The determination of cooling time is an element of practical importance because it allows time to find a product to achieve a given temperature in the thermal center starting from an initial temperature, a cooling medium temperature, geometry, type of packaging, etc. This result can be used in the calculation of the product load for the thermal load.
One way this time can be determined is by the graphical method. Graphs for each of the simple geometric shapes (e.g., spheres, parallelepipeds and cylinders) are related to a temperature factor, the Fourier number that relates the thermal diffusivity, the size of the product and the cooling time, and the Biot number, which relates to the heat transfer coefficient, conductivity and thickness of the product.
The method described above assumes that the heat transfer is in unidirectional. When the heat transfer takes place in more than one direction, obtaining the time leads to infinite possibilities, making necessary to limit it to only the first of its terms. In practical terms, tables and figures have been prepared that quickly and easily determine the cooling time.
This method is based on a combination of unidirectional heat transfer developed in simple geometric shapes such as sphere, cylinder and parallelepiped. Thus, for a cylinder of finite length where the heat transfer takes place in the radial and longitudinal directions, the method combines the solution of the first cylinder and the plane for the second. In the case of rectangular solutions, three planes are combined.
The latter will provide more accurate results as the geometric figure is closer to a regular figure. It illustrates the application of these methods to different systems.
Characteristics of Water
Water is the most abundant element in most foods in their natural state. Therefore, water plays an essential role in the structure and other characteristics of products of plant and animal origin.
Water in food can be free standing or bound with other elements, sometimes in a complex way. That is why the state of water in a food is important to its stability and its entire contents, since it impacts the ability to decay.
Water is a solvent for many chemical species that can be disseminated and react with each other. Water can also expand and participate in various reactions, especially hydrolysis. The introduction of different chemical species into water in a solution or colloidal suspension leads to the so-called colligative properties which, depending on the number of molecules present, include the decrease in vapor pressure, boiling point elevation, freezing point depression, decreased surface tension, increasing viscosity and osmotic pressure gradients across semi-permeable membranes, among others.
Colligative properties determine the behavior of food. Water molecules in the solid state are linked together by hydrogen bonds, giving rise to the formation of a polymer crystalline structure in which each molecule is linked to four others.
Various agents have different influence on the water structure. For example, electrolytes such as Na+, K+, Cl-, hydrated strongly in a solution, decrease the number of hydrogen bonds between water molecules. Substances in a solution can form hydrogen bonds themselves and can modify the association between water molecules according to their compatibility with the existing geometry network.
The water in turn modifies properties such as structure, diffusion, reactivity, and so forth. Of substances in a solution, water activity is a measure of the degree of water availability in various foods, which is defined by the decrease in partial pressure of water vapor:
aw = pw / po
where “pw” is the partial pressure of water vapor from the food and “po” is the vapor pressure of pure water at the same temperature.
Water activity is a relative measure with respect to a standard state taken as a comparison. The standard state selected is pure water, which is considered the unit, so the activity of a food is always less than this unit. This is because the chemical species present in the product diminish the ability of water to vaporize.