Yesterday morning was our usual rendez-vous at Counter Culture Coffee's espresso lab. Ironically one "problem" with our Friday coffees is their freshness. They are too fresh, often roasted the prior day. That was the case yesterday and everyone was struggling to pull a decent shot. What followed was an interesting discussion of why the "rest period" is important for espresso.

My observation was that no matter how carefully I prepared the puck, the extraction would accelerate to blonding for the last third of the pour. Peter commented this is a common problem with overly-fresh beans, citing the "wettability" of the coffee as the cause. He explained that recently roasted coffees (and young greens) offer barriers to extraction. Brewers incorporate delicate pre-infusion wetting to let the coffee "bloom" a bit before being hit with the full flow for this reason. The consequence when making espresso is what I noted: An extraction that begins wonderfully and then accelerates.

Bob Barraza is a retired chemist and offered other interesting commentary. He explained that the trapped carbon dioxide under high water pressure is ripe for entering into solution, producing carbonic acid. The pH lowers and contributes to the acidic (bright) taste of overly-fresh coffees, plus the extraction profile is changed. That is, acidic water won't extract solubles the same way as normal pH water, and the solubles it favors aren't the ones we prize.

For most freshly-roasted coffees, the rest period is three to five days. Peter mentioned that they paid dearly for being one day off in competition: The extractions were harsh and bitter, yet the next day the espressos prepared by the same barista using the same coffee were dreamy. Now they give careful attention to how the characteristics change as the coffee degasses.

No doubt. Corrected.


From the EPA's Students Site