> Here's a question I've often wondered and finally remembered to ask
I for one, I am not willing to experiment much with this. My reasons are
simple (...and I in NO WAY intend to denigrate the question. The question is
an excellent poser, and, look, it isn't even about CO2 and Climate change!):
"Resting," I think is a quaint way of saying: "waiting for all the
additional chemical and physical changes to occur inside the bean that we
started when we popped those little critters into the roaster and added
To put it another way and to repeat what some others have said: resting is
much more than out-gassing.
1. You might note that certain varieties "require" different resting times
to reach what most (well, at least "many") would agree to be peak flavor or
2. Also note that differing roast levels "require" different resting periods
3. And also note that different preparation (brewing) styles "require"
differing resting times.
* I am changing "requires" to "suggests" from this point onward.
In general... (and remember, these are gross generalities. Also remember,
your taste may vary [YTMV]).
Espresso suggests longer resting time than the same coffee and same roast
used in drip brewing; just as the grind makes a difference, so does the
North Africans and Indo-Asian coffees (Ethiopia, Yemen, Sumatra, etc.) often
fare better with more extended rest times when compared to South Americans
and most Central American coffees.
Lighter roasts often do better with longer rests than darker roasts.
The question then becomes: "What's happening in there?"
It comes down to chemical processes that *mainly* (not exclusively) involve
carbohydrates, lipids, and proteins.
When you roast, the heat will partially break apart long chains of
carbohydrates (some are properly called starches) into smaller
carbohydrates. The smaller carbos are usually referred to as "sugars." If
you heat long enough without scorching or burning, you can caramelize those
sugars, producing the smoothness characteristic in certain varieties like
the Brazilians and many of the Bourbon varietals.
If you roast at a higher temperature or for a longer period of time, you
will continue to the break down or caramelize these sugars and then begin to
break down the lignin (protein) and cellulose (carbohydrate) structure that
makes up the cell walls of all the cells in the bean. Keep going and the
lignin turns to a kind of plastic, then loses its ability to hold the cell
wall together and the cellulose loses almost all of its oxygen and hydrogen
and become something close to pure Carbon. Some would call the coffee
"charcoal" at this point. If you have gone this far, there is little left of
any sugar, much less any other delicate flavor lent by lipids/oils or
Sometimes a "shorter" roast won't physically break down as many of those big
carbos and the result is a larger number of mid-sized carbohydrates. They
are less "sugary sweet" and less caramelly right at the end of the roast.
Give that coffee several days and some of these carbos will continue to go
through chemical changes that result in smaller chains and a sweeter,
Similar things happen with lipids (fats, oils, and waxes). Some lipids
consist of 3 long chains of fatty acids. Heating can alter the length and
shape of these fatty acids - some becoming more pleasantly flavorful and
other less flavorful or even having an "off" taste. Too high heat or
extended heating is more likely to make the lipids break down and oxidize
into stuff we don't really like. Roasted "properly" (depending on purpose
and your own taste) and left to rest in a more or less oxygen-free
environment, these fatty acids will join with one another to form new oils
that are quite nice. It takes time. The oils already present after the roast
also get a chance to be absorbed or be adsorbed into or onto the cellulose
and lignin matrix that hold the shape of the bean - and so, the oils are
spread throughout the coffee when you grind it and don't just sit on top of
the outer layer. Carbonized beans adsorb the oils and other organic
chemicals fairly readily and quickly; faster than a lighter roasted coffee
bean. It takes longer to get the lipids absorbed and adsorbed into the
cellular matrix of a bean with intact cellulose and lignin.
If you rest a bean with oil on its surface too long, you risk the bean
tasting bitter and rancid. Beans will oil on the surface tend go rancid much
more quickly for two main reasons: 1) the fatty acids have been broken into
much smaller fragments and are already on their way to oxidation and the
formation of some of the chemicals we associate with bad smells & tastes
(butarols, for instance - peeyew!), and 2) these smaller fatty acids having
been driven out of the bean and now sitting on top, are more likely to react
with oxygen in the environment and with each other...both resulting in
Do it "right" and let it sit for just the right time and you may even end up
with some esters that smell like flowers, ripened fruits, or even taste like
blueberries (love that Harar Horse!)!
The proteins in the bean come in a huge a variety of shapes and sizes. They
are quite complex. Under heating, they denature - mostly change their shape,
rather than change their size by "breaking down." Some do break down in size
and leave us with short chains of amino acids that lend acidic flavors and
even some sweetness. (Aspartame/Nutra-Sweet, after all, is simply two
naturally occurring amino acids stuck together.) Some of the proteins in the
bean are enzymes that have certain jobs like tearing apart carbohydrates
into sugars that the itty-bitty coffee-embryo can use. On heating,
especially high heating, that enzyme is rendered inactive. Other enzymes
have jobs of putting things together or eliminating nasties from the system
by breaking the nasty stuff apart or transferring it to a safe place. All
this is lost when the bean is roasted. Some of these proteins get changed
into other protein-like compounds that lend flavors. Some good, some bad
(the sulfur in some proteins can lend a "skunky" smell/taste, for instance).
Again, heat makes the initial changes and then time and the composition of
neighboring chemicals continue the job.
OK...so where are we? We've just taken a simplified look at the three big
players: carbohydrates, lipids, and proteins. Now add out-gassing of Carbon
dioxide and other aromatic gases resulting from the breakdown and
recombination of the big three, throw in some minerals and trace elements
that can link with any of the big three - and which vary with geographic
region of growth of the parent coffee tree - and you have a huge experiment
in probability. The possibilities are mind-numbing -- and when your mind
goes numb...it's time for another cup of coffee!
There is enough variability and chance in this coffee roasting journey for
everyone to enjoy!!
Grinding immediately to allow faster out-gassing probably works for just
that - more rapid out-gassing, but it certainly would NOT help the processes
of progressive chemical changes in the bean and grind/wait/brew would in all
likelihood increase dramatically the chances (almost certain to happen) of
oxidizing many of the chemicals that would normally develop into pleasant
flavors and the "mellowing" we love in a well-rested cup.