Faruk

The reason age hits ground coffee so hard is worth understanding, because it changes what you buy.

Coffee goes stale through oxidation and through the loss of volatile aromatic compounds. Grinding increases the surface area enormously — a huge multiple of whole beans — so a ground coffee stales in weeks where whole beans take months.

What you taste as it declines is fairly specific: the aromatics go first, so it smells of less; then the acidity flattens; and eventually you get a dull, cardboard-like quality. It is not usually sour or rancid, it is absent, which is why people describe stale coffee as tasting of nothing much.

A three-month difference in shelf time for pre-ground coffee is easily enough to produce the gap you are describing.

Also worth checking: whether the bags have a one-way valve, and whether either had been opened or was stored somewhere warm. A bag left in sunlight in a shop window ages far faster than one in a stockroom.

Since you bought it used, there is a diagnostic worth doing before anything else: check whether the previous owner had it apart.

Signs are a missing or mismatched screw, burrs that do not sit flush, or a calibration that is obviously off the factory position. A grinder that has been disassembled and reassembled slightly wrong will work acceptably for weeks and then shift when something settles — which fits your timeline exactly.

Also worth weighing your dose and timing your brew before and after any change, and writing it down. A coarser grind and a faster drawdown are easy to confirm objectively, and it tells you whether a fix worked without relying on taste memory.

The variables, in order of how much they matter:

The mass of coffee used. This dominates everything. Caffeine per gram of dry coffee varies within a fairly narrow band, so how many grams went into the cup is most of the answer. Weigh your dose and you have the biggest term.

Extraction efficiency. Caffeine is highly soluble and comes out early, so most brewing methods extract most of the available caffeine. This means the differences between methods are smaller than folklore suggests.

Serving size, obviously, and this is where café drinks diverge wildly.

The consequence that surprises people: a large filter coffee usually contains substantially more caffeine than a single espresso. Espresso is more concentrated per millilitre and much smaller. A double shot is a fraction of the dry coffee that goes into a mug of filter.

So weigh your dose and count your shots, and you can compare your own drinks reasonably well without any instrument.

What to actually look for, since "when it stops working" needs a definition:

• Blocked holes that will not clear with brushing. Reduces flow and can cause pressure problems.
• Distortion. The plate should be flat. A bent or dished plate does not sit properly against the gasket and leaks grounds into the brew.
• Cracks, particularly radiating from the holes. These do appear eventually — I have had a plate crack after several years of daily use — and a cracked plate lets grounds through.
• Deep pitting or corrosion, which is more common on cheaper pots and on ones washed in a dishwasher.

If none of those apply, keep using it. Mine have generally outlasted several gaskets each.

The specific adjustments, and they compound:

Lower the water temperature. This is the single biggest one. Well off the boil — let the kettle stand for a minute or two. In a press, adding a little cool water to the grounds before the hot water works too. Bitter compounds extract preferentially at high temperature.

Grind coarser. Less surface area, slower extraction. Go noticeably coarser than you would for the same method with a medium roast.

Shorten the contact time. In a press, four minutes becomes two and a half. Plunge and decant immediately rather than leaving it sitting.

Use more coffee, less time. A higher dose with a shorter, gentler extraction gives you strength without pulling the harsh compounds. This is counter-intuitive and it works.

For the stovetop pot specifically: start with warm water in the base, take it off the heat as soon as it flows, and run it on a low flame throughout. Most stovetop bitterness is the last spluttering phase, and stopping before it happens fixes a great deal.

You are approaching it wrongly, and the reason is a category error worth spelling out: coffee grounds are cellulose and they do not dissolve in anything you would put in a kitchen.

The cleaning products people recommend for coffee equipment target oils and soluble residues — the brown film and the rancid taste. Vinegar attacks mineral scale. Neither is a solvent for plant fibre.

So no soak will remove them, however long you leave it. What you need is mechanical.

The effective approach is to take the filter assembly apart. Press filters unscrew from the plunger rod into separate discs — usually a mesh, a cross plate, a spiral spring and a base. Once separated you have access to both faces of the mesh with nothing behind it, and a brush or a rinse from the clean side pushes the particles straight out.

Most people have never taken theirs apart and are trying to clean a sandwich from the outside.