Freezing Rate

The quality of frozen foods is influenced by the rate at which freezing occurs. Various quality characteristics are related to the size of the crystals, which is a consequence of the speed at which freezing occurs. The main effect of freezing on the quality of food is the damage caused in cells from the growth of ice crystals. Freezing has virtually no effect from a nutritional point of view.

 

The resistance of various animal and plant tissues to freezing is very diverse. Thus, fruits and vegetables, for example, have a very rigid structure so that the formation of ice crystals can affect them more easily than the meat.

 

The freezing of tissues is initiated by the crystallization of water into the intercellular space since the concentration of solutes is lower than in the intracellular spaces.

 

When freezing is slow, intercellular crystallization increases the local concentration of solutes which causes, by osmosis, progressive dehydration of the cells. In this situation, large ice crystals will form, increasing the intercellular spaces, while cells’ plasmolyzed volume decreases considerably. This movement of water and mechanical action of ice crystals on the cell walls cause effects on the texture and give rise to the appearance of exudates during thawing.

 

When the freezing is rapid, crystallization occurs almost simultaneously in the intercellular and intracellular spaces. The water displacement is small, producing a large number of small crystals. Therefore the damages on the product were considerably lower compared with slow freezing. However, very high speeds of freezing in some foods can create internal tensions that can cause cracking or breaking of their tissues.

 

There are different ways of defining the rate of freezing, including: the characteristic time of freezing, the nominal duration of freezing and the average speed of freezing.

The freezing causes the increased concentration of solutes present. The speed of reaction increases despite the decrease in temperature and despite the laws of action of masses. This increase in speed of reaction is between -5 º C and -15 º C.

 

The increase in the concentration of solutes causes changes in viscosity, pH, redox potential of the unfrozen liquid, ionic strength, osmotic pressure and surface tension, among others. The action of these factors, associated with the effect of the disappearance of a portion of liquid water, causes adverse changes in the food. One example is the aggregation of proteins. These effects may be limited when the passage through the aforementioned temperature range is done quickly. This range is known as the danger area or critical area.

 

Because the volume of ice is higher than that of liquid water, the freezing of food causes it to dilate. This expansion may vary in correspondence with the water content, the cellular arrangement, the concentration of solutes and the freezing temperature.

 

These variations arise from internal stresses and can cause a tissue volume of great magnitude, which can cause internal tears (and even complete rupture of the plant tissues) leading to fluid loss during thawing.

 

The main effect of freezing on food is the damage in the cells caused by the growth of ice crystals. When the freezing rate is slow, the ice crystals grow in the intercellular spaces, which deform and break the contacting walls of the cells. The vapor pressure of ice crystals is lower than inside the cells, leading to progressive dehydration of the cells by osmosis and thickening of the ice crystals. This in turn leads to enlargement of the ice crystals and increase in the intercellular spaces.

 

Plasmolyzed cells, however, decrease considerably in size. This cellular dehydration decreases the chances of an intracellular nucleation. The breakdown of cell walls results from the mechanical action of the large ice crystals and excessive cell shrinkage.

 

During thawing, the cells are unable to recover their original shape and firmness. The food is softened and cellular material is lost through leakage. The expulsion of the contents of a cell can cause contact between enzymes and their substrates sometimes found in separate compartments. For example, polyphenol oxidases and polyphenols in foods not previously treated by scalding cause an acceleration of enzymatic browning during the thawing and during storage.