Frozen storage

Thawing below — 10°C, then packaged and frozen—will reduce the growth of bacteria. However also in this case microbiologic growth can be recorded. If the products are instead frozen in-line immediately after the heat treatment and then packaged, there will be almost no increase in the number of bacteria present.

In-line freezing is, of course, even more essential when prepared foods are processed without heat treatment prior to the freezing process.

A low microbiologic load in the frozen state will directly influence the preservability of the food product after thawing. All measures taken to arrest growth of microorganisms prior to freezing are therefore beneficial.

Compared to batch freezing in a tunnel, the modern inline freezer provides much faster freezing, and even more important, the in-line process itself minimizes the delays in product flow from preparation to freezing and through the temperature zone, which is critical from a microbiologic point of view.


During the freezing process it is unavoidable that a certain evaporation of water from the surface takes place, resulting in both a quality loss and a weight loss. Only if the product is tightly enclosed in a water vapor inpermeable packaging material, evaporation can be completely avoided. If there are small spaces between the product and the packaging material ice is deposited in these.

Freezers that are poorly designed and improperly used may cause a weight loss in the order of 5-7%, while properly designed and used equipment will cause no more than 0.5-1.5% weight loss. As the total freezing cost often is no greater than 3-5% of the product value, it is obvious that dehydration losses are of great importance in any comparison of different freezing methods.

There is a definite correlation between the degree of dehydration loss and the rate of freezing. In Figure 4, the temperature gradient in a product with a thickness of 2 b and in the air surrounding the product is plotted against the partial pressure of water vapor in moist air.

The size of the dehydration is influenced by a number of factors related to the biological materials as well as to physical handling, temperature of the heat-transfer medium, dimensions of the product, etc. The rate of evaporation from the product is determined by the conditions at the surface.

In a simplified consideration the evaporation rate may be regarded as proportional to the vapor pressure Ap2. The curve corresponds to the "relative" humidity of the product surface. This concept is introduced in order to represent the diffusion resistance that may exists in cell walls, etc.

During infinitely slow freezing the surface temperature approaches the average temperature ts. Then the evaporation rate increases to the value Aps. Hence it is clear that ts — t2 should be as great as possible in order to obtain low dehydration losses. Three principal factors contribute to this:

Figure 3. Growth pattern of bacteria during different steps in the processing of precooked frozen foods. Source: Ref. 7.

• b large. In thick products the surface temperature will be low during most of the freezing process.

Figure 4. Dehydration mechanism during freezing. Source: Ref. 8.

• t1 low. The lower the temperature of the ambient air, the more curved the temperature gradient in the product will be.

• High heat-transfer rate. A low air temperature has no appreciable effect if the heat-transfer rate is too low. Therefore, it is extremely important that the heat-transfer conditions be favorable. This is more important for thinner products.

Wet products generate water vapor at a rate proportional to the difference between the vapor pressure at the surface of the product and that of the surrounding air. In a product that has a more or less dry surface there is a resistance in the cell walls against diffusion of vapor from the interior of the product to the surface and the air. This results in a reduction of vapor pressure at the surface of the product.

The water vapor pressure decreases rapidly when the temperature is reduced, which means that the dehydration will be less the colder the heat-transfer medium is. The importance of a low temperature during freezing is well illustrated in Figure 5, where the accumulated weight loss during different freezing tests with a temperature varying from —13 to - 35°C is plotted against the core temperature of the product. The diagram illustrates both the importance of the latent heat zone and the drastic influence of the air temperature.

Temperature (°C)

Figure 5. Accumulated dehydration during freezing at different air temperatures. Source: Ref. 9.

Temperature (°C)

Figure 5. Accumulated dehydration during freezing at different air temperatures. Source: Ref. 9.

Glazing for Protection of Product Quality. In order to improve shelf life by preventing desiccation and oxidative changes, it has become standard practice to glaze certain individually frozen products, eg, shrimp, after freezing. The product quality is greatly improved as the thin ice layer prevents the product from the changes mentioned above. The glazing is carried out by spraying the frozen product with cold water, which immediately freezes on the surface. Even if the product leaves the freezer at a low outfeed temperature, the product temperature may increase considerably after the glazing operation. This highend temperature may cause clumping of the product when packed, and the slow decrease in temperature during subsequent storage and distribution may lead to noticeable texture changes on thawing.

To avoid these problems special equipment has been developed—GLAZoFREEZE—that is designed to lower the product temperature of the glazed product immediately after the glazing operation. The equipment utilizes the flu-idization technique. In GLAZoFREEZE the fluidization must be specially gentle to the product in order to avoid damage of the glazed surface.

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