Freezing represents a preservation process for food where the product temperature is decreased to a temperature range resulting in the formation of ice crystals within the product structure. The purpose of the process is to reduce the temperature of the product as much as is economically feasible in an effort to reduce product quality deterioration reaction rates within the product.
The engineering aspects of the freezing process are numerous.
They deal with the computation of refrigeration requirements needed to accomplish the desired reductions in product temperature.
Involve removal of thermal energy in the form of both sensible and latent heats.
The design of processes for food freezing requires knowledge of the time needed to reduce product temperature to desired levels.
The efficient design of frozen food storage requires knowledge of changes occurring within the food product during the time it is exposed to the environmental conditions within the storage systems.
Science behind food freezing
Thermodynamics can be used to describe the physical changes in water within a food product during the freezing process.
the decrease in product temperature during freezing occurs gradually as the latent heat of fusion is removed from water within the product.
The food freezing process has two unique characteristics compared to the freezing of pure water.
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The equilibrium temperature for initial formation of ice crystals is lower than the equilibrium temperature for ice crystal formation in pure water. Although supercooling may occur in the product before the initial ice crystal is formed, the temperature will be below that of a pure water system. The magnitude of the depression in equilibrium freezing temperature is a function of product composition.
In simple terms
when water freezes and forms ice crystals, the temperature at which this happens is usually lower than the temperature at which pure water freezes. This means that the water can become colder than its freezing point without actually turning into ice. This phenomenon is called supercooling.
Now, when we have other substances or impurities present in the water, such as salts or minerals, these can affect the freezing process. They can lower the temperature at which the first ice crystal begins to form. So, even if the water is supercooled, it will freeze at a lower temperature compared to pure water.
The amount by which the freezing temperature is lowered depends on the composition of the substances present in the water. Different impurities can have different effects on the freezing temperature. This depression in the freezing temperature caused by impurities is called the depression in equilibrium freezing temperature.
2. The second difference between freezing of the food product, as compared to pure water, occurs after the initial ice crystals are formed. In the food product, the removal of phase change energy occurs gradually over a range of decreasing product temperatures. The temperature–time relationship during phase changer is a function of the percent water frozen at any time during the freezing process.
In simple terms
The temperature and time relationship during this phase change is influenced by how much water in the food product has frozen at any given time. If only a small percentage of the water has frozen, the temperature will decrease slowly. However, as more and more water freezes, the temperature will continue to decrease until the entire product is frozen.
PROPERTIES OF FROZEN FOODS
The food product properties of interest when considering the freezing process include density, specific heat, thermal conductivity, enthalpy, and latent heat. These properties must be considered in the estimation of the refrigeration capacity for the freezing system and the computation of freezing times needed to assure adequate residence times.
PRODUCT DENSITY
when a liquid substance freezes, its density tends to increase. This is because as the substance cools and forms a solid structure (such as ice crystals), the molecules become more closely packed together, resulting in a higher density compared to the liquid state.
it's important to note that not all substances follow this pattern. Some substances, such as water, actually have a lower density in their solid state (ice) compared to their liquid state. This is why ice floats in water. When water freezes, the molecules arrange themselves in a way that creates empty spaces, making the ice less dense than the liquid water.
PRODUCT SPECIFIC HEAT
The specific heat of a product refers to its ability to absorb or release heat energy. During the freezing process, the specific heat of a product can also be influenced.
When a substance undergoes phase change from a liquid to a solid, such as during freezing, there is typically a change in specific heat. Specifically, the specific heat of a substance tends to decrease as it transitions from a liquid to a solid state.
In simple terms, as a product freezes, it requires less heat energy to change its temperature compared to when it was in its liquid state. This is because the energy is now being used for the phase change from liquid to solid rather than just increasing the temperature.
The change in specific heat during freezing is often associated with the release of latent heat. Latent heat refers to the energy involved in the phase change process itself, rather than changing the temperature of the substance.
THERMAL CONDUCTIVITY
the thermal conductivity of a substance tends to decrease as it transitions from a liquid to a solid state. This means that the frozen state of a product typically has lower thermal conductivity compared to its liquid state.
When a substance freezes, its molecular structure changes as it transitions from a disordered liquid state to a more ordered solid state. This change in structure can result in a reduction of thermal conductivity. The molecules in the solid state are usually more closely packed together, creating fewer pathways for heat to transfer.
As a result, during freezing, the product's ability to conduct heat may decrease. This can affect the freezing process itself, as slower heat transfer within the product can result in longer freezing times.
PRODUCT ENTHALPY
The enthalpy of a product refers to the amount of heat energy absorbed or released during a process, such as freezing. In the case of freezing, there are changes in the enthalpy of the product.
During the freezing process, the enthalpy of a substance decreases. This means that heat energy is being released from the product as it transitions from a liquid to a solid state.
When a substance freezes, it undergoes a phase change where the molecular arrangement changes from a disordered liquid state to an ordered solid state. This phase change involves the release of latent heat, which is the energy required to change the state of the substance without changing its temperature.
The release of latent heat during freezing results in a decrease in the enthalpy of the product. The heat energy that was initially absorbed by the product to maintain its liquid state is now being released as the product freezes and transitions to a solid state.
Some aspects of Freezing will be discussed further.
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