Water is the secret to brewing a perfect cup of coffee. British scientist Christopher Hendon at University of Bath recently published his research, “The Role of Dissolved Cations in Coffee Extraction,” in the Journal of Agricultural and Food Chemistry by the American Chemical Society in May and has attracted a lot of attention in how to prepare the perfect coffee. He discovered that the chemical composition in water is key to bringing out different flavors from the coffee bean. A perfect cup of coffee can be achieved through the control of the minerals, such as sodium, magnesium and calcium cations in water.

Coffee’s popularity, effects and preparation

Coffee is a part of most everyone’s daily lives and gets the brain into gear and gives pleasure. It contains over 1,000 aroma compounds, including antioxidants such as chlorogenic acids, as well as carcinogens including 4-methylimidazole and acrylamide. The absorption of these antioxidants and carcinogens is complicated and depends on many factors. Recent studies show that coffee could help reduce risk of acquiring Type II diabetes and Alzheimer’s disease. Drinking coffee has been associated with a variety of beneficial and harmful health effects.

Water is not only the main ingredient of coffee (98.5 percent to 99 percent), but it is also a key to bringing out different flavors from the coffee bean. To make the best cup of coffee possible, the National Coffee Association offers the guidelines found in Table 1.

Table 1: National Coffee Association guidelines

Variables Guidelines
Equipment Needs to be thoroughly cleaned
Coffee bean Fresh roasted coffee within one to two weeks
Coffee grinding Grinding the coffee bean as close to the brew time as possible
Water quality Very important to the quality of the coffee
Ratio of coffee to water One to two tablespoons of ground coffee for every six ounces of water
Temperature Coffee extraction temperature is 195-205o F
Brewing time Drip system: Five minutes; plunge pot: Two to four minutes; espresso: 20-30 seconds
Brewed coffee Drinking immediately or no more than 45 minutes; never reheat.

 

Water quality matters

Concerning water quality, using previously boiled water is bad to brew coffee. Fresh and cold water with dissolved oxygen makes coffee taste better.

The Specialty Coffee Association of America has determined the following standards for the water used to brew specialty coffee. For a superior quality extraction of coffee solids, the brewing water should have the characteristics found in Table 2.

Table 2: Water standards from the Specialty Coffee Association of America

Characteristic Target Acceptable Range
Odor Clean/fresh, odor-free
Color Clear color
Total chlorine 0 mg/l
TDS (Total Dissolved Solids) 150 mg/l 75-250 mg/l
Calcium hardness 4 grains (68 mg/l) 1-5 grains (17-85 mg/l)
Total alkalinity 40 mg/l At or near 40 mg/l
pH 7.0 6.5-7.5
Sodium 10 mg/l At or near 10 mg/l

 

Science behind coffee’s flavor

Based on Hendon, the binding energies between dissolved cations in water and flavors in coffee are proportional to the ionic charge and inversely proportional to the ionic radii. He concludes:

  • Magnesium-rich water is most suitable to bring out most of the coffee flavor compounds, i.e., instant coffee
  • Both calcium and magnesium do a comparable job to have the best balance of flavors
  • Sodium-rich water will make no difference to bring out flavor from coffee.

According to Hendon’s finding, TDS defined in the Standards is inherently flawed. If water contains 200 ppm NaCl as TDS, it will taste terrible. Cation-rich water has the most extracting ability to bring out coffee flavors. Therefore, TDS defined in the Standards shall be specific for cation-rich water, not something else.

Hendon’s study explains why mineral content in water is important. Hard water will generally produce a better tasting coffee compared to soft water; however, hard water will cause scale buildup inside the coffee equipment such as pumps, boilers and valves. This will increase long-term maintenance costs. In order to balance the taste of coffee and maintenance requirements for the coffee equipment, pros and cons for different water treatment have been summarized below:

