The answer to your question is in fact very simple.
The appauling conditions in the trenches between 1914-1918 had a great effect on the durability of food.
The British Army during this period ate a concoction called 'Bully Beef' (corned Beef) and Hard Tack (this probably refers to your question)
Further to this, ready fresh ingredients were uncommon at the Front as food was cheaply mass produced at Mess Time
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