Fundamental Changes During Freezing

The importance of early freezing is related to the need to decrease the rate of the deterioration processes caused by chemical, biochemical, and physical reactions as well as microbiological activity. Chemical and biochemical reactions influence the product quality not during the freezing process, but during subsequent storage. The importance of quick freezing is some times argued from a sensory point of view (4). It must not, however, be forgotten that quick freezing is most important from a technical-economical-operational point of view. With reference to quality, the rate of freezing determines the size of weight loss and in some cases also the microbiologic quality of the product. The drip loss or loss of product juice on thawing is determined by the rate of freezing as well.

The freezing process may be seen as a lowering of the product temperature from its original value to the storage temperature in question. However, from a technical-economical-operational point of view a more strict definition is needed.

Ice Crystallization

The major component of most foods is water, which in the freezing process is transferred from a liquid to a solid state. This transfer obviously result in numerous changes.

Most food products consist of or contain animal and/or vegetable cells forming biological tissues. The water solution of the tissue is contained between the cells—intercellular fluid—and within the cells—intracellular fluid.

When the food product is cooled below 0°C ice begins to form at the initial freezing point. The temperature at which freezing starts depends on the concentration of dissolved substances—salts and other solubles—present. The concentration is higher within than outside the cells. The cell membrane acts as an osmotic barrier and maintains the difference in concentration.

When the product is frozen the first ice crystals are formed outside the cell since the freezing point is higher because of the more diluted fluid here than inside the cell. Once started, the rate of ice crystallization is a function of the speed of heat removal as well as the diffusion of water from within the cell to the intercellular space. If the freezing rate is low, few crystallization centers—nucelli—are formed in the intercellular space. During the freezing process the cell looses water by diffusion through the membrane, and this water crystallizes to ice on the surface on the crystals already formed outside the cell. As in slow freezing, there are few nucleins formed; those existing as crystals grow to a relatively large size.

As the cells loose water, the remaining solution within the cells becomes increasingly concentrated and the cell volume shrinks causing the cell wall to partly or entirely collapse. The large ice crystals formed outside the cell wall occupy a larger volume than does the corresponding amount of water, and therefore they will exert a physical pressure on the cell wall. In some cases this pressure can be sufficient to damage the cell wall and contribute to an increased drip loss on thawing.

By increasing freezing rates a larger number of ice crystallization nuclei are formed, which results in a much smaller size of the final crystals as compared to slow freezing. However, even in the case of high freezing rates most of the crystals are formed outside the cells. Only at extremely high freezing rates not obtainable in commercial freezing of food products, small crystals are formed uniformly throughout the tissue both externally and internally with respect to the cell.

As the food products are cooled down below the initial freezing point, an increasing amount of water is turned into ice and the residual solutions become more concentrated. The relation of water frozen out as ice and the concentration of the remaining solution have an impact on the preservability of a number of food products (5).

The size of the ice crystals has long been regarded as crucial for the quality of the frozen product. It appears, however, from experience as well as a number of investigations that the differences in ice crystal size and distribution have little effect on the sensory properties of the food product when presented to the consumer, provided up-to-date equipment and good commercial practice have been used (6).

The freezing rate is not negligible, however. On the contrary, in good commercial practice the freezing time must be determined for each product in order to safeguard against microbiologic growth, which is most important from a safety point of view and low controlled weight losses, which is important from an economic point of view.

The practical result of different freezing rates—size and locations of ice crystals—can be seen as a difference in drip loss of water or "product juice" when a product is thawed. Loss of juice results in a more or less pronounced loss of texture, flavors, and—in most cases—nutrients. For this reason the drip loss is often used as an indication of the quality loss during freezing and subsequent storage. The relation between speed of freezing and drip loss for two different foods is illustrated in Figure 1.

For strawberries it is rather obvious that the consumer will not readily accept the product with a 20% drip loss if a product with only 8-10% drip loss is available. The latter

100 Time (min)

Figure 1. Drip loss during thawing of sliced beef and strawberries, frozen at various rates, to an equalization temperature of - 18°C. Source: Ref. 6.

100 Time (min)

Figure 1. Drip loss during thawing of sliced beef and strawberries, frozen at various rates, to an equalization temperature of - 18°C. Source: Ref. 6.

level of drip loss can be achieved in modern freezers usually employing the fluidization technique.

When comparing slices of beef even very slow freezing rates gave a small drip loss hardly noticeable to the consumer. The small improvement achieved by quick freezing is normally not observed.


