Brewing a great cup of coffee depends on chemistry and physics

There are many ways, though, to achieve a drink containing 1.2 to 1.5 percent coffee. A pour-over, Turkish, Arabic, Aeropress, French press, siphon or batch brew (that is, regular drip) apparatus – each produces coffee that tastes good around these concentrations. These brew methods also boast an advantage over their espresso counterpart: They are cheap. An espresso machine can produce a beverage of this concentration: the Americano, which is just an espresso shot diluted with water to the concentration of filter coffee.

There are two families of brewing devices within the low-concentration methods – those that fully immerse the coffee in the brewing water and those that flow the water through the coffee bed.

From a physical perspective, the major difference is that the temperature of the coffee particulates is higher in the full immersion system. The slowest part of coffee extraction is not the rate at which compounds dissolve from the particulate surface. Rather, it’s the speed at which coffee flavour moves through the solid particle to the water-coffee interface, and this speed is increased with temperature.

Pour-overs and other flow-through systems are more complex. Unlike full immersion methods where time is controlled, flow-through brew times depend on the grind size since the grounds control the flow rate.

The water-to-coffee ratio matters, too, in the brew time. Simply grinding finer to increase extraction invariably changes the brew time, as the water seeps more slowly through finer grounds. One can increase the water-to-coffee ratio by using less coffee, but as the mass of coffee is reduced, the brew time also decreases. Optimization of filter coffee brewing is multidimensional and tricky.

Even if you can optimize your brew method and apparatus to precisely mimic your favourite barista, there is still a near-certain chance that your home brew will taste different from the cafe’s.

There are three subtleties that have a tremendous impact on the coffee quality: water chemistry, particle size distribution produced by the grinder and coffee freshness.

Given coffee is an acidic beverage, the acidity of your brew water can have a big effect. Brew water containing low levels of both calcium ions and bicarbonate (HCO₃⁻) – that is, soft water – will result in a highly acidic cup, sometimes described as sour. Brew water containing high levels of HCO₃⁻ – typically, hard water – will produce a chalky cup, as the bicarbonate has neutralized most of the flavorsome acids in the coffee.

Every coffee enthusiast will rightly tell you that blade grinders are disfavored because they produce a seemingly random particle size distribution; there can be both powder and essentially whole coffee beans coexisting. The alternative, a burr grinder, features two pieces of metal with teeth that cut the coffee into progressively smaller pieces. They allow ground particulates through an aperture only once they are small enough.

Roasted coffee contains a significant amount of CO₂ and other volatiles trapped within the solid coffee matrix: Over time these gaseous organic molecules will escape the bean. Fewer volatiles means a less flavorful cup of coffee. Most cafes will not serve coffee more than four weeks out from the roast date, emphasizing the importance of using freshly roasted beans.

So don’t feel bad that your carefully brewed cup of coffee at home never stacks up to what you buy at the café. There are a lot of variables – scientific and otherwise – that must be wrangled to produce a single superlative cup. Take comfort that most of these variables are not optimized by some mathematical algorithm, but rather by somebody’s tongue. What’s most important is that your coffee tastes good to you… brew after brew.