Maintain the cold chain it is essential to ensure food safety frozen and deep-frozen. And it is also important defrost carefully. That's how.
Frozen and deep-frozen, advantages and precautions
Freezing and deep freezing they allow to preserve food products well beyond the period foreseen by the simple refrigeration process. (1) During freezing, most of the water contained in food forms ice crystals. This blocks microbial growth and significantly slows down enzymatic activity.
The size of the ice crystals it is inversely proportional to the speed of the freezing process. The more this is fast, like 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 (eg muscle fiber cells in meat products).
The freezing, however, should not be considered a means in itself suitable for bacterial destruction (with the exception of parasites, eg. Anisakis in fish). In fact, microorganisms have different sensitivities to lowering the temperature below the freezing point.
The most sensitive bacteria at low temperatures they are Gram-negative (Enterobacteriaceae, for example, such as Salmonella ed Escherichia coli, Besides Pseudomonas e Campylobacter). Gram positive bacteria (Staphylococcus aureus, Listeria monocytogenes) instead tend to resist thermal breakdown.
Defrosted food therefore they often have a residual bacterial load that is able to withstand the freezing-thawing process, and requires some basic precautions. First of all, frozen and deep-frozen products must always be stored, without interruptions, at temperatures equal to or below -18 ° C.
Frozen and deep-frozen, defrost carefully
Defrosting of products frozen (or 'chilled') and deep-frozen should be performed gradually, at refrigeration temperatures between + 2 ° C and + 4 ° C.
The graduality indeed allows the slow and progressive melting of the ice crystals that formed during the lowering of the temperature, with less damage to cell membranes and greater possibility of reabsorption of cellular juices rich in proteins, vitamins and mineral salts.
Maintain a low temperature in the defrosting phase it also prevents the multiplication of those bacteria that have maintained their integrity, which could start multiplying again after having reached a suitable temperature (generally, above 10 ° C).
The defrosting time of food products depends on a number of factors. In any case, it is a good rule not to let too much time pass. So as not to allow any psychrophilic bacteria (i.e. those capable of multiplying at refrigeration temperatures), possibly present, to resume their life cycle and damage the products.
(1) For the difference between frozen and deep-frozen foods, see the article https://www.greatitalianfoodtrade.it/tecnologia-alimentare/una-bella-differenza
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.