With reference to temperature requirements, bacterias are divided into four basic groups according to type of growth: thermophilic, mesophilic, psychrophilic, and psychro-trophic. Of those the two latter groups are of special interest in food spoilage at low temperatures. Psychrophilics are often an important cause of spoilage of protein-rich foods such as meat, fish, poultry; psychrophilic bacteria grow well at temperatures above 0°C.

The optimum temperature for growth, however, is much higher. Psychrotrophics are also able to grow close to the freezing point but the optimum temperature is higher than that for psychrophilics. Lowering the temperature will slow down the growth rate of all bacteria, and at temperatures used in commercial storage of frozen foods all microbiologic growth ceases completely. The time to decrease the temperature to below the freezing point is critical. A product temperature of — 10°C is normally considered safe with regard to microbiologic growth.

During freezing and frozen storage some bacteria are impaired and even destroyed. Under certain circumstances the death of bacteria will cause a considerable decrease in the total number of viable cells in the frozen products. Since some species are more susceptible to freezing injuries than others there may also be a change in the relation between various species (7).

From a microbiologic point of view the total flow from the production to the consumption must be regarded. During this flow, food products are subjected to various temperatures and other growth-affecting factors. Large variations occur from product to product; meat and meat products have been chosen as examples in the following discussion. The general concept is obviously valid for most food products.

During slaughter and subsequent handling the surface of the meat is infected by microorganisms originating from the animal itself and from the environment.

The number of organisms present is dependent on the hygienic conditions, but the flora consists to a great extent of spoilage organisms that thrive on the meat surface and multiply.

Rapid cooling reduces the rate of growth substantially, but many of the psychrophilic and psychrotropic organisms will grow even at chill temperatures. These organisms depend on free oxygen for their metabolism, and since oxygen is available in the surface layer only, no growth will occur in the interior of the meat.

Freezing of carcass meat therefore normally will not cause any serious problems from a microbiologic point of view. In commercial freezing the freezing rate is fast enough to stop the growth at the surface. As the microorganisms cannot develop under the surface layer, the freezing rate is of less importance microbiologically.

Processing, like cutting and mincing, increases the microbiologic contamination as the surface/volume ratio increases. The freezing rate becomes more critical. A common pack in the industry today is the 30-kg carton normally frozen in a traditional air-blast tunnel. It is then essential that a good air circulation be provided in order not to prolong the freezing time.

Freezing times for cartons subjected to both adequate and inadequate airflow are compared in Figure 2.

Inadequate airflow has been achieved by placing the meat cartons directly on pallets with only a small wooden spacer (30 mm) between each layer instead of being placed on freezing racks or with rigid layer separators with a minimum height of 50 nm.

If spacers used as in the improper airflow system do not cover the total carton area, some cartons may collapse, which will prevent airflow through the different layers.

On racks With spacer

On racks With spacer

Figure 2. Recorded temperature decrease during the freezing of cartons in an air-blast tunnel (air temperature -38°C, front air velocity 1.5 m/s, size of cartons 160 X 400 X 600 mm). Source: Ref. 7.

Figure 2. Recorded temperature decrease during the freezing of cartons in an air-blast tunnel (air temperature -38°C, front air velocity 1.5 m/s, size of cartons 160 X 400 X 600 mm). Source: Ref. 7.

The freezing time is obviously prolonged. There is also a risk that cartons are removed from the air-blast tunnel before the freezing is completed if blocking of the air channels is not visible from the outside. In the latter case there may be substantial growth of microorganisms during subsequent storage, ie, until a sufficiently low temperature has been reached.

In most cases a freezing time of 24-36 h down to — 10°C in the center of wholesale cut meat will not cause any microbiologic problems. If the degree of cutting is increased to smaller cuts, such a long freezing time may become a major problem. Those products should preferably be frozen integrated in the processing line before packaging or in very small packages that allow for a much faster freezing.

As most prepared foods involve a high degree of processing as well as mixing of different ingredients the freezing becomes very important. The general pattern of the growth of microorganisms in the production of prepared foods is illustrated in Figure 3.

During storage of raw material as well as during handling and preparation, microorganism growth will take place. If the preparation is followed by heat treatment, the total number of microorganisms will be reduced.

At this point the product could be handled in two different ways: either packaged and frozen in batch-operated equipment or frozen in-line and then packed. If the products are placed on racks and then transported to a freezing tunnel for freezing, there may be a time lapse, resulting in a marked growth of microorganisms. If the product has been heat treated, it will pass through the temperature zone of optimum microbiologic activity.

A chilling operation immediately after the heat treatment—which means that the products are cooled down to

Raw material


Raw material

Processing, handling

Heat treatment



Frozen storage

Processing, handling

Heat treatment



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The Mediterranean Diet Meltdown

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