Cansu

Almost certainly age, and probably not for the reason you are assuming.

Those codes are usually a production stamp: a letter or digit for the year, something for the month, something for the day, and often a factory or line identifier. They are not standardised across companies and are not really meant for consumers, which is why they need deciphering at all. Comparing the codes on two bags from the same producer will usually show you which fields change together over time, and that is enough to work out the date positions without any documentation.

But here is the thing about your case: the two bags may well have been roasted within a day of each other. Large producers roast continuously, and a shop restocks on its own schedule. Two bags bought three months apart can come from nearly identical production and have spent very different amounts of time on a shelf.

So the variable is usually not roast date. It is how long the bag sat, and under what conditions.

One variable people forget when a brew suddenly changes: the coffee itself.

A new bag, a different roast level or a noticeably fresher batch all change how the beans fracture and how fast the bed drains. Darker roasts are more brittle and produce more fines at the same setting; fresher coffee degasses more and can slow a pour-over dramatically.

So before dismantling anything, check whether the bag changed at the same time. It is not the answer here often, and it costs nothing to rule out.

Direct measurement is possible and it is not a consumer proposition. The instruments that do it properly — laboratory chromatography, or the dedicated bench analysers used by producers — cost thousands, and that is the honest answer to the equipment question.

There are consumer test strips marketed for this. Reviews from people who have compared them against known references are not encouraging; they tend to give an order-of-magnitude indication rather than a number you could do arithmetic with.

So the realistic route is estimation from the variables that actually drive it, which turns out to be more informative than people expect, because the main driver is not the one everyone assumes.

Worth keeping a spare gasket and plate in the cupboard regardless of schedule.

They cost very little, they take no space, and the moment you need one is a morning when the pot is leaking and you have no coffee. That is the actual argument for the multi-pack, and it is a decent one.

Some of it is unavoidable and a lot of it is not, so it is worth being clear about which is which.

Unavoidable: at very dark roast levels, some of the material has genuinely carbonised. Those compounds are in the bean and they will end up in the cup. You cannot brew out something that is physically present.

Avoidable: most of what people experience as harsh bitterness in dark roasts is over-extraction. Dark roasted beans are more porous and more brittle than lighter ones, so they give up their solubles much faster. Brewing parameters tuned for a medium roast will over-extract a very dark one badly, and over-extraction is where the acrid, drying, ashy character comes from.

So the strategy is to extract less, deliberately, at every lever you have.

Preventing it is easier than curing it, and it comes down to two things.

Grind coarser. A press should be at the coarse end, and fines are what work into the mesh. If your grinder produces a lot of fines — common with entry-level or worn burrs — you will clog filters no matter how you clean them.

Rinse immediately after use, before anything dries. Wet grounds rinse out; grounds that have dried into the mesh with oil around them are what you are dealing with now.

The habit that makes the biggest difference is discarding the grounds and rinsing the mesh straight away rather than leaving the pot until later. Ninety seconds, and the filter never reaches this state.