Thaw carefully


Maintaining the

cold chain

is essential to ensure the safety of food

frozen and deep-frozen
. And it is also important thaw carefully. Here’s how.

Frozen and deep-frozen foods, advantages and cautions

Freezing and deep-freezing allow food products to be stored well beyond the period provided by the simple refrigeration process. (1) During freezing, much of the water in food gives rise to ice crystals. This stops microbial growth and greatly slows down enzyme activity.

The size of ice crystals is inversely proportional to the speed of the freezing process. The faster this is done, as in the case of frozen foods, the smaller the size of the crystals, which will cause less damage to the cellular structures of the product (e.g., muscle fibrocells in meat products).

Freezing, however, should not be considered a suitable medium per se for bacterial destruction (with the exception of parasites, e.g., Anisakis in fish). In fact, microorganisms have different sensitivities to lowering the temperature below the freezing point.

The most sensitive bacteria at low temperatures are Gram-negatives (Enterobacteriaceae, for example, such as Salmonella and Escherichia coli, as well as Pseudomonas e Campylobacter). Gram-positive bacteria (Staphylococcus



Listeria monocytogenes

) tend instead to resist thermal killing.

Thawed foods therefore often present a residual bacterial load that is able to withstand the freeze-thaw process, and requires some basic precautions. First, frozen and deep-frozen products must be stored at temperatures of -18°C or below at all times, without interruption.

Frozen and deep-frozen foods, thaw carefully

The thawing of products
frozen (or ‘blasted’) and deep-frozen products should be done gradually, at refrigeration temperatures between +2°C and +4°C.

Graduality invero allows for the slow and gradual melting of ice crystals that have formed in the temperature lowering phase, with less damage to cell membranes and more opportunity for reabsorption of cellular juices rich in proteins, vitamins, and minerals.

Maintaining a low temperature
at the thawing stage also prevents the multiplication of those bacteria that have maintained their integrity, which may resume multiplying after reaching a suitable temperature (generally, above 10°C).

The thawing time of food products depends on a number of factors. In any case, it is a good rule not to allow too much time to elapse. So as not to allow psychrophilic bacteria (i.e., those capable of multiplying at refrigeration temperatures), which may be present, to resume their life cycle and damage the products.

Silvia Bonardi


(1) For the difference between frozen and deep-frozen foods, see

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Graduated in Veterinary Medicine and Specialist in Inspection of Food of Animal Origin and in Veterinary Public Health, she is Professor of Inspection and Control of Food of Animal Origin at the University of Parma.