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So, the coffee is perfectly ground for the brewing system you are using, what is the next variable we can investigate and optimize?


You may have a bag of the most premium Gesha coffee you can afford, but if you are brewing with tap (or even worse, over filtered) water, that coffee will come out with significantly less balance and nuance. The important thing to keep in mind here is that not all water is created equally, and some waters serve the purpose of coffee extraction better than others. Considering that your final cup will be upwards of 99% water, it is important to utilize some with the optimal characteristics to effect brew flavor. 

The SCAA provides official guidelines on water quality for proper extraction.

Of course the water you use should be free of any discernible odors as well as completely clear in color. There is a great deal of information included, but the things this article will focus on are the total dissolved solids, calcium composition and sodium. Alkalinity is a measure of alkali molecules, which includes calcium and magnesium. pH, which is an abbreviation of the potential of hydrogen, is a measure of the acidity of a solution by way of concentration of H+ (hydrogen) and OH- (hydroxide) ions, and water should ideally always sit at perfect neutral (7). While it is unreasonable to assume that the layperson would have equipment to measure these aspects, there are certainly filtration products and bottled water that deliver on some or all of these characteristics. But, to garner greater understanding, let’s look at each aspect individually, so we can understand how they affect coffee extraction.

Total Dissolved Solids

This represents the amount of matter dissolved in a volume of water. Oftentimes measured in parts per million or mg/L, this number will tell you how dense your water is with minerals and salts, without specifying what those minerals might be. Distilled water ideally represents 0 dissolved solids while spring water can exceed 450 parts per million. Typical tap water concentrations are maintained between 100-150 ppm, and bottled waters can vary from 0 to 500 ppm. Bear in mind, however, that this number does not have a direct correlation to the initial flavor of the water as it does not specify WHAT is dissolved in the water. 

Aside from direct impact on brewed coffee’s flavor, the number of total dissolved solids will give you a good idea of how much coffee material you will extract out of the ground beans. Lower TDS quantities tend to over extract coffee as there is more diffusive potential in the water. Strictly speaking, diffusion potential refers to an accumulated charge along a membrane due to unequal ion distributions about either side of the membrane, but here it will be used as a descriptor of a particular solution’s ability to absorb dissolved solids. A lower concentration of particles in the brew water means there is more space for aromatic and flavor compounds to diffuse from the ground coffee. 

Higher TDS quantities oftentimes lead to under extraction for the opposite reason; if the brew water is chock full of minerals, there is no space for coffee compounds. 

Calcium and Hardness

Inherently included in, but not specified by a TDS reading, is the measure of a water’s hardness. This correlates to dissolved calcium (and somewhat also magnesium) concentration, and has a broader range of acceptable values according to the SCAA. Beyond contributing to a water’s overall TDS, the chemical structure of each molecule effects brew flavor. Both compounds are positively charged, and work to pull negatively charged flavor ions out of the ground coffee. Too high a concentration of calcium, and the flavor becomes over extracted and harsh. Too low and the flavor is barely discernible. 


A final contributing factor to total dissolved solid concentration is sodium. Sodium is also a positively charged ion that will attract negatively charged molecules, but due to sodium’s low binding energy, most sodium ions will gravitate toward the hydroxide ions inherent in water. Therefore, sodium does not facilitate much aromatic diffusion in the cup. The important contribution of sodium in water is the ability to draw in bicarbonate ions which act as acidity neutralizers. Too many bicarbonates, however, and your coffee will just taste flat and dull. 

While this is a cursory look at water composition for coffee brewing, it should serve as a good initial guide as to what type of water to look for. In the next post in our water series, we will cover varieties of water’s available, and their effects on coffee extraction.

Brandon Nylund

Hill Tree Roastery Coffee Education Advisor


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