----- Original Message ----- 
From: "Marilyn Harris" <[log in to unmask]>

>I have read that freezing meat for a minimum of two weeks kills parasites.
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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