Why is expired frozen chicken safe to eat?
It seems to be a widely held belief that that freezing chicken, and then defrosting, cooking, and eating it much later (well past the expiration date) is a perfectly fine thing to do.
What makes it safe? And are there any food safety hazards to watch out for when doing this?
"Expiration" is not a date. It's only so in food packaging for legal and labeling purposes, but in reality food expiration, decay, and safety is a complex process. Storage methods affects how long food can last.
Here in UK the label says to freeze the chicken on the day of purchase and use within a couple of months.
Just for clarity: If the food was unsafe due to bacterial hazards when frozen, it will be just as unsafe after thawing, since the bacteria will also have stayed fresh :) Parasites are a more diverse matter, there seem to be some freezing-based methods that a) make certain seafood safe that was not before freezing (and not before catching for that matter), b) keep grains safe from insect infestation (if not recontaminated) even after unfreezing.
It's safe because freezing greatly slows (if not completely arrests) the growth of the bacteria that would otherwise make the meat spoil. It doesn't kill them, it just puts them in 'stasis'. The expiration date is given based on the meat only being refrigerated. If you intend to store the meat past its expiration date, best practice is to freeze the meat ASAP, and to use it within 24 hours of defrosting.
The main hazard is in that defrosting. At no point should the chicken (any part thereof) be at room temperature for more than 2 hours. The safest method is to defrost the meat slowly in the fridge. You can also defrost it under cold running water, providing you keep it under the 2 hour mark.
Of course, you can also cook the meat straight from frozen. There is absolutely nothing wrong with doing this for small cuts like chicken breasts. Slicing and cooking a partially defrosted chicken breast is perfectly safe, especially as it often easier to make really thin strips with semi-solid meat. You just have to be extra careful that the meat is at a safe temperature (170°F/75°C).
It may be worth noting that meat which has never been frozen will often have a *refrigerator* shelf-life much longer than meat which has been frozen. While moving meat to the refrigerator is a good way to thaw it, one should generally keep frozen meat frozen until the intended day of use (or the night before, if it will be needed in the morning).