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Freezing foods
Do you know the main processes by which food spoils? Do you know the appropriate amount of time to refrigerate food?
Subject: Refrigeration and freezing of food
The application of cold temperatures is significant because food temperature is reduced below its freezing point, meaning that a high proportion of the water contained in the product forms ice crystals.
This freezing of water into ice, and the increment in the concentration of water solutions that does not freeze, cause the reduction of the water activity in the food. Therefore, the preservation of the food in this way is the consequence of the cold temperatures and the decrease of water activity.
Not all water present in the food forms water crystals as a result of the freezing. In food, there exists a fraction of the water (up to 0.3%) that does not freeze. This type of water can be found in strongly bound molecular structures and is called bound water, which does not freeze up to -30º C.
It is believed that this water forms a monolayer that is attached to polar groups such as NH3 and the COO-protein and HO-groups of starches, among others. Bound water is between 5 and 10% of the total mass of water contained in food.
Water from this layer is very difficult to extract since it is not available to act as a solvent or reagent.
The free or unbound water, in turn, accounts for most of the water contained in food. However, this water does not come spontaneously from the tissues. This water is in the form of gels in the interior of the cell and in the intercellular spaces, with retention influenced by pH and ionic strengths.
During freezing, this water is removed from its normal position within the tissues and converted into ice. This process is partially reversed during the thawing resulting in the formation of exudates; this increase in the concentration of cell contents can create undesirable products.
Freezing Curve
The freezing process in food is more complex than just the freezing of pure water. Foods contain other dissolved solutes in addition to water, which display a similar behavior when frozen.
The temperature evolution with time during the freezing process is called freezing curve. The freezing curve of a typical solution is shown in the figure below.
This curve has the following sections:
AS: The food is cooled below its freezing point of 0º C. At point S, which corresponds to a temperature below freezing, the water remains liquid. This subcooling can be up to 10 ° C below the freezing point.
SB: The temperature rises rapidly to the freezing point, because ice crystals will form and at a higher speed release latent heat, which is extracted by freezing the food.
BC: The heat is removed at the same rate as in previous phases, eliminating the latent heat with the formation of ice and the temperature remaining constant. Increasing concentration of solutes in the unfrozen water fraction causes the decrease of freezing point, so the temperature decreases. This is the phase in which most of the ice forms.
CD: One of the solutes reaches saturation and crystallizes. The release of the corresponding latent heat increases the eutectic temperature of the solute.
DE: The crystallization of water and solutes continues.
EF: The temperature falls for the mixture of water and ice.
In fact, the freezing curve for food is somewhat different from simple solutions, with the rate at which freezing occurs for food being higher.
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