The Impact of Dissolved Minerals on Coffee Flavor
Water purification refers to the process of removing impurities from raw water through sedimentation, filtration, and other treatments, in order to produce water that meets safety and quality standards. The sources from which water is drawn are called intake sources or water reservoirs. In Korea, there are numerous such water sources distributed across the country. Water obtained from these reservoirs in its initial state typically contains a variety of harmful substances, which must be removed through purification—a process known as water treatment.
The treatment process generally consists of chemical dosing, sedimentation, filtration, and disinfection. Because Korea’s water treatment infrastructure is highly advanced, tap water is considered safe for immediate consumption. However, the chlorine and other disinfectants used in the final treatment stage often produce odors or tastes that many people find unpleasant. Furthermore, even after treatment, the chemical composition of tap water can change as it travels through the distribution system—most often due to aging pipelines. For these reasons, households often either boil water to remove off-flavors or use water purifiers to make it more palatable. Many also prefer bottled water for drinking.
Tap water used in homes or cafés begins its journey at the intake stage, where raw water is drawn from rivers, lakes, or dams. Most tap water in Korea originates from surface water rather than groundwater. The purpose of purification is to make water clean and safe to drink—it does not fundamentally alter the chemical composition of the water. Thus, the chemistry of tap water is primarily determined by the characteristics of the source.
The next step involves adding coagulants to aggregate and settle suspended particles and colloids. Common coagulants include aluminum sulfate (Al₂(SO₄)₃) and ferric chloride (FeCl₃). This stage removes over 90 % of turbidity and suspended matter. Any remaining fine particles or dissolved organic matter are then removed during filtration, which typically employs sand, gravel, and sometimes activated carbon. Finally, disinfectants such as chlorine (Cl₂) or chlorine dioxide (ClO₂) are added, and the pH is adjusted appropriately. The result is the treated tap water we use daily.
Although Korea’s tap water is safe for direct consumption, additional purification is often performed for specific purposes. Activated carbon filters are sufficient for removing odors and chlorine. Ion-exchange filters are required to reduce hardness. To remove nearly all dissolved minerals, reverse osmosis (RO) purification must be employed.
Types of Water Purification Systems
From a chemical standpoint, purification systems can be broadly divided into two categories: direct-flow systems and reverse-osmosis systems.
A direct-flow system purifies water simply as it passes through inline filters connected directly to the plumbing. In contrast, a reverse-osmosis system uses a semi-permeable membrane and external pressure to separate pure water from contaminants.
In commercial café environments, the direct-flow system is typically preferred. Reverse-osmosis systems are less suited to café use and are more commonly found in household applications.
Types of Purification Systems
- Reverse Osmosis (RO)
- Direct Flow
Reverse Osmosis (RO)
Reverse osmosis operates by reversing the natural process of osmosis. To understand this, one must first consider osmosis itself: when two solutions of differing concentrations are separated by a semi-permeable membrane, water molecules naturally and spontaneously move from the less concentrated side to the more concentrated side.
Reverse osmosis, as the name suggests, applies external pressure to force water to move against this natural gradient—from the concentrated side toward the dilute side. Originally developed as a method of desalinating seawater, the technology is now widely used to purify groundwater and other sources containing high levels of impurities.
A reverse-osmosis purifier includes a pre-filter (to remove large particles), a high-pressure pump (to create pressure greater than the natural osmotic pressure), and the essential semi-permeable membrane—a barrier with pores as small as approximately 0.0001 μm. This membrane allows only water molecules to pass through, blocking bacteria, viruses, heavy metals, organic matter, and over 99 % of dissolved minerals.
However, the RO system has several drawbacks for café use:
- Excessive water waste. Reverse osmosis often produces significant wastewater as the membrane must be periodically flushed to prevent fouling. This cleaning process consumes large volumes of water.
- Slow purification rate. Because the RO membrane is extremely fine, water passes through it slowly, resulting in limited flow rate. Most RO systems therefore require a storage tank, which is impractical in commercial environments.
- Over-purified water unsuitable for coffee. RO filtration removes not only contaminants but also beneficial minerals essential for coffee extraction. The resulting water is often nearly de-ionized, slightly acidic (pH 5.0 – 6.0), and potentially corrosive to metal pipes. Since minerals such as calcium and magnesium play key roles in extraction chemistry and flavor development, RO water tends to reduce extraction efficiency and dull the cup’s flavor complexity.
In cases where raw water contains excessively high mineral content, a partial remedy is to blend RO-treated water back with a portion of the untreated source water to restore mineral balance.
Direct-Flow Purification
A direct-flow system purifies water as it passes directly through a filter without intermediate storage. Because the pore size of the filter is much larger than that of an RO membrane, it allows minerals to remain in the water. This makes it particularly well-suited for café use.
Its advantages include:
- High purification speed. Water flows rapidly through the filter.
- Selective mineral control. Depending on the filter media used, certain minerals can be selectively removed.
- Compact system design. With no storage tank required, the system occupies minimal space.
