In other words, not every solvent is going to extract every solute because they are chemically different. Think oil and water. It is very hard to extract oil from a matrix if you’re using pure water. A concentration gradient is also required so that the solute in the matrix will diffuse to the solution. A concentration gradient exists if in one location there is a high concentration of a particle and in a nearby location there is a lower concentration of the particle. Particles tend to move down the gradient, from high to low concentration, until the gradient ceases to exist and the concentration is the same everywhere (the particles never actually stop moving in any direction, rather, they just fill up the space they’re in and are spread across it evenly). Thus, if the solvent is already saturated with solute, additional solute will not be removed from the matrix.
The coffee matrix is very complex and the molecules we hope to extract come in all sorts of shapes, sizes, and electrical charge densities. Water is a great solvent (especially hot water) because it has the capacity to hold on to (dissolve) all kinds of molecules. Pure water (completely distilled, with nothing else dissolved in it) will extract coffee solids differently than water with impurities (ions, metals, other molecules—basically things that make water hard, soft, or distasteful). This is because the impurities influence the concentration gradients or alter the electrical conductivity of the water. In short, not all water is equal!
The first rule of thumb about using water for your coffee is that if it tastes good as plain water, it might be good for coffee. Unfortunately, that isn’t always a guarantee. If you think water is a problem for you, procure filtered water or get a filter system that moderates the contents of the water. You can always check with your municipality’s water provider for a report on the quality. This chart, supplied by the Specialty Coffee Association of America, is a recommended guide to water quality for brewing coffee.
Recent research suggests an additional recommendation: brewing water should have 1 part bicarbonate (HCO3) to 1—2 parts double-charged cations (Ca2+ and Mg2+).
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