During a food inspection check, we were told the following:
“You must first keep the ‘witness dish’ in the cold store for 72 hours, then in the freezer.” The reason would be that otherwise you kill certain bacteria.
Answer
This is a complex question that I answer here as a “general microbiologist” :
In normal freezing (-20°C) many bacteria die, including the pathogenic species, but not all; endospore formers (Bacillus sp. Clostridium sp , .. ) and their endospores can nevertheless survive in frozen conditions; so the message is only to freeze hygienically treated food. In pre-chilled (around 0° to -5 °C ??) food, this bacterial flora present has already been able to adapt to some extent / form spores / form toxins, and thus increase their chance of survival, as well as their later detection.
Many other bacteria (not known as food pathogens) occur naturally in the very cold regions of Earth (North and South Poles), in the permafrost, glaciers, etc., and metabolize very slowly.
On the other hand, most bacterial species in culture collections (ATCC,..) are kept frozen for years in liquid nitrogen!
You should also consult a food microbiologist!
Answered by
Prof.dr.ir. Erick Vandamme
general microbiology ;industrial microbiology ; fermentation processes ;biocatalysis ;enzyme technology; biochemistry ; biochemical technology
http://www.ugent.be
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