The structure of enzymes is slightly denatured by freezing according to scientific reports, however they are not destroyed, at least not instantly. I suppose much lower temperatures are needed. I've also heard it claimed by some RVAFers that freezing meats for more than a few weeks does damage enzymes over time, but this is just anecdotal.
Re bacteria/pathogens:- People who eat raw meats invariably find, sooner or later, that the concerns re bacteria/parasites have no real basis. I don't think one can exactly guarantee the absence of all so-called "dangerous" bacteria without cooking at extremely high temperatures. I should add re the salmonella bacterium mention that a study showed that salmonella is present in 38% of US households, yet you don't find massive salmonella epidemics going on - same goes for e coil etc.. In other words, the presence of such bacteria is not relevant to any particular heath-problem, it's more of a question of the environment rather than the pathogen .
Geoff
> Date: Sun, 31 May 2009 12:09:16 -0600
> From: [log in to unmask]
> Subject: how long should raw meat be frozen?
> To: [log in to unmask]
>
> ----- Original Message -----
> From: "Marilyn Harris" <[log in to unmask]>
>
> >I have read that freezing meat for a minimum of two weeks kills parasites.
> ---------------------------------------
>
> Thanks to those of you who are giving me guidance regarding how to kill the
> parasites in raw meat.
>
> Question: if all biological activity in the meat is stopped (not just
> slowed) at low temperatures, is there a danger of killing the beneficial
> enzymes as well?
>
> I have read that in order to kill the parasites in meat it is necessary to
> leave the meat at " -4° or lower for 7 days or secondly, frozen at -31°F
> or below until solid and stored at that temperature for 15 hours." I do
> have a refrigerator that will allow me to set the freezer and refrigerator
> at specific temperatures.
>
> But... I've also read that different parasites need to be frozen at
> different temperatures and for different time links in order to be actually
> killed.
>
> Do the list members have any thoughts on where I should set the freezer
> temperature to kill parasites in raw beef?
>
> From the New Zealand Food Safety Authority: (this has some interesting
> information that doesn't really answer my question)
> Does freezing kill all pathogens and parasites?
> No. A common misunderstanding is that freezing makes food sterile (ie, it
> kills all microbes including bacteria, yeasts and moulds) - this is NOT the
> case. The best way to kill pathogens and parasites in food is to cook food
> thoroughly.
>
> Although pathogens don't multiply in the freezer, certain species can
> survive the freezing process by going into 'hibernation' or forming
> resistant cells (eg, spores2) and up to 70% may 'come back to life' again as
> the food begins to thaw.3 Salmonella have been known to survive for 7 years
> at -23°C in ice cream and Campylobacter can survive freezing if the initial
> contamination levels are high. Also, freezing does not affect toxins left by
> certain bacteria (eg, staphylococcal enterotoxin and botulinum toxins).
>
> What about rapid or slow freezing?
> Slow freezing (lowering the temperature by 1°C/minute) kills some pathogens
> as ice formation draws up the usable water which makes the solutes more
> concentrated. Microbes cannot control their water loss, and they dehydrate
> by osmosis. Large ice crystals and shards damage the cell walls of both the
> pathogens and the food.
>
> Snap freezing or fast freezing causes less damage to the structure of the
> food (but also kills fewer pathogens) because ice crystals don't have time
> to grow in size before all the liquid is frozen. Clarence Birdseye pioneered
> the technique in the 1920s and it is the basis of modern commercial food
> freezing. Approximately 75% of ice in frozen food forms at temperatures of
> 0.5°C and -5°C, and during snap freezing, this temperature range is quickly
> surpassed. Unfortunately this also means there is less osmotic pressure on
> pathogens so more tend to survive.
>
> What temperature should my freezer be, and why is that important?
> Your freezer should be operating effectively at -18°C or below. Check your
> make and model to determine the optimal operating temperature range.
>
> Even if meat and poultry appear frozen at slightly warmer temperatures than
> this, 'psychrotrophic' moulds (black spot, white spot or feathery mould) can
> grow between -5°C and -10°C, causing the food to spoil. A temperature
> of -18°C is easily achieved by a home freezer and effectively prevents the
> growth of these moulds and stops other micro-organisms from multiplying."
>
> Kath
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