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Permalink Reply by Chris DeMarse on August 4, 2008 at 7:19pm

Dale,
Though this question is a bit broad and covers a lot of material, let me break down some of the science behind extraction that might help you adjust the variables in extraction.

Extraction, by definition, is a process of removing the essence of any one material. Therefore, we are attempting the remove the essence of a ground coffee bean to remove three basic categories of chemical compounds:
Volatiles: Chemicals that are released a largely gasious state that affect the olfactory lobes in the body, stimulating them to give a perceiveable scent. They are the most unstable of the chemical compounds and quickly dissipate.
Colorant: Chemicals that affect the material doing the extraction (in our case water) to give the water a semi-translucent to opaque appearance. (Black coffee)
Flavor: Compounds (typically linked with volatiles) that give the coffee a level of perceived acidity, body, and specific taste as well as aftertaste.

The principles of any extraction are based on time and frequency of contact. This means that both the time that the water interacts with the coffee and the temperature of the water critically affect the extraction rate. The colder (slower moving) the water is and/or the shorter the amount of time the coffee is in contact with the water, the less extraction happens, and it works vice versa.

Temperature is stable in a dual boiler machine, but a heat exchanger will often need to be flushed significantly to achieve a somewhat stable temperature (that's a whole discussion in and of itself)

In extraction, the first thing extracted is typically dissolved solids. These contribute to the body and, if the beans are blended to reflect such a characteristic, a sweetness. Next come the emulsified oils which contribute to a large component of the flavor and acidity level.

The commonly accepted dosing rule is 14-18 grams for a double espresso. By dosing up, if the grind is "correct", you will get more solids in your extraction time. Dosing down lends itself to more oils.

If you are within the parameters of dose, the fineness of your grind affects the contact time. Finer= longer time whereas coarser=shorter conact time. This will again affect how much extraction occurs. Underextraction will be thin and grassy with possible astringency. Overextraction can taste chalky, bitter, astringent and even burnt.

Time, again, is a factor. The commonly accepted window of time is 20-30 second from initiation of the brew cycle with a relative aim around 25. Long shots are heavier on the oils whereas shorter shots are more solids

Properly filtered water is huge is flavoring (again another conversation)

Volume is also a factor. The typical parameters for a double shot of espresso range anywhere from 1.5-3 oz. of extracted liquid with the crema. Longer (3 oz. (lungo)) shots are less concentrated in solids by volume and exhibit more of the emulsified oil whereas shorter (1.5 oz.(ristretto)) shots are heavier bodied and sweeter with less discernable flavor characeristics.

I hope I have thoroughly and accurately covered some of your questions that can define your shot quality. I hope I've done more than just confuse or overwhelm you.

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Permalink Reply by Lee Wardle on August 5, 2008 at 9:40am

What he said! :P

Seriously though. To take one point from the above, play around with extraction times. You would be amazed at the difference 1 second either way with make to the flavour. Star around 19 and work your way up in 1 second intervals.

Lee

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Permalink Reply by Brady on August 5, 2008 at 10:56am

Great post Chris, thanks!

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Permalink Reply by Dale Harris on August 5, 2008 at 1:45pm

this is great and certainly defines the variables really well but what effect does adjusting just one in isolation have on extraction and flavour in the cup

e.g. i get a new blend i've never tasted before with no notes suggestions etc.
i go for my standard extraction so grind &dose 15g, purge group, attach and brew, adjust grind to get 25sec extraction for 2oz - this results in taste 'x'
can i adjust this to get taste 'y' highlighting specific flavours of this blend?

if i up the dosage to 16g/adjust brew temp/ what difference should it make? is it the same with all blends coffees or will one become more acidic and one more bitter

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