keep biscuits in a tin closed for decades

[tali1]

frien claims you can keep biscuits in a tin closed for decades and as long as thet are not exposed to air - threy will not ahve gone off - true?

[effervescenc]

Of course. Decay cannot occur without oxygen.

[theprof]

No.

[xud]

Explorers in Antarctica have found tins of cocoa embedded in the ice/snow which were left behind by Scott in 1912

[chazza]

There will be air inside the tin though, unless the biscuits take up every available space in the tin

[theprof]

Although it�s not tali1�s fault, the problem with this question is that it invites further questions before we can sit down and answer it properly. These questions include:

Was the tin evacuated of air and then made airtight?

Were the biscuits enclosed by some form of manufacturers
sealed outer wrapping in the tin?

Were the biscuits loose in the tin?

In what sort of environment was the airtight tin kept?

What was the average external temperature of the tin?

Were the biscuits fresh?

[theprof]

Decomposition in this case would be carried out by bacteria and fungi. For this decomposition to occur, ideally a number of requirements should be met. These include:

Warmth

Oxygen (from air)

Moisture

Organic matter

Suitable time period

Suitable pH

In the absence of some of these factors, decay and decomposition may not begin at all. However, there are exceptions and in this case, there is ample scope for decay to occur. Here�s why.

(continued)

[theprof]

It�s incorrect to assume that decay cannot occur without oxygen. There are many species of anaerobic bacteria that do not require oxygen to thrive, and although decay may not be complete and rapid, it will certainly occur in the presence of anaerobic microorganisms. Sufficient decay by anaerobic organisms over decades would make the biscuits inedible. The anaerobic bacteria could have been introduced from those previously in contact with the biscuits.

Similarly, mould spores or other fungi would have been introduiced into the tin with the biscuits. Although this is more likely had the biscuits been loose in the tin, they could be in the sealed packaging. Mould spores of hundreds of species are present in every cubic centimetre of air that we breathe, and it would be impossible to exclude them from the tin without specialised filtration. Moulds require only a very limited amount of moisture to thrive and sufficient would be present on the biscuits. In the right circumstances, condensation could form on the internal walls of the tin providing ample moisture anyway. The darkness in the tin would not prevent mould growth and air contained in the tin would allow mould to survive for many decades.

(continued)

[theprof]

An airtight container is merely one in which air cannot enter or leave. The microorganisms in the tin box would have sufficient air present in their environment and would have no need for extra air from the outside. Consequently, an airtight tin would be a suitable environment for growth and reproduction for many decades.

With regard to xud�s point about the tins of cocoa, the reason these products are preserved without spoilage is down to the sub-zero environment in which they were found. Microorganisms do not reproduce at such temperatures and many cannot withstand the extremely low temperatures. Consequently, spoilage does not occur. In addition, because it�s a dry foodstuff without moisture, it won�t go off. Such products rapidly decay when returned to a warm, moisture-laden atmosphere.

[theprof]

Incidentally, removal of oxygen from the tin could have been easily accomplished by placing a small lit candle or tee-light inside the tin before putting the lid on. The flame would use all the available oxygen as fuel before being extinguished.

All the same, this technique would not affect anaerobic microorganisms and the decomposition caused by them.