Why coffee needs to ''rest'' before making espresso

Discuss flavors, brew temperatures, blending, and cupping notes.
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HB
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Postby HB » Sat Mar 11, 2006 3:45 pm

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.
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Postby AndyS » Sat Mar 11, 2006 4:41 pm

HB wrote:Bob Barraza is a retired chemist and offered other interesting commentary. He explained that the trapped carbon dixoide under high water pressure is ripe for entering into solution, producing carbonic acid. The pH rises and contributes to the acidic (bright) taste of overly-fresh coffees, plus the extraction profile is changed.


Undoubtedly Bob said that the carbonic acid lowers the pH.
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Postby HB » Sat Mar 11, 2006 4:58 pm

No doubt. Corrected. :oops:

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Postby another_jim » Sat Mar 11, 2006 9:25 pm

A minor dissent here, not about the facts of Dan's post, which are correct, but about the conclusion that espresso should rest a few days. There's an equipment based work-around.

LM baskets extract evenly, with the colors lightening very slowly; whereas Faema and Cimbali baskets are much more "frontloaded," and have a rather sudden change from dark to light. This has nothing to do with double boilers versus HXs, but rather that the LM baskets seem to have more holes of a smaller size. The effect remains the same when the baskets are exchanged on the same machine. Moreover, since all three designs (I'm talking real LM ridged) hold roughly the same 18 to 20 grams of coffee, the baskets are very comparable.

If the color gets light fast on an LM basket extraction, the shot **will** be bad, period, regardless of the coffee's age. It's almost impossible to make a ristretto with one that isn't far harsher than a normale from the same coffee. The other two baskets work the other way, it's actually tough to make a good normale with a Cimbali or Faema basket.

This gets us to out of the roaster espresso. The taste variations when making ristrettos with a Faema basket are actually quite minor, the out of the roaster shots are slightly brighter and more aromatic, but no more so than if the coffees are brewed normally. Using LM baskets, yech!

Here's my guess: the conventional wisdom is that the later part of the shot is weak and bitterish, which is true for degassed beans. My guess is that the later part of an out of the roaster shot is coffee-seltzer, sourish rather than bitterish (although I've never tried this). This would explain why the change in taste balance is so dramatic pulling normales on LM baskets; while the dark stopped shots from a Faema basket are hardly affected.

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Postby barry » Sat Mar 11, 2006 10:11 pm

i've never been a big fan of the idea of "resting". i recognize that coffee shifts in taste and performance as it ages, and so pulling great shots can be a bit like hitting a moving target. as an industry/hobby which pays so much lip service to the idea of "fresh" coffee, it is astounding to me the number of folks who readily let their coffees begin to stale before using them. really folks, letting the coffee sit for a couple of days is staling it... "a rose by any other name..." and all that. why do people freak out about letting ground coffee sit for a few minutes/hours, and yet they think nothing of letting the beans sit for a few days? fwiw, approximately 75% of the CO2 in a bean is liberated during grinding. if you have fresh coffee, have you tried letting the grounds sit for a couple of minutes before use to allow that CO2 to dissipate?

i really have to laugh sometimes when folks talk about home roasting to get fresh coffee, and then letting it sit for 3 to 5 days before using it. really.

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Postby Nick » Sun Mar 12, 2006 12:35 am

barry wrote:i've never been a big fan of the idea of "resting".

So you don't believe in degassing?

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Postby barry » Sun Mar 12, 2006 5:24 am

Nick wrote:So you don't believe in degassing?


i believe that degassing is the first stage in staling, as the more volatile aromatics are lost with the CO2.

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Postby AndyS » Sun Mar 12, 2006 1:20 pm

barry wrote:fwiw, approximately 75% of the CO2 in a bean is liberated during grinding. if you have fresh coffee, have you tried letting the grounds sit for a couple of minutes before use to allow that CO2 to dissipate.


No, I haven't tried it, but I will at the first opportunity. Are you really suggesting that all it takes to make delicious espresso with beans right out of the roaster is letting the grounds sit for a couple minutes before packing? That would be nice.
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Postby King Seven » Sun Mar 12, 2006 4:30 pm

What about the possibility that not all staling reactions produce a negative effect?

I know most staling reactions are bad - but surely some of the chemistry going on in the rest phase makes it taste better?

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Postby barry » Sun Mar 12, 2006 5:12 pm

AndyS wrote:Are you really suggesting that all it takes to make delicious espresso with beans right out of the roaster is letting the grounds sit for a couple minutes before packing? That would be nice.


a finer grind helps, too. ;)


the espresso will be more acidic than after it's staled for a few days, but that can be figured into the blend.