  • Polyphosphate/siliphos treated water: A polyphosphate/siliphos filter only releases polyphosphate to the treated water in order to form a thin, protective layer on the metallic surfaces of the coffee machine. As a result, this technology inhibits scale deposition. There is no removal of chlorine and/or chloramine from tap water. Thus, polyphosphate treated water will not be a good choice for brewing coffee.
  • Distilled water: Distilled process will remove all dissolved solids, including minerals and will keep all VOC (volatile organic compounds) and any gases in water (such as chlorine if distilled from tap). Since no minerals are in the water, this leads to under-extraction of coffee aroma compounds. As a result, the brewed coffee is flat. However, using distilled water will protect the equipment without scale out.
  • Reverse osmosis (RO) water: RO treated water is close to distilled water and basically is a pure and corrosive form. There are no minerals, causing the coffee to taste flat. Although RO water can prevent scale, it will rust the equipment due to its aggressiveness. Since TDS is very low, this may cause the equipment sensor to stop functioning. A blending RO system has a built-in, by-pass tubing and a valve to mix incoming water and treated water as well as provides a mechanism for the customers to dial in the precise TDS level to bring some hardness back and to make water less corrosive. Thus, if the cation concentration is right, blending RO water will make good coffee.
  • Softened water: Softening uses ion exchange technology to take out calcium and magnesium ions and introduce sodium ions into treated water. Therefore, softened water causes a flat taste for brewed coffee. Softened water will protect the equipment without scale out. Water softener itself will not remove chlorine and/or chloramine from water. Rather, chlorine and chloramine damages the resin beads due to its oxidation power.
  • Activated catalytic carbon treated water: Granular carbon, or carbon block filter, generally takes out bad odor and taste, such as chlorine and other organic contaminants and leaves the minerals in treated water. This would be the ideal water for a perfect cup of coffee if hardness is not very high (see Table 2). However, this technology will not prevent the coffee machine from scale out.
  • Activated catalytic carbon with nano-crystal scale control: Except for the benefits mentioned above for activated catalytic carbon treated water, nano-crystal technology is to control the scale out. Although there is no change in the hardness concentration before and after treatment, nano-crystal formation significantly reduces scale deposition. The combination of catalytic carbon with nano-crystal scale control will balance the great taste and optimal coffee equipment maintenance.
  • Activated catalytic carbon with ion exchange scale control: There are mixed carbon and ion exchange resin bead products specially designed for coffee, tea and espresso applications. This technology improves not only bad odor and taste, but also reduces hardness in proper range. Therefore, treated water makes coffee taste great.
  • Bottled water: Commercially available bottled water could be from different sources and/or different water treatments, including tap water, spring water, reverse osmosis, distilled and minerals. Spring bottled water can be hard or soft depending on the water sources. Mineral water is extremely hard and can make coffee bitter (over-extraction of coffee).

As Hendon pointed out, brewing coffee might be the most practiced chemical extraction in the world. It is clear that the chemical composition in water plays a very important role for brewing a perfect cup of coffee. Hendon’s finding helps water treatment professionals better understand which water treatment products are the best for coffee taste and the protection of coffee/espresso machines. Table 3 summarizes the benefits for different types of treated water that have been discussed.

Table 3: Pros and cons of water treatment for a perfect cup of coffee

Water treatment Mineral removal Scale control

 

Odor/Taste

removal

Perfect coffee
Polyphosphate X
Distilled X X Maybe with blending
RO X X X Maybe with

blending

Softened X X
Catalytic carbon X X
Catalytic carbon/NanoScale X X X
Catalytic carbon/Resin X X X X
Bottled water Maybe Maybe X Maybe

References:

  1. Christopher H. Hendon, etc. “The Role of Dissolved Cations in Coffee Extraction”, Journal of Agricultural and Food Chemistry, 62, 4947-4950, 2014.
  2. Emma Davies, “Chemistry in every cup”, Chemistry World, 36-39, May 2011
  3. National Coffee Association, “How to Brew Coffee”, http://www.ncausa.org
  4. Specialty Coffee Association of America, Water Standard, Version 21NOV2009A.

 

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Cang currently is director of R&D/QC at Selecto. He joined Selecto as a senior research chemist in March 1999. His wide range of duties in product development and testing for drinking water filtration, separation and purification materials are all directed toward quality control/regulatory, manufacturing, technical customer services, product development and international sales. He has 25 scientific and technical publications and one U.S. patent.