Geoff Benge, of Waikanae Beach, asks :-

For disaster preparedness we are told to keep a supply of food. What is the life of canned food?

Ian Shaw, a biochemist at the University of Canterbury and author of "Is it safe to eat?" (Springer 2005) and "Food Safety" (Wiley-Blackwell, 2013), responded.

This is an interesting question and I agree it is a minefield of confusion. What a pity even the Ministers of Food Safety didn’t have a simple answer!

In short, canned food is put into a metal can, and sealed following high temperature treatment. This process kills all bacteria and viruses and their spores. Therefore, providing the process is carried out properly, once sealed nothing can grow in the can and spoil the food inside. In theory the food would keep forever.

The limiting factor is the can itself. If the can degrades and pin holes appear microorganisms might gain access to the food and begin the decay process. If the microorganisms are pathogenic, the contents of the tin would then be unsafe to eat.

The next question is how are shelf lives set? It’s simple; food is left until it goes off and the time taken for this to happen is recorded and a safety margin included to determine the shelf life. The problem is, if a shelf life is very long many companies don’t wait for the time necessary to demonstrate the ‘real’ shelf life, but instead use a shorter time. This means the food is definitely safe to eat, but could have been kept for much longer and still be safe. For this reason many foods have shelf lives that are shorter than their ‘real’ shelf lives, but still comply with the regulations.

In the case of canned food, the shelf life is usually so long that most companies will not take the time necessary to determine the ‘real’ shelf life, but instead will give a guideline based on manufacturing date.

Bearing all of this in mind, as you say, you need the packing date to apply the Ministry guidelines for definitive safety of the product. My feeling (and I must emphasise that this is my opinion) is that providing the can is intact (no signs of rust, pinholes or dents) and not swollen its contents are OK to eat. I certainly would.

I mentioned swollen cans above because a significant food safety risk results if the canned food is contaminated with the bacterium Clostridium botulinum. C. botulinum grows in anaerobic (oxygen free) conditions (i.e. the conditions inside a can of food) and produced botulinum toxin which is amongst the most toxic chemicals we know - it is lethal.

In addition to botulinum toxin, C. botulinum produces the gas, hydrogen sulphide which makes the can swell. This is why swelled canned food should never be eaten.

This has not definitively answered your question, but a definitive answer simply is not possible.