Freshness is one of those difficult terms in coffee because it is often considered quite subjective.
However I was thinking about brewing stale coffee as espresso, and then thinking about measuring filter brewed coffee and an idea cropped up.
To me, when I brew stale coffee as espresso it seems that there are a lot less solubles in the brew – the pour looks very pale very quickly, if not all the way through the shot. So based on this observation I propose an experiment using filter coffee.
Roast up 5 kilos of a coffee and then set up to brew as filter coffee so that the percentage of extraction is around 20% for the chosen weight of coffee. (The brewing charts may differ on ideal strength but they all agree that 18-22% of solids extracted is considered ideal.) Let’s say we use a Nordic dose of 70g per litre.
Once this grind has been set for that dose then 3 brews a day are done for that recipe with a fixed grind (the grinder kept cool – no back to back grindings). The resulting brews are then measured for TDS, as well as the brew water so an accurate log of extraction percentages could be kept. This daily experiment is then repeated for 30 days and the results graphically logged.
The coffee could be kept a number of different ways for each type of experiment – craft packaged, valve packed, valve packed and nitrogen flushed.
I would hypothesize that though everything was kept constant (grind/dose/brew temp and volume) as the coffee aged it would start to be more difficult to extract. Whether you chose to see the point at which the coffee drops below 18% as the point it is stale, or perhaps another point that has some repeated statistical significance I don’t know. However I think that one could likely find enough data to give a reasonably accurate shelf life for truly fresh coffee.
It would also be interesting to couple the metering tests with cupping, but I worry that it would be too difficult to be objective/difficult to set up well.
I am sure this has flaws and would welcome any input on this before I start to play with it. (Plus I need a TDS meter!)
Thoughts in the comments please?