Baristas strive to control each element of coffee production, from processing methods and roasting through to grind consistency and temperature profiling. Water is another element not to forget. If you use the water straight from your tap and let it run through a coffee machine, chances are that rare, expensive Panama Gesha you’ve just added to the menu will be lacking in the flavour profile it deserves. That’s why most cafés implement water filtration systems – to control water’s impact on coffee flavour and maximising the lifetime of the espresso machine at the same time.
But what really goes on behind the filtration doors? What are the different options available and what do they do to turn ordinary tap water into coffee’s best friend? Let’s find out.
Carbon block filters
Carbon block filters are the most common kind of filter to remove residual chlorine, sediment, or small organic compounds that could taint water without affecting water hardness.
Carbon filters are made from carbon-rich organic material, such as bamboo or coconut shells, which have been incinerated at high temperatures. The result is a super-porous powder with a very large surface area available for contaminant particles to stick to. Just one gram of active carbon typically has a surface area of 1000 square metres – that’s the size of three IMAX screens.
One of the main uses of carbon filters is to get rid of the strong chlorine taste often found in areas where a higher level of residual chlorine is needed to keep drinking water safe from microbial contamination (refer to December 2017 issue of BeanScene). Reservoir water, especially from a protected catchment, usually needs less residual chlorine whereas surface water, such as river water, needs more. A carbon filter helps turn chlorine molecules into tasteless and odourless chloride ions. Over time, the fine pores of the carbon clog up, making the filter’s performance degrade, so you will need to replace it.
Ion exchange resins
Water hardness is a concern in many areas, such as regions of South Australia, Queensland, or Western Australia (refer to the water hardness map in the December 2017 issue). To soften the water, ion exchange resins can be used.
Ion exchange is the process of capturing undesirable ions from the water and replacing them with less objectionable ones. Say you have very hard tap water filled with minerals like calcium bicarbonate and magnesium sulphate. The ion exchange resin will replace the positive calcium and magnesium ions with sodium. Meanwhile, the negative sulphate or carbonate ions are replaced with chloride. Over time, the sodium and chloride will get used up and will need to be “regenerated” by soaking the resin in a concentrated sodium chloride (salt) solution. The end result is a softer and saltier water. Using potassium instead of sodium can reduce our perception of saltiness.
To avoid the added salt, you can use a special type of ion exchanger called a de-ioniser, where positive ions are exchanged with hydrogen ions (H+) and negative ions with hydroxide ions (OH-). These two combine to form pure water — hardness is removed without added saltiness. The resulting water can be too low in minerals, however, meaning a flat, hollow taste in coffee. Its maintenance requires handling of strong acids and bases, which can potentially be dangerous – best leave it to a professional.
Reverse osmosis
Another filtration strategy is to break the water down to its purest possible state and then build it up again by adding minerals. The first step is often reverse osmosis, which indiscriminately pulls out ions from the water.
Osmosis is a trick nature uses to move water across a membrane, especially from a low salt region to a high salt region. It’s how plants take water from soil.
Reverse osmosis is the opposite of the natural process. It means extracting pure water from a salty solution (see image 1). Reverse osmosis also uses a special membrane that stops any dissolved material, but lets pure water flow through. We need to apply pressure to force the pure water through to the other side of the membrane. This is why reverse osmosis can only be operated using external energy. For example, at desalination plants, seawater is purified using 40 to 80-bar pressure. A café unit can run simply on mains water pressure (typically around four to five bar), or, if desired, their performance can be increased using a booster pump.
Water filtered through reverse osmosis membranes should be almost completely free of minerals. While this avoids the problem of limescale build-up, it can create another problem, such as corrosion. Corrosion is much harder to repair then limescale. There is a fine balance between scale-forming and corrosive water. Higher levels of general and carbonate hardness, higher pH, and temperature push the balance towards scale formation, while lower pH, low bicarbonate levels, and more salts lead to corrosion. This complex system is best described by the Langelier saturation index, which I suggest studying if you would like to learn more, or ask your water professional. A good practice to overcome this problem is making sure that a small level of hardness is replenished into the treated water so that it can form a thin protective layer on the pipes.
The pure water base from reverse osmosis gives us a chance to redesign the mineral make-up of the water from scratch for optimal taste outcomes. Pre-prepared mineral concentrates, or simply a fraction of the salt solution left behind in the process, are commonly mixed back into the treated water using a dosing pump.
Water testing
To be able to make the best choices for water filtration equipment, tap water at your location should be tested first.
Total dissolved solid (TDS) measurements provide general information about how much dissolved material is in the water altogether (as discussed in the October 2017 issue). It can give a good starting point to determine if the water supply is quite pure or rich in minerals.
Water pH is related to carbonate hardness and is crucial in determining if the water is likely to cause limescale build-up in the coffee machine. pH meters need to be calibrated before each use using buffer solutions with known pH values. General hardness and carbonate hardness can be quickly tested on the spot using appropriate aquarium titration kits. These use indicator dyes that change colour upon reaction with the minerals in the water (see image 2). You can find many instructional videos on how to use them online. It is good to remember though that their resolution is around ±10 parts per million and they generally can’t separate calcium and magnesium. Some kits do, but these are slightly more complicated to use and don’t provide high accuracy. My advice is to test three times before you accept a result. If it is important to have a more exact picture of general hardness, carbonate hardness, and calcium-magnesium ratio, laboratory testing could be necessary.
A good background understanding of the parameters of your water and the available filtration options should help you make an informed decision about the system that works best for you.
This article is written by Dr. Monika Fekete, Founder of Australian Coffee Science Lab.