Changes in Food during Storage

Degradation reactions are affected by storage of frozen products. Chemical and biochemical changes during frozen storage take place slowly. If the enzymes were not inactivated previously, disruption of cell membranes by ice crystals may favor this action. Changes include: degradation of pigments, vitamin loss, residual enzyme activity and lipid oxidation.

 

Recrystalization of ice is a phenomenon where the average size of the ice crystals increases because larger ice crystals grow in place of the smaller ones from before. This increases the surface energy that is created by the crystals in contact with each other. However, the highest incidence of recrystalization is migratory, resulting from fluctuations in storage temperature.

 

The temperature of the frozen product is increased by the partial thawing of the crystals. If, after that, the temperature drops again, the freezing of the melted water does not create new crystal nuclei, but rather increases the growth in existing crystals. This causes a loss of product quality similar to that which would occur if the thawing had been slow.

Knowledge of freezing time is of great importance in the design of the process. The information is needed to determine the cooling rate required in relation to the capacity of the freezing system.

 

The prediction of freezing time may be based on numerical methods and on approximate methods. Numerical methods are based on solving the general differential equation of energy. Approximate, or analytical, methods take into account simplifications in solving the differential equation.

 

The first approximate solution proposal corresponds to the Plank equation, which takes into account a number of assumptions. Despite its limitations, this equation has been widely used and many of the subsequent equations that have been developed are modifications of it.

When food is thawed, the surface layer of ice melts, forming a layer of liquid whose thermal properties are inferior to those of water in solid state. As a result the rate at which heat is transferred into the food changes, increasing the insulating effect to the extent that the layer slows the thawing. That is why the thawing of a food at the same temperature gradient is slower than freezing.

 

The cellular damage caused by slow freezing and recrystallization lead to loss of cellular components, which manifests as an exudate in which various compounds lose nutritional value.

 

Thawing should be designed to minimize the following phenomena: microbial growth, fluid loss, dehydration losses and losses from spoilage reactions.

 

Defrosting usually takes place at a temperature slightly above the point of thawing.

 

As indicated in advance, the prolonged maintenance of the product at temperatures slightly below 0 ° C is unfavorable because the product is exposed to relatively high concentrations of solutes and promotes the development of psychrophilic microorganisms.