In general, frozen foods have an excellent safety record – it is extremely rare for a foodborne illness to be traced back to a frozen food. Freezing preserves food by either stopping microbes (bacteria, fungi etc) from multiplying or halting the foods own enzyme activity that would otherwise cause the food to rot.
This fact sheet outlines some safety recommendations about handling and thawing frozen food to reduce the growth of pathogens and reduce the risk of them contaminating other foods via drips. (Pathogens are microbes such as Salmonella and Campylobacter that cause illness in people.)
How does freezing preserve foods?
Most pathogens don’t multiply at freezer temperature and many of them perish because their enzymes don’t work properly to maintain normal cell activity. Also, pathogens need water to grow and freezing turns the available water into solid ice crystals. The slower the freezing process the larger the crystals become and the more cells they damage.
How quickly the food freezes also depends on the amount of solutes (soluble salts, proteins and carbohydrates) which affect the temperature at which ice crystals are formed1 . The higher the level of solutes the lower the temperature must be for the food to freeze.
Does freezing kill all pathogens and parasites?
No. A common misunderstanding is that freezing makes food sterile (i.e., 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 (e.g., 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.
What foods can I freeze?
You can freeze almost any food although some foods require special treatment before they can be frozen safely. If defrosted correctly, frozen foods are generally as safe as their original condition and become perishable again.
In vegetables, some enzymes remain active at freezing temperature and continue to ripen and spoil them unless stopped with mild quick cooking (blanching). Blanching involves boiling or steaming vegetables very quickly for 1–10 minutes at temperatures between 75°C and 95°C, depending on the type, and chilling them rapidly. Refer to a good quality cookbook for exact instructions.
Most types of meat, fish and fruits do not need to be blanched before freezing because their enzymes do not affect them in the same way when frozen.
Have frozen foods been linked with foodborne illness?
Overseas, there have been reports of illness involving ice cream, frozen raspberries and strawberries, and contaminated ice. Most of these reports are linked to home made ice cream made from eggs infected with a strain of Salmonella that is not found in New Zealand. The good news is there have been no reports of illness linked to home made or commercially made ice cream in New Zealand although it is always wise to take extra care to use clean eggs (without cracks) when making ice cream at home and wash your hands thoroughly after handling the egg shells. Commercial ice cream manufacturers use pasteurised milk and pasteurised egg ingredients to prevent their product being contaminated by bacteria.
How long can I store frozen food?
If the temperature in a freezer fluctuates, the length of time you can keep frozen foods is considerably reduced. Freezer doors should be kept closed as much as possible, and only a small amount of unfrozen food should be added at one time.
Is freezer burn a safety concern?
No. Freezer burn is a result of ice forming in air pockets under the packaging causing the food to dry out. The cold air sucks moisture out of the surface of the food leaving a leathery patch which is dry, tainted or tasteless and unpleasant to eat. Cut away any affected parts or throw away all of the food if too much is affected. Make sure you wrap food tightly and thoroughly before freezing.
Is accidentally frozen food still safe to eat?
The consistency of some foods are altered with freezing. Cucumber and lettuce become mushy, cream sauces tend to separate and curdle and become inedible (although not unsafe). Frozen egg yolks become very thick and don’t blend with other ingredients once thawed. Tins or eggs that have been accidentally frozen should be discarded as the tins and shells may have swollen and become damaged, letting in air and pathogens.
If you have a glut of eggs and really must freeze them, use only clean eggs with no cracks. Remove them from the shell and beat the egg yolk and white together before freezing. Remember to label the container with the number of eggs inside and date for later reference – use within 12 months.
Can I cook food from frozen?
Yes. Small pieces of meat can be cooked from frozen provided the following steps are taken:
For more information, please Chat with us Ask The Expert.
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