The amount and nature of solids already dissolved in water are referred to as Total Dissolved Solids. The source of the water supply chiefly determines the type and concentration of TDS.
Ground water retained in aquifers or dispersed throughout soil may have a high concentration of minerals, depending upon the composition of the aquifer, soil, or rock in which it’s resting. For example, limestone will dissolve easily under acidic conditions and create calcium-infused (i.e., hard) water. By contrast, granite will not.
Biological metabolism of various microorganisms in soil can also contribute carbon dioxide, which can lower the pH and give the water an increased capacity to dissolve minerals.
On the other hand, surface water, which includes sources from rivers and lakes, typically has less contact with soil and minerals, and usually has lower TDS. Worth mentioning, is that these sources are rarely unaffected by human activity. There are a variety of conditions that can change the TDS in water from day to day. For instance, acid rain from air pollution can lower pH and higher mineral contents in poorly buffered lakes. Industrial and agricultural wastes can add more solids to the surface water supply. Although municipal water treatment eliminates most (but not all) of this activity, water may still carry musty-smelling products of lichens, moss and molds, as well as the nitrogenous products of mammals or fertilizers added to soils in agricultural areas.
Surface water from rivers and lakes is often recycled in municipalities multiple times as they take water in for their use and discharge treated water. Further downstream (in the case of rivers) the next municipality will do the same. Municipalities on lakes may be recycling their own discharge, as well as that from other municipalities bordering that lake. A single municipal use of water treated and discharged into a lake or river usually adds to the total dissolved minerals. And all of these conditions change the TDS from day to day.
A water source is considered safe if it has less that 1,000 mg/L TDS. If dissolved solids are present in high concentration, the flavor potential of the coffee being brewed will not be fully realized. The reason is water has a reduced capacity to solubilize flavor-producing substances. Additionally, the type and concentration of these substances affect the hydrodynamic dispersion of water as it penetrates the bed of coffee and the individual coffee particles. As a result, dwell time and the level of extraction are affected. In water testing scenarios, it was found that a TDS of 120-150 mg/L brewed a coffee of noticeably higher quality than water with a TDS of 300 mg/L.
Some ions and compounds, when dissolved in distilled water, can have a great effect on extraction during brewing than others. For instance, studies have shown extraction ranging from 1.04% TDS with calcium chloride, to 1.29% with sodium bicarbonate. The reason for this is calcium binds with the negatively charged soluble organics and carbohydrates in coffee, and sodium has a significantly lower positive charge and, thus, very little binding or even dispersion of the soluble coffee compounds.
This phenomenon can be observed similarly by how hard water containing calcium forms a soap scum in the bathtub. The calcium binds with the soluble soap compounds forming (precipitating) the soap scum ring (solid and less soluble). When the water is softened – meaning sodium ions are exchanged for the existing calcium ions – the soluble soap compounds remain soluble and flow down the drain, leaving no soap scum ring in the tub, but leaving the water saltier.
Other dissolved solids have other effects on brewing. Bicarbonate tends to expand the coffee particles and acceleration extraction. While doing so, the calcium is binding to specific substances, creating a unique flavor that would not have happened during brewing with another water chemistry.
When we measure TDS we gain an overall picture of the amount of dissolved solids present in the water supply. Increasing the ionic loading (TDS) of the water supply will limit the level of soluble compounds that will be extracted. The higher the level of TDS in water, the lower level of soluble flavor compounds. One of the primary goals of water treatment must be to limit the level of TDS to an acceptable level.
