Hello,

I would guess that solubility of gases (Nitrogen, CO2), which are dissolved in the paste, are influenced by pressure. The decreasing of pressure decreases solubility and gas releases form the product. As soon as the product is very viscous, the released gas does have enough time for separation and it exists as a small boubles in the product. This increaeses the volume and creates the overflow afterwards.

In the case of temperature influence, the solubility of gases in products is not defined in details. The solubility of gases in water, for example, increases with decreasing of temperature, so the process will drive in opposite direction. But, in your case you have lots of proteins and harbohydrates, which can behave differently.

Regards

Ignat

And does the same thing happen to Vegemite? And you get more Marmite than you originally thought or bought!

Thanks Ignat for your cogent explanation. Although bubbles were not evident, Marmite is almost black and they may be too small to see; I'll have a closer inspection next time. Alan raises an intriguing question. And besides vegemite, similar savory spreads include Cenovis (Switzerland) and Vitam-R (Germany), among others(https://en.wikipedia.org/wiki/Yeast_extract), although these may differ in viscosity and texture (http://justhungry.com/2006/11/marmite_vegemite_andcenovis_a.html). Ketchup, mustard and peanut butter also come to mind, not to mention toothpaste. Perhaps yeast-extract-loving jet-setting RGers can assist with corroborative follow-up research and report the results. The potentially messy implications are probably well known to logistics boffins, but not all unsuspecting travelers.

One experiment would be to out gas the Marmite under vacuum (cap off - into a vacuum desiccator) before departure. This would also answer the question of whether it expanded due to decompression (indeed invalidating the purpose of the air journey). I don't see similar out gassing issues with the sachets of material (e.g. butter) they serve on aeroplanes, although such packs are often tight and expanded due to the reduced pressure in the cabin. Perhaps the Fine Bubble (was Nanobubble) Committee of ISO (TC281) can obtain some governmental funding for experimentation.

BTW, easily obtainable in the US, although not a single American that has tasted it professes to like it (and they don't know what lbw means either...).