A coffee bag is ground coffee sealed inside a filter sachet, steeped in a mug of hot water like tea.
How good are they
Better than instant, by a wide margin, and below anything freshly ground. From a specialty roaster who nitrogen-flushes the packaging, they are genuinely pleasant — clean, recognizably origin-specific, and a real cup of coffee.
The limitation is freshness
Coffee is ground weeks or months before you steep it. Nitrogen flushing and sealed sachets slow the loss considerably; they do not stop it.
That is the whole difference between a bag and a brewer.
Getting the best from one
Water just off the boil, not boiling.
Steep three to five minutes, agitating once or twice — squeezing the bag at the end forces fines through and adds bitterness.
One bag per 200 ml (7 oz). Two bags in a mug is better than one bag steeped twice as long.
Where they earn their place
Hotel rooms, offices, camping, and travel where a grinder is not practical. They occupy the niche that instant coffee used to own, and they do it far better.
The alternative
A drip bag — a small paper cone that hooks onto a mug — is a pour-over rather than a steep, and generally produces a cleaner cup. Same freshness limitation.
Choosing a brand
Look for a roast date, or at least a packing date, rather than a best-before date alone, and for individually sealed sachets with nitrogen flushing, usually stated on the box. Single-origin bags from specialty roasters taste noticeably cleaner than supermarket ones, at several times the price per cup. A medium roast is the safest choice, since steeping in a mug is an imprecise brew and light roasts can taste thin.
Cold brew bags
Larger bags made for cold brew work on the same principle: drop one in a pitcher of cold water, refrigerate for 12 to 18 hours and lift it out. With no filter to clean, they are the easiest way to make cold brew at home, and a long steep hides the freshness problem better than a hot one.
