Until the 2020s, the importance of water in coffee brewing was rarely emphasized. Many people overlooked its significance. However, as the specialty coffee industry entered the 2020s, an increasing number of professionals began to stress the critical role of water. This shift occurred because many had experienced firsthand how the chemical composition of water profoundly influences coffee flavor.
As interest in water grew, so did the demand for water filtration systems, eventually leading to the emergence of cafés that produce their own brewing water. The right water can make an underdeveloped coffee taste pleasant, while the wrong water can make a well-developed coffee seem underdeveloped. It can alter the perceived intensity of acidity, thereby reshaping the overall impression of the coffee, and certain minerals within the water have a direct impact on flavor.
Yet, water still holds many mysteries. The precise roles of individual minerals remain largely undefined. While the functions of some minerals can be scientifically interpreted, the majority are still poorly understood—despite the fact that even trace amounts, at parts-per-million or lower, can dramatically alter our sensory perception.
When studying water, it is essential to have a clear objective. The purpose is not to identify a single “perfect water” for coffee—such a concept cannot exist in absolute terms. Each coffee possesses unique flavor compounds, and minerals in the water interact with these compounds differently. A coffee with bright acidity requires different water than one with low acidity; a coffee with strong chocolate notes will benefit from different water than one dominated by floral aromas. Moreover, personal taste preferences vary widely, making it virtually impossible to determine one universally optimal water for all people and all coffees.
The real reason to study water is to expand our understanding of possibilities. When problems arise, we must be able to consider a wide range of potential causes. We must identify the most likely cause through logical reasoning, then test each possibility in order of likelihood. Without knowledge of water, we may never resolve a series of issues that are, in fact, water-related. A coffee may taste flat solely due to the water, yet we might wrongly blame the roast profile or the brewing method—or even doubt our own sensory abilities.
Water is fascinating. Water is exciting. Even the slightest changes in its composition can lead to dramatic shifts in coffee flavor. We must remember that water has a profound influence and often stands as one of the most probable causes when coffee flavor changes unexpectedly.
In this section, we will examine the chemical composition of water—specifically, the types of minerals it contains. We will also explore the indicators used to measure them: TDS, hardness, pH, and alkalinity. Finally, we will investigate how these factors influence coffee extraction and the resulting flavor profile, and conclude by discussing water treatment methods and practical applications of filtration systems in the coffee industry.
This section draws upon the results of sensory evaluation experiments conducted over the past five years with approximately 100 professional baristas (hereafter referred to as the “CoffeeLink Experiment”). A general overview of the experimental procedures and conditions is provided here, with full details available in the appendix.
TDS in Water
The concept of total dissolved solids is also applied when measuring coffee concentration. However, because coffee is a complex mixture of solution, colloids, and suspended solids, TDS is not an entirely accurate descriptor in that context. When discussing the concentration of substances dissolved in water, however, the TDS concept is entirely appropriate, since—unlike coffee—most substances present in water are fully dissolved.
The water we use contains countless dissolved minerals, which in this context typically refer to the ions present in solution. The chemical characteristics of water are determined by these ions. Drinking water typically contains cations such as calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), and hydrogen (H⁺), along with anions such as bicarbonate (HCO₃⁻) and chloride (Cl⁻). These ions enter the water through various pathways, and their concentrations vary significantly depending on the properties of the water source.
Dissolved minerals can create distinct and perceptible sensory differences, even to the average consumer. Some minerals may also negatively impact water quality for domestic use. For this reason, such characteristics are considered when evaluating water quality.
Because the amount of dissolved ions is extremely small, it is measured in ppm (mg/L). The most accurate method for determining TDS is to evaporate the water and weigh the residue left behind. However, measuring electrical conductivity offers high enough precision that it has become the more common approach.
The Relationship Between TDS in Water and Coffee Flavor
It might be tempting to assume that if TDS is low, the water has more “empty space” to accommodate coffee solubles, and thus extraction would be higher—leading to the claim that using pure or distilled water results in over-extraction. Conversely, some argue that if TDS is high, the water is already saturated with ions, preventing proper extraction. In reality, both claims are incorrect.
Saturation is substance-specific; the presence of many minerals does not inherently prevent coffee solubles from dissolving. In fact, there is no significant difference in extraction yield between coffee brewed with low-TDS water and coffee brewed with high-TDS water. Paradoxically, coffee brewed with low-TDS water can give the sensory impression of lower extraction, often tasting lighter and less intense in flavor.
Ultimately, TDS does not reliably explain extraction dynamics or the final flavor profile of coffee. TDS alone cannot define the nature of water, as it measures only the total mineral content. It tells us nothing about which minerals are present or in what proportions—and at a point where the role of each individual mineral becomes increasingly important, TDS is far from a sufficient indicator.
The only cases where TDS may provide limited practical guidance are when values are extremely low (below ~20 ppm) or extremely high (above ~200–300 ppm). For reference, the TDS of municipal tap water in South Korea is typically around 70 ppm; Jeju Samdasoo mineral water ranges from 20–30 ppm; and Evian mineral water often exceeds 250–300 ppm.