The dissolving of calcium and magnesium in natural waters makes water hard, and hard water is responsible for scaling, liming and other undesirable mineral deposits on brewing equipment. When water is heated and then cooled, these dissolved substances can deposit scale on metal surfaces. As scaling proliferates, these compounds restrict flow through spray heads and plumbing, and coat thermostats and auto-fill sensors, causing them to fail. The coating of the heating elements and talks make heating less efficient. This not only affects brewer operation, but also the flavor of the brew.
The Chemistry of Scale
They occur when negatively charged bicarbonates lose a hydrogen atom and combine with calcium or magnesium to form chalk scale. Oxidation of dissolved silicon or sulfites can also cause silica or calcium sulfate to grow on metal surfaces under these conditions and form a much harder and more insoluble compound.
We need some of these chemicals
Though they can be problematic, we don’t was to remove all of these hard chemicals from coffee brewing water. Calcium, when balanced with bicarbonate, sulfate or chloride in the water, chemically reacts with coffee to create favorable flavor extraction. However, excessive calcium is what causes scale build up in equipment. A balance in the relationship between quality of coffee flavor and equipment maintenance must be determined.
Like calcium, magnesium can cause scaling and have a negative impact on flavor. Silica can also cause scaling, but does not impact flavor.
The Langelier Index
The tendency of a water supply to form scale is a function of its hardness chemicals, total alkalinity and pH (together, forming the carbonate system). We can use the Langelier Index to estimate the tendency to scale or corrode (in the case of acidic waters). This index takes into account all measurable aspects of the carbonate system (pH, calcium hardness, total alkalinity, temperature) to determine the likelihood of corrosion or scale formation. By computing the results, we can reasonably predict the physical effect that our water quality will have on equipment. The Langelier Index calculator can be found at http://www.csgnetwork.com/langeliersicalc.html.
Using the Langelier Index is necessary because different aspects of the carbonate system influence the tendency to develop scale in different ways. For example, water with a pH between 7.0 and 6.5 results in lower scale formation in any piece of brewing equipment, especially espresso machines due to the pressure applied during brewing. A pH between 7.5 to 8.5, however, will tend to produce scale. The latter range is common in tap water.
