Why does food spoil in the freezer?

Question

Many recipes suggest storing perishable food, such as left-over chicken stock, in the freezer for up to 2-3 months. Why could one not store it frozen for longer, surely bacteria and mould cannot grow in a deep freezer to spoil the food?

Answer 1

Most freezers are frost-free, which means that they occasionally cycle above freezing to prevent formation of frost on the walls of the freezer. This makes them easier to maintain but it shortens the storage longevity of the food.

This makes products stored in them vulnerable to freezer burn, which is a loss of moisture. It can also cause ice and IQF products like frozen fruits or vegetables to solidify into a single mass. If you have ice cube trays, you may even notice them becoming empty after a few months of sitting unused.

Freezer burn is a loss of quality, though, not a health issue. The food isn't spoiling.

If you have a "deep freeze" or a non frost-free freezer that you only access occasionally, you can certainly keep foods for nearly indefinite periods of time without worrying as much about freezer burn.

You can also reduce the likelihood of freezer burn by following better freezer practices.

Resources available here and here and here.

Answer 2

Actually, it's a matter of physics. More heat = more energy = all actions and reactions happen more quickly.

Freezing slows down but does not stop chemical processes. Those would not stop unless your freezer could chill things to absolute zero. If your food is in the refrigerator, processes take place in a matter of days, because the food is relatively warm. In the freezer, the processes still take place, but they do so much more slowly. Bases and acids still react; they just do it in slow motion, so to speak. Many bacterial agents are killed by freezing, while others just go dormant; a few even still remain active, but at such a slow rate that they are not considered a risk. Because everything happens slowly, freezing offers a long shelf-life, but on the other hand, because processes still occur, the shelf-life is not indefinite. The processes can result in something as minor as freezer burn or as major as actual spoilage (usually chemical rather than bacterial), if given enough time and the right chemical reactions.

When a limit is given for freezer storage, it is usually based on the chemical composition of the substance, its tendency for ingredients to react to other ingredients within it, its tendency for compounds in the substance to react with outside molecules, and the chemical stability of various compounds (i.e. their ability to remain stable in the freezer environment). Most freezer storage limits assume packaging with a single barrier that is air-tight but not vacuum-sealed, and water content percentages that are such that the food is not noticeably damaged by freezing for a certain length of time. This is why vacuum-sealing or multi-layer wrapping can extend freezer storage limits, and why nuts keep longer than veggies in the freezer without damage.

Answer 3

I was under the impression materials deteriorate due to oxydation. Bacterial and fungal growth stops, but degradation due to oxydation still happens. The reason why vacuum sealing is effective is due to the fact that you remove all oxygen from the package. Afterwards, as oxygen can permeate most packaging, it is hard to prevent the oxydation effect from happening.

This next to the technical limitations ofcourse.