I have a vacuum-packed truffle in the fridge since a few weeks now. The mushroom was fresh when it has been vacuum-packed.
Does anyone know how long will it last? Is it still ok to eat it?
It looks ok from what I can see....
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Truffles are a low acid food, and they grow underground, so they are at risk to have botulism spores present.
Vacuum packing creates a low oxygen environment, which is what the botulism bacteria requires to grow and produce its toxin.
Unless the truffles have been salted (to 5% by weight), acidified (to a pH of 4.6 or lower), or frozen (at all times after being vacuum packed), they are at risk for the growth of botulism. Some strains can grow and produce their toxins at refrigerator temperatures (and most refrigerators have a temperature cycle that peaks in the more general growth range). While the growth is slow, it can occur. The risk is increased if the truffles have not been continuously refrigerated since they were vacuum packed.
For this reason, mushrooms and truffles should never be vacuum packed and stored. You should discard these truffles.
In the future, obtain your truffles from a reputable source (which does not vacuum pack) and use them fresh, or freeze them as botulism is not active at freezer temperatures.
See also: University of Colorado Extension fact sheet on Botulism