When friends urged me to visit them in Alaska it turned out that they were using the invitation as opportunity to re-stock with Marmite. It's simply not available in the back country (nor anywhere in the States, for all I know). I duly packed 12 of the largest-size jars, and set forth.

TIP: If anyone else tries this, it would be advisable to remove a large spoonful of the precious sludge from each jar before checking your bags. I didn't, with the result that all 12 jars exploded during the flight (cargo holds being unpressurised). At least I discovered that the entire contents of a backpack are rendered unserviceable by this kind of incident.