A little while ago we froze a packet of crumpets. At the time, the sealed plastic packet seemed to contain a lot of air.

A little while ago we froze a packet of crumpets. At the time, the sealed plastic packet seemed to contain a lot of air, but after four months in our freezer it had contracted tightly against the crumpets, which had also shrunk. After two more months at room temperature, the whole package appeared to have shrunk still further, though with no sign of mould or decay.The ingredients are listed as flour, water, yeast, raising agents, E450, E500, salt, sugar, preservative, calcium propionate. What's
going on?

^^^^That's the full comment^^^^

Firstly, EEEWWWWW! Who keeps a packet of pre-frozen crumpets for 2 months? If I'd done that there's no way I'd announce it to the world! You can bet the world reads this too....because it's mine & I'm amazing!

Anyway, my first thought on this would be chemical reaction....that somehow the yeast reacted with the E500 (sodium bicarbonate), but they're both rising agents that work independently of one another. Yeast likes a warm environment & E500 doesn't really care I don't think. Water is quite high on the list of ingredients so maybe that has something to do with it too?? Hmmmm.... high water content usually means mould though.......So, maybe there's a reaction with something that sucks out the water, preserves the crumpets by drying them out & uses up any air in the packet in the process making it constrict round the crumpets.....(these dots are my mind working, each one is 500 brain ticking overs).

What uses oxygen up.........sec, need to Bing......Yeast metabolizes sugar to produce carbon dioxide and ethanol, also known as alcohol, in the production of wines, beers and other spirits, but also in the production of bioethanol:
C6H12O6 → 2 CO2 + 2 C2H5OH

Biogas typically refers to a gas produced by the biological breakdown of organic matter when there's no oxygen. Foods packed in air don't store as well as in oxygen free gasses. This is because air contains oxygen which oxidizes many of the compounds in food. Bacteria, one of several agents which make food go rancid also needs oxygen to grow. Food storage companies have a couple of different processes for removing the oxygen:

1. Displacing the oxygen: This is done by purging out all the air in the product with an inert gas. Nitrogen is almost always used because it is the most inert gas known. People doing their own packing occasionally use dry ice which gives off carbon dioxide gas, and probably works just about as well.
2. Absorb the oxygen: Oxygen absorber packets do just that. Air contains about 78% nitrogen and 21% oxygen, leaving about 1% for the other gasses. If the oxygen is absorbed, what remains is 99% pure nitrogen in a partial vacuum.

OK, now I've looked into it a bit more I reckon it has to do with the packaging. If there's no oxygen, there can't be mould because it can't grow. Also, if an oxygen sucking gas was used to package the crumpets, some of the water content would disappear as well...any leftover moisture found in the crumpets could interact over time with the E500 and create a small amount of carbon dioxide gas too.

Right, to answer the question, I think perhaps the gas used to pack the crumpets sucks the moisture out of them & causes the packet to constrict (perhaps because of the extreme temperature) or, there's a carbon dioxide producing reaction in there somewhere. I'm NOT a rocket scientist though as I keep mentioning :p

P.S. Stray, I'd thump him for insinuating you're not good at chucking things away. He probably hid the packet somewhere just so he could do an experiment! lol