. The greater this difference, the more intense the heat exchange in the first minutes.
⚠️ Attention: Do not place hot foods directly into the refrigerator. A sharp jump in temperature can disrupt the operation of the compressor and lead to a local increase in temperature around other products, accelerating their deterioration.
The process of heat transfer continues until the temperatures of the product and the air in the chamber are equalized. This state is called thermodynamic equilibrium. At this moment, the change in internal energy stops, and the product retains its properties until the storage conditions are violated.
Heat exchange mechanisms in the refrigeration chamber
In order for the internal energy of the product to decrease, heat must be effectively removed from it. In household refrigerators, this process is implemented through several mechanisms. The main way of transferring heat from the product to the walls of the evaporator or the air is convection.
The air, cooling at the evaporator, becomes heavier and falls down, displacing warmer air masses upward. This circulation ensures uniform cooling of the chamber volume. In models with a system No Frost this process is enhanced by forced air circulation using fans, which significantly accelerates the reduction in the internal energy of products.
- 🌬️ Convection: heat transfer by air or liquid flows washing the product.
- ❄️ Radiation: heat transfer in the form of electromagnetic waves (infrared radiation), relevant for products on open shelves.
- 🧊 Thermal conductivity: energy transfer during direct contact of the product with a shelf or other cold surface.
The second important mechanism is thermal conductivity inside the product itself. The outer layers cool faster than the inner ones. Heat from the center of the product flows to the surface and is then released to the environment. This is why large pieces of meat or whole poultry carcasses require more time to stabilize internal energy.
The third mechanism is the evaporation of moisture from the surface of the product. During evaporation, the product loses its most energetic water molecules, which also helps to reduce its internal energy. However, this process can lead to weathering and drying out, which is why sealed packaging is so important.
The influence of phase transitions on the energy of products
The case of particular interest is when the temperature of the product drops below the freezing point of water (0°C). At this moment, a first-order phase transition—crystallization—begins. The internal energy continues to decrease, but the temperature of the product remains constant until all the water turns into ice.
The energy that is removed from the product during this period is called latent heat of fusion (or crystallization). It is spent on restructuring the crystal lattice of water molecules. Only after complete freezing of the bulk of the moisture, the temperature of the product begins to fall again, and the internal energy decreases due to a decrease in the kinetic energy of ice molecules.
| Type of product | Temperature of the onset of crystallization | Features of energy change |
|---|---|---|
| Clean water | 0°C | A sharp jump in energy output at a constant temperature |
| Meat and fish | -1...-2°C | Gradual crystallization due to dissolved salts |
| Vegetables and fruits | -1...-3°C | Risk of damage cells with ice crystals |
| Fats and oils | Various | They freeze at lower temperatures, change structure |
For products with a high content of dissolved substances (sugar, salt), the freezing point is lower than that of pure water. This phenomenon is called cryoscopic effect. The internal energy of such systems changes more smoothly over a wide range of temperatures.
Why is ice cream soft in the freezer?
Ice cream remains soft at subzero temperatures due to the high content of sugar and fats, which lower the freezing point of water, as well as due to the whipped structure containing a lot of air.>
The role of humidity and packaging in energy conservation
Packaging plays a critical role in managing the processes of change in internal energy. It creates additional thermal resistance, slowing down heat transfer. On the one hand, this increases the initial cooling time, on the other hand, it stabilizes the temperature when the door is opened briefly.
The most important aspect is humidity control. If the product is not packaged, water from its surface evaporates, taking with it a significant amount of internal energy. This can lead to drying out. In a sealed container, a dynamic equilibrium is established between the liquid phase and steam, and mass loss stops.
- 📦 Vacuum packaging: almost completely eliminates heat transfer by convection and evaporation, keeping the internal energy stable longer.
- 💨 Perforated bags: allows vegetables to “breathe”, removing ethylene, but slow down the loss of moisture.
- 🥫 Glass containers: has low thermal conductivity, which protects the contents from sudden temperature changes.
