Saturday, January 26, 2013

Dialing In Your Coffee Grinder

Brewing coffee presents a unique problem. It is extremely sensitive to a variable that most people have no way of measuring: extraction yield. Tea, by contrast, is tasty across a wide range of extractions, as evidenced by the fact that multiple steepings of the same tea leaves can produce delicious results. Bitter tea is over-strong, not overextracted.

So when you brew coffee, you're aiming at a very small target (18% - 22% extraction) which you have no way of measuring. To hit that target, most people rely on recipes handed down by a trusted source. But the problem with just following a recipe is that even if you can control water temperature, coffee dose, and brewing time, there's no way for a recipe to specify exactly how fine your grind should be. Typically, the recipe will specify, say, 'medium-fine' or 'the consistency of table salt', but this is far too inexact. Even a small difference in grind size can have a major impact on extraction yield. Furthermore, no grinder produces a perfectly even grind; every grinder produces a slightly different grind profile. What that means is that even if the average grind size is identical, the grounds produced by two different grinders will extract slightly differently.

Now, I do think recipes are a useful starting place, but everyone needs to dial the recipe in on their own grinder. The only way to do this, short of a coffee refractometer, is to rely on taste. So you should be able to taste underextraction and overextraction, and know how to correct for both. All this assumes that you're starting with good, fresh coffee, good water, and clean equipment.

1. Overextraction. This is relatively easy to taste. Overextracted coffee is harshly bitter, often astringent or solvent-like. If you want to know what overextraction tastes like, try boiling your coffee. Often times overextracted coffee will taste as if it is roasted darker than it actually is, because the bitter compounds that are extracted last are mostly dry distillates—burnt sugars and heavy maillard compounds—which are more concentrated in darker roasts. If your coffee tastes overextracted, use a coarser grind.

2. Underextraction. This is a bit more subtle. Underextracted coffee generally lacks sweetness and complexity, and sometimes has a tea-like flavor. Underextracted coffee is often described as being overly acidic or sour; this is not strictly true. Think about it: why would a cup with less extraction have more acid? However, it is true that acids extract more quickly than most compounds in coffee, so an underextracted cup may have a higher proportion of acids, which might lead to higher perceived acidity. If your coffee tastes underextracted, use a finer grind.

In order to dial-in your brewing technique, you need to make sure that you have control of all variables. That means weighing your coffee. Measuring by volume will make it much more difficult to dial-in your grind. Start with a middle-of-the-road dosage, say, 60 grams of coffee per liter of water (14g per 8 oz). If you're brewing manually (pour over, French press, Aeropress), you need to measure water temperature and brewing time. 200˚F is a good place to start for water temperature, but more importantly, be consistent with temperature and brew time.

Brew, taste, adjust grind if necessary, repeat. Take notes. Since overextraction is easier to taste, it can be a useful exercise to push the grind finer and finer until you taste overextraction, and then back off a bit. There are a lot of variables that affect coffee extraction, but most of the time you can get a decent cup just by manipulating the grind. In general, people worry far too much about the brewing device, and far too little about the grinder. Yes, you need an adjustable burr grinder, but once you have one, you can brew very good coffee even in a cheap automatic coffee maker—or a clean cotton sock—provided you know how to dial in your grind.

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