Update Feb 2019: We’ve just published brand new research on the science of water for coffee. Click here to check it out.
Wherever water flows on earth, you are sure to find life. From humans to plants, water provides the perfect carrier of nutrients to cells. The slight negative charge from its oxygen, and slight positive charge from its hydrogen makes water an attractive molecule to bond to. Because of this polarity, our world’s most abundant solvent is never found in its pure form, unless we have manufactured it that way. From its path from the sky when it rains, to our homes, water comes into contact with many elements, dissolving minerals and changing its nature of interaction. In cities we treat it for consumption and domestic use, but for specific commercial applications, water needs to be measured and manipulated to work efficiently and prevent damage to equipment.
For coffee, this can be a challenging feat. We have this machine that has high power, creates and delivers hot pressurised water and steam, contained in metal boilers that flows through really small jets, just to be able to make this delicious, concentrated brew we call espresso.
There are common elements in water that when put under these conditions can cause damage to the machine, or even worse – destroy tasty flavours in your coffee. In order to make tasty coffee and happy machines we need to test the water to ascertain the general hardness minerals, carbonate hardness and chlorides, along with the pH.
What is general hardness in water?
General hardness are elements in the water such as calcium, magnesium, sodium, copper, iron, etc. The ones in the highest concentrations, and the ones we really need to pay attention to, are calcium and magnesium. These two positively charged ions increase the water’s ability to dissolve organic molecules, meaning they help to extract flavour from coffee.
What is carbonate hardness in water?
Carbonate hardness is the measure of the water’s ability to stabilise the pH when acids or bases are introduced. The main buffer in water is bicarbonate, which is formed from the carbon dioxide the water molecule picks up in the atmosphere on its way down to earth.
Bicarbonates are amphiprotic molecules, meaning that they can act both as acids or bases, gaining or giving protons. When carbon dioxide dissolves into water it becomes carbonic acid, and will typically lose a proton to become bicarbonate, and lose one more to become carbonate. If there are high concentrations of carbonate and calcium in the water then the system will favour the formation of calcium carbonate – limescale.
Remember those really small jets the water flows through in the espresso machine? Limescale loves to deposit in them. The pressure change of the water at the jet promotes the formation of calcium carbonate. Also, those high powered heating elements used to keep the brewing water and steam at a stable temperature love to bring these components together. These deposits of scale will lead to reduced water flow and also heating inefficiency in your machine.
Luckily, we can remove protons from this bicarbonate system by adding acids. Typically a citric acid solution is used to remove the calcium carbonate from the espresso machine. There are different water filtration systems that look to remove the calcium and other general hardness minerals from the water so we don’t get this build up of scale in the machine. As we mentioned before, calcium and magnesium help to extract flavour from coffee. Water that has had too many of these minerals removed will result in weak, low yielding extractions of the coffee’s flavour. Too much will result in scale. The balance must be found!
Chlorides in water
Chlorides are commonly found in water and will bind to magnesium, calcium and sodium to form salts. Chlorides can be highly corrosive to metals when the pH of the water is acidic. If there are impurities in the manufacture of the metal, this is the most likely place for corrosion to occur. It’s important to mention that water filtration technologies, such as softeners that reduce the calcium and bicarbonate content of the water, reduce the water’s ability to buffer. This in turn reduces the pH of the water which can lead to water becoming corrosive. A pH below 6.5 is not recommended for this reason, also because it is unfavourable for extraction.
pH scale in water
The pH scale indicates the potential of the Hydrogen in the water. The pH of the water is an important measurement as it can be a good indication of how reactive the system is. A pH of 7 is neutral, and with the addition of an acid or a base, this will move. A lower number indicates a more acidic solution, a higher number a more basic solution. Each whole number difference in measurement is one order of magnitude, meaning a measurement of 6 has a 10x difference in hydrogen ion concentration than 7. Large changes in pH will cause the mood of the water to change, making the push and pull of reactions of certain elements more aggressive with measurements either side of 7.
Bicarbonates buffer this reaction as mentioned before, but they also buffer tasty acids in coffee, turning them into their conjugate base, which can taste flat.
To achieve the right balance of water to make tasty coffee and maintain the health of your espresso machine, water must be measured on a site-by-site basis, to see what technology will best suit the conditions. Water needs to be retested periodically as the water will change depending on how it’s being treated and the source it’s coming from.
For an excellent guide, and more in depth information on water we recommend reading Water for Coffee. We also suggest working with a good water filtration company, such as WFS in Melbourne.