How to store and reheat bagels so they stay good
A bagel bought in the morning is at its best within a few hours of baking. By evening the crust has softened and the crumb has started to dry. This is not a flaw in the bagel; it is the nature of a low-fat, high-gluten bread with no preservatives. The question is what to do about it.
Why bagels go stale quickly
Staling in bread is caused primarily by retrogradation: the starch molecules in the crumb recrystallise over time, making the bread feel dry and firm. This process happens faster at refrigerator temperatures than at room temperature, which is why refrigerating a bagel is generally a bad idea unless you plan to eat it within a few hours.
Bagels stale faster than enriched breads like brioche or milk bread because they contain no fat or eggs to slow the process. The same dense crumb that makes a bagel satisfying to eat also means there is less moisture buffer against staling.
The right way to freeze a bagel
Freezing stops retrogradation almost completely, which is why a frozen bagel reheated correctly can taste close to fresh. The key is to slice the bagel before freezing. A whole frozen bagel takes too long to reheat evenly, and the crust tends to steam rather than crisp.
Wrap sliced bagels individually in cling film or place them in a zip-lock bag with the air pressed out. They will keep well for up to four weeks in a standard home freezer. Beyond that, freezer burn starts to affect the flavour.
Reheating from frozen
Toast the slices directly from frozen in a toaster or toaster oven. Do not defrost first. The rapid heat from the toaster drives off the surface moisture quickly, which is what restores the crust. A microwave will make the crumb soft but will not restore the crust at all.
If you do not have a toaster, a dry frying pan over medium heat works reasonably well for the cut face. Place the bagel cut-side down for two to three minutes until the surface is warm and slightly crisped, then finish in an oven at 180 degrees Celsius for four minutes.
Sliced and frozen the same day, a Bold Citadel bagel reheated from frozen is a reasonable substitute for the morning version. It is not identical, but it is considerably better than a day-old bagel left on the counter.