It should be taken into account that different materials have different effects on speed temperature changes. Metal trays, which are sometimes found in professional equipment, speed up heat removal due to high thermal conductivity. Plastic shelves, on the contrary, act as insulators.
⚠️ Attention: Condensation inside the package can become a breeding ground for bacteria. Make sure the product is sufficiently cool before sealing to avoid a "steam bath" effect that will speed up spoilage.
Thermodynamics of defrosting and temperature fluctuations
The life cycle of a product in the refrigerator is not always linear. Periodic defrosting of the refrigerator or temperature fluctuations due to frequent opening of the door lead to cyclical changes in internal energy. As the ambient temperature rises, the product begins to absorb heat and its internal energy increases.
This process is dangerous because upon re-freezing, large ice crystals are formed, which destroy the cellular structure of the products. The internal energy of the system changes, but the quality of the product is irretrievably lost. Biochemical reactions stopped by cold are resumed with each defrosting cycle.
☑️ Checking storage conditions
Modern refrigerators are equipped with stabilization systems that minimize these fluctuations. Temperature sensors monitor changes and adjust the operation of the compressor. However, the physics of the process is such that the surface layers of the product react to changes faster than the central ones.
If the temperature in the chamber rises above the critical point, the internal energy of the molecules becomes sufficient to activate enzymatic processes. This leads to the beginning of damage. Therefore, maintaining a stable low temperature is more important than extremely low values.
Practical recommendations for optimizing storage
Understanding the physics of processes allows you to optimize the storage of products. In order for the change in internal energy to occur most efficiently and safely, the products should be positioned correctly in the chamber. Cold air is heavier than warm air, so the temperature at the bottom of the refrigerator is usually lower.
Do not overcrowd the refrigerator. For effective convection, air must circulate freely around the food. If the shelves are filled to capacity, “heat pockets” are formed where the temperature remains high and the internal energy of the products does not decrease to the required level.
- 🥩 Meat and fish: store on the bottom shelf, where it is coldest, to slow down the growth of bacteria.
- 🥛 Dairy products: it is better to place on the middle shelves, away from doors where the temperature fluctuates.
- 🥬 Vegetables: require high humidity and a temperature just above zero, so they are ideal for lower drawers.
It is also important to consider the thermal load. If you load a lot of warm food at once, the compressor will have to work harder to compensate for the influx of internal energy. This can lead to a temporary increase in temperature throughout the entire volume of the chamber.
Conclusion and main conclusions
Changing the internal energy of food in the refrigerator is a fundamental physical process underlying modern food storage. Reducing the kinetic energy of molecules allows you to slow down biochemical reactions and keep products fresh.
The effectiveness of this process depends on many factors: temperature, humidity, packaging and location of products. Understanding these principles helps not only save energy, but also preserve the maximum benefit and taste of food.
Remember that a refrigerator is a complex thermodynamic unit, and its correct use requires attention to detail. Following simple rules of heat transfer will ensure that your products remain of high quality for as long as possible.
Why does internal energy change more slowly in the center of the product?
This is due to the low thermal conductivity of most food products. Heat must be transferred from molecule to molecule from the surface to the center, which takes time. The outer layers cool quickly, creating a temperature gradient.
Does the color of the package affect the change in internal energy?
Yes, but only slightly in refrigerator conditions. Dark surfaces absorb and emit heat better (infrared radiation), but the main cooling mechanism is still air convection, where color plays a secondary role.
What happens to the internal energy during defrosting?
During defrosting, the product absorbs heat from the environment. Its internal energy increases: first, the kinetic energy of the molecules (heating) increases, then the energy goes to breaking bonds when melting the ice, and the temperature rises again.
Is it possible to cool the product faster if you turn on the "Super Freeze" mode?
Yes, this mode forces the compressor to work continuously, lowering the temperature in the chamber below normal. This increases the temperature difference between the product and the environment, accelerating heat transfer and reducing internal energy.
Why can’t food be stored in aluminum foil without packaging?
Aluminum has high thermal conductivity, which is good for cooling, but it does not protect against drying out. In addition, acidic foods can react with metal, changing the taste and composition, although this has little effect on the internal energy.