Fractional coffee brewing

December 29, 2010 11:43 PM

Exactly how are the first drips of coffee different than the last? Chemistry welcomed.

I use a simple 4-cup coffee maker and a blade grinder. The first cup of coffee that drips out tastes more like Intelligensia, and the last tastes more like Starbucks. I generally steal that first cup before all the brewing is done because I like my coffee less hot than the heating plate imposes.

I recall reading that most of the caffeine is extracted early, but what about the compounds that provide flavor, aroma, and mouthfeel? Is anything worthwhile coming through in those last few ounces? Am I ruining the first cup if I allow it to mix, or are coffee makers designed with the expectation of a full-pot mixture?

posted by RobinFiveWords to food & drink (8 answers total)

I don't know if you are "ruining" the first cup if you allow it to mix, but I use a Melitta #6 cone and a blade grinder, along with a tea kettle, which allows me to "listen" to various "boil points" prior to full out "steaming," (full whistling of my kettle). Accordingly, I can apply "near boil," "just on the boil," "full boil," and "after steam water" to any kind of filtered coffee grounds I like.

In the main, I find that less roasted coffee requires more (longer), exposure to hot water (and to hotter water, like "full boil"), than does more roasted coffee (which may pull out great flavor at "near boil" sounds from my kettle), to develop full flavor. Some of the best coffee I make is from some long roasted coffee (expresso roast) from Tampa roasters, that is very oily in the bag, ground very fine in a blade grinder, and then made in normal Melitta fashion, in a #6 cone, with whistling "boiling" water, from a common tea kettle.

Really, I've gotten new lovers, after I proffer a cup of that stuff....
posted by paulsc at 12:17 AM on December 30, 2010 [1 favorite]

I've certainly seen academic research studies on this very topic. For example, this one (behind a paywall) suggests that temperature as well as time determines the rate at which different components are extracted. In general though, caffeine and nice-tasting compounds are extracted early and bitter compounds extracted late.

People selling coffee machines seem to agree that flavour, colour and aroma are extracted early, bitter compounds (e.g. tannins) extracted late.

There's a number of links from this wikipedia article inlcuding to "brew charts", although they're plotted using extraction yield rather than extraction time.
posted by firesine at 3:13 AM on December 30, 2010

As far as I can tell, the blade grinder is what's causing your extraction woes.

Blade grinders grind (or rather, chop) very, very, very unevenly. Add to that that they make it very hard to achieve a consistent grind level from session to session, and you have a recipe for bad extractions.

Smaller grounds require less steeping time; conversely, larger grounds require more.

The reason your first cup tastes good is because the smaller grounds haven't had time to oversaturate yet. There isn't any inherent magic in the "first cup".

Solution? Get a reasonable burr grinder.

As an experiment, try sifting your coffee through a fine meshed sieve before you brew your coffee next time.
posted by flippant at 6:18 AM on December 30, 2010

(That said, there are always more extracted solids in the first cup. In a proper extraction, the balance of the end result - of the entire extraction - is what you're going for)
posted by flippant at 6:20 AM on December 30, 2010

The solubility constant defines how much of a given compound will dissolve for a given volume of solution at a given temperature. At equilibrium, it represents a fraction between solute (that which is dissolved) and precipitate (that which is not).

If you mix one cup of water with one tablespoon of coffee, a certain fraction of the total caffeine will dissolve into the water. If you were then to remove that water and add another cup of water to the used coffee grounds, the same fraction (of the remaining caffeine) will dissolve. So each cup of water that runs through the grounds will contain less total caffeine—the amount in each cup will drop off in a neat curve. The same goes for all soluble compounds in the coffee; each will drop off, though at different rates because each will have a different solubility constant. Compounds with a higher solubility constant will be depleted from the grounds faster. Thus, the amounts of the different compounds relative to each other will vary from cup to cup.

If the temperature of the water were to vary over time, that would affect the solubility constants (in general, higher temperatures equal more dissolution), but the temperature of the water being added to the grounds should be fairly constant.
posted by dephlogisticated at 7:52 AM on December 30, 2010

Moral of flippant's story: Please be thoughtful of your house/office mates* and don't swipe the first cup before the rest of the pot has brewed.


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*And if that doesn't cut it, be thoughtful of your future, second cup self.
posted by notyou at 8:25 AM on December 30, 2010

It depends on how much coffee you are making. The first cup out of a multi-gallon commercial Bunn-o-Matic is going to be different from the first cup out of yours.

I'm not even sure it counts as chemistry as much as physics? The first few drops out of the coffee maker have gone through dry grounds. It will have only the compounds that are most easily/quickly dissolved or mixed with water. The last few drops will have the stuff that takes the longest to dissolve. Also, the grounds will be going through the process of fully getting wet. The last cup will have a lot of the stuff from the 'inside' of the ground-kernels, too.
posted by gjc at 10:02 AM on December 30, 2010 [1 favorite]

Both water temperature and time in contact with water will change how much of the various flavoring compounds/oils, caffeine, and bitter tannins dissolve. This wikipedia article on ristretto shots of espresso gives a fairly good practical example of how changing different aspects of the brewing process can effect the flavor - for a similar effect, you can change the brew time, the grind, or the amount of space between grounds.

For coffee that tastes more like the first cup through the machine, I like cold brewing it. You let very coarsely ground coffee steep with cold water for 12 hours, and it pulls very little of the bitter and acidic compounds into the coffee. It creates a concentrate that you can mix with hot water, or with cold water for iced coffees. We have a Toddy coffee maker, but it's easy enough to make a batch in a mason jar or the like, as long as you have a good way to filter it.
posted by polymath at 3:12 PM on December 30, 2010