I’m aware that some bacteria are not killed by freezing; example, frozen raw meat must still be cooked so as to render it safe to eat.
What about other bacteria, such as the live cultures in yogurt? Do they also survive in freezing temps?
Freezing does kill bacteria, it’s just that it’s not so good at killing them that it can be considered a method of sterilization. It doesn’t kill ALL of them. I well remember when I was taking food microbiology, we visited a nearby factory that made frozen meals. They told us that sometimes, if a batch of meals tested slightly too high in some category of bacteria, they’d stick the batch in the freezer for a few months and then retest them, and usually they’d then pass.
Freezing bacteria results in the same thing in anything frozen: the water in the bacteria expands when it turns to ice, breaking the cell walls. This can lead to texture problems in food, but in a single-cell organism, it’s fatal.
But it’s not a sure thing. There may not be enough water in the bacteria so that expansion is a problem, for instance. Frozen meat may have some bacteria dormant inside it and not cooking it thoroughly can lead to disease.
IIRC, freezing is not supposed to kill bacteria, at least not as its main purpose. What freezing does is retard their life processes, slowing their growth/reproduction rate. It prevents the bacteria from multiplying (or greatly reduces the rate thereof).
What killing does occur is incidental to that function.
Freezing will kill bigger critters though. Parasites that cause trichinosis for instance, will be killed by freezing. It’s the usual way to make raw fish for sushi safe. Many local health codes now require all raw fish to be frozen for 48 hours prior to serving.
Many of the bacteria you may be interested in are spore formers. I have never heard of freezing killing spores.
Also, for bacteria that are toxin producers (e.g., botulism), if they’ve produced the toxin, freezing may kill the bacteria, but the toxin will still be there. So, don’t use freezing as a antiseptic after the fact. It’s supposed to be done before any significant bacterial growth.
Do bacterial really multiply when frozen? At least, that is what your statement implies to me, that bacteria can grow and multiply while frozen (I know there are bacteria that live in ice, but common food spoilage bacteria?).
Listeria monocytogenes has been known to grow at temperatures as low as 28°F.
At some temperature the growth rate hits 0. But when it warms up, it starts up again.
I once had the pleasure of scromboid poisoning - bacteria specific to coldwater fish. This was after freezing and cooking on high temp. My blood pressure dropped to 80/40, heartrate in the 40s. In the e.r., the doctor said *nothing *kills it - once the fish meat is infected, it must be thrown out. It was scary a couple of hours, but got back to normal after Benadryl. Since it tasted fine, I don’t know if it’s detectable beforehand.
If Wikipedia is right, then scombroid food poisoning isn’t caused by bacteria. histamine is produced when some fish is stored in air at warm temperatures; that’s why Benadryl (an antihistamine) took care of you. Histamine is tough stuff, immune to heat or cold - but it’s nothing to do with bacteria.
ur right - the e.r. doc did specifically say it was histamine, I was assuming it was related to bacteria.