Why Leftover Biscuits (The British Kind) Get Soggy, According To Mary Berry
When you have Mary Berry's wealth of culinary experience, you pick up a few pointers as it pertains to desserts. Storing biscuits (the English variety, not to be confused with their bready American cousin) requires due care and attention. In particular, one mistake everyone makes with biscuits is letting them go soggy in storage.
According to Berry's website, several factors can cause biscuits to lose their ideal crunchiness. These include storing the biscuits when they are still warm, keeping them in the same container with items such as cake, placing them "in a moist environment," or having biscuits that "contain glace cherries, apricots, etc." The one linking factor throughout is moisture.
Berry mentions this explicitly, stating that when biscuits contain fruits, "the moisture of these seep[s] into the biscuit". This also comes into play when they are stored with cake. When "the biscuits absorb the mixture from the cake" as Berry describes, they absorb moisture, as well.
Storing biscuits for maximum crunch
To keep your leftover biscuits from going soggy, it's important to keep them in a cool, dark, and dry place. Where you keep them depends on how long you plan on storing them. Biscuits can last for three or four days without refrigeration if they're sealed in a plastic bag. If you can manage it, this is the best way to avoid excess moisture; fridges are prone to condensation due to humidity that can't be regulated in its sealed environment.
However, if you need to keep your biscuits for longer than four days, the fridge is a perfectly fine solution. Biscuits can survive in the fridge for up to a week as long as they're kept in an airtight container. For a seriously long haul, whip out a freezer bag. You can keep biscuits in the freezer for up to three months, but make sure to thaw them in the fridge for a day before you're ready to eat them.
Biscuits don't necessarily become dangerous if left out (although bacteria can grow on any perishable food after too long), but they do turn unpleasant over time. The best way to test your biscuits is by touching or, if need be, tasting them. If the biscuits are soggy to the touch or if they have a stale taste, it's best to avoid them.