- No wastewater. Unlike RO systems, it does not generate reject water.
Direct-flow systems may be less effective when used with heavily contaminated groundwater, but they perform excellently with Korea’s clean municipal water supply.
| Purification System | Advantages | Disadvantages |
|---|---|---|
| Reverse Osmosis (RO) | Produces highly pure water | Generates wastewater; removes beneficial minerals; slow purification rate; increases risk of pipe corrosion |
| Direct Flow | Fast purification rate; adjustable mineral composition | Less effective for highly contaminated sources; dependent on raw-water quality |
Types of Purification Filters
Purification filters can be classified into three fundamental types based on their mechanisms: adsorptive filters, mechanical filters, and chemical-conversion filters. Most commercial filter cartridges are composites that integrate several of these functions. For instance, an ion-exchange cartridge may also include a mechanical pre-filter and an activated-carbon layer for odor removal.
Types of Filters
- Adsorptive Filters (Activated Carbon)
- Mechanical Filters
- Chemical-Conversion Filters
Adsorptive Filters (Activated Carbon)
Adsorptive filters remove contaminants by binding them to the surface of porous materials. The most common example is the activated-carbon filter, typically made from coconut shells or other plant-based carbon sources. Activated carbon has a remarkably large surface area—up to 2,000 m² per gram—which enables it to effectively trap organic compounds, chlorine, and odor-causing molecules.
Once the adsorption sites become saturated, the filter loses its effectiveness, and in some cases, previously trapped odors can even leach back into the water. Because the filter captures organic matter, microbial growth can occur on its surface if not replaced regularly. Therefore, timely filter replacement is critical to prevent bacterial contamination.
Activated-carbon filters do not remove minerals and thus are not suitable for softening water. However, they play an essential role in eliminating residual chlorine, which is crucial for preserving coffee flavor. Residual chlorine is a strong oxidizing agent capable of degrading delicate aromatic compounds such as fruity or floral volatiles. Moreover, chlorine can react with phenolic compounds in coffee to produce chlorophenols, which emit harsh, plastic-like odors even at trace concentrations. These off-odors interfere with olfactory perception and mask desirable aromas.
For those using tap water to brew coffee, it is recommended to boil the water with the lid open until it reaches 100 °C. Although a substantial portion of chlorine is removed between 85 °C and 95 °C, only a full rolling boil ensures complete dechlorination.
Mechanical Filters
Mechanical filters remove suspended solids according to particle size using membranes with defined pore diameters. They are manufactured in various forms—hollow-fiber, spiral-wound, tubular, or monolithic modules—with the hollow-fiber type being most common due to its large surface area. While such filters may clog easily with heavily polluted water, they are ideal for use with Korea’s generally clean tap water.
Depending on pore size, filtration membranes are classified as follows:
| Filter Type | Pore Size | Removes Bacteria | Removes Viruses | Removes Minerals | Characteristics | Typical Use |
|---|---|---|---|---|---|---|
| Reverse-Osmosis Membrane | 0.0001 μm | Yes | Yes | Nearly all | Produces de-mineralized water | Industrial or household RO systems |
| Nano-Filtration Membrane | 0.001 μm | Yes | Yes | Partially | Slightly lowers hardness | Premium direct-flow systems |
| Ultra-Filtration Membrane | 0.01 μm | Yes | Yes | No | Retains minerals | Standard direct-flow systems |
| Micro-Filtration Membrane | 0.1 μm | Yes | No | No | Pre-treatment filter | Industrial pre-filters |
Chemical-Conversion Filters
Chemical-conversion filters actively alter the water’s chemical composition. The most representative example is the ion-exchange filter, which contains resin beads that exchange specific ions in the water for others. Ion-exchange resins are divided into cation-exchange resins and anion-exchange resins, though in the coffee industry, cation-exchange resins are primarily used to reduce hardness by removing calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
The two major types of cation-exchange resins are hydrogen-ion exchange and sodium-ion exchange.
A sodium-ion exchange resin has a stronger affinity for divalent ions like Ca²⁺ and Mg²⁺ than for Na⁺. When hard water passes through, it captures these hardness ions and releases sodium ions in their place, thus softening the water.
In coffee applications, complete hardness removal is undesirable because calcium and magnesium contribute positively to flavor extraction. Therefore, professional coffee ion-exchange filters are typically engineered to achieve only 30–50 % ion exchange, maintaining a balanced mineral profile.
A hydrogen-ion exchange resin replaces calcium and magnesium ions with hydrogen ions (H⁺). Since each divalent ion is replaced by two hydrogen ions, the higher the hardness, the greater the release of H⁺. This process lowers both pH and alkalinity, resulting in water that enhances acidity and brightness in coffee—sometimes to the point of sharpness.
Conversely, a sodium-ion exchange resin replaces calcium and magnesium ions with sodium ions (Na⁺), which lowers hardness but does not alter pH or alkalinity. Such water often produces a smoother acidity and accentuates sweetness in coffee.