After all, if it is the caffeine we want, we might as well figure out how to get the most into each cup! With brewing, there are two main things to consider: brew method and filter type. Before we discuss those, let’s discuss solubility a bit. For every molecule, we can measure its solubility. Solubility is the amount of the molecule that can dissolve in a set amount of liquid at a given temperature. We’re interested, of course, in caffeine’s solubility in water. Caffeine is a little soluble in water at room temperature 73°F (23°C), about 16 mg caffeine/1 ml water. As the temperature of the water increases to 176°F(80°C) and 212°F (100°C) (boiling), the solubility jumps up to approximately 200 mg/ml and 666 mg/ml, respectively! What a great demonstration about how important temperature is to discussions of solubility!
When trying to figure out how much caffeine can potentially end up in a cup of coffee, it helps to know how much is in the roasted coffee to start with. The values for this vary quite a bit. In one study of an arabica collection in Ethiopia, the caffeine content ranged from .42 to 2.9 percent! Let’s use 1.15 percent for our calculation, using 20 g of coffee and 360 ml water. That 20 g of coffee contains 230 mg of caffeine. Since 360 ml of water at room temperature can dissolve 5760 mg of caffeine, it is possible to get all of that caffeine into the resulting approximately 11 oz cup of coffee. For myriad reasons, all the caffeine doesn’t come out during brewing, though most of it does. The two main reasons not all the caffeine is removed are that some of the caffeine will be preferentially attracted to the bean matrix over the solvent and that some water always remains in the bean mass (and will retain the same dissolved solids that the water in the cup also has).
Knowing we can extract most of the caffeine from the beans, is there a difference between brew methods in how effective they are? The answer is yes, although, unfortunately, we can only hypothesize as to why some of the differences exist; no researchers have published the “why” of caffeine differences, only the “what”.
The espresso brew method results in a brew that has a higher concentration of caffeine than any other brew method. One study shows a concentration of nearly twice as much compared to American drip, Neapolitan flip, and the Moka pot. Unpublished research by this author showed an almost seven times increase in concentration using espresso brewing! In general, methods that use hot water and no additional pressure don’t differ in their caffeine concentrations from each other. Coffee made using an Aeropress has a concentration somewhere between espresso and the other methods.
This leads one to think that pressure may be the important difference—it removes more caffeine and/or forces more water out of the beans. However, both the Aeropress and espresso use much lower water-to-coffee ratios than nonpressurized brew methods; they may have more caffeine because more coffee was used! However, other brewing parameters are at play here, too. Finer grinding, when producing small batches of coffee (single cup versus full pot), increases the concentration.
Filters also influence the concentration by intercepting the caffeine as it passes through them. One Brazilian study compared the effect using five different types of filters had on the caffeine concentration of the final brew. The results indicated highest concentrations using a nylon filter, followed by white paper, then brown, unbleached paper and cotton, with flannel yielding the lowest concentrations. In some contradiction, unpublished research by this author showed no difference in concentration using a paper filter or a gold metal filter.
For the practical coffee drinker, an important thing to keep in mind is not just the difference in caffeine concentrations but the total caffeine intake per unit. The classic case is espresso verses drip. In the United States, most serving sizes of espresso are 1 to 2 ounces (29.5 to 59 ml) whereas a cup of drip can be 12 to 16 ounces (355 to 473 ml). So, while espresso may have more caffeine per unit of brew, a normal serving of drip is so much larger that its final content of caffeine is typically much greater than that small serving of espresso. The same is true for the filters. While there was a mathematical difference in the concentrations, sometimes the difference was small enough that a person’s body might not recognize the difference as being physiologically important.
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