MARCO WELLINGER of ZHAW (Wädenswil, Switzerland) explains why alkalinity is more important than pH and why you should adjust your water recipes to your beverage ratio and extraction method.
Even as raindrops are falling to earth, gases already diffuse into it, most notably CO2. The dissolved CO2 combines with water to form carbonic acid, which results in rainwater being always acidic at around pH 5.5. While this makes any rainwater by definition acidic, it should not be confused with the phenomenon of acidic rain if the air is polluted with sulfur and nitrous oxides at pH 4.5 or below, posing a threat to the environment and buildings.
Why am I writing about rainwater and pH in the first place? It turns out acidic rain is the perfect example to demonstrate the power of alkalinity compared to pH. One liter of average US tap water with 150 ppm CaCO3 alkalinity can neutralize 100 L of acidic rain at pH 4.5. Extending the same logic to water and coffee, the impact of alkalinity is several hundredfold times higher than that of pH on the final acidity of the beverage, provided that pH is close to neutral (6–8) and alkalinity reasonably high (above 20 ppm CaCO3).
pH vs. Alkalinity
What does the pH measurement of a water really tell you as a user? The most common interpretation – “pH tells you how acidic a water is” – is broadly correct, but hides the important fact that measuring pH does not give you the total amount of acidity present. It only provides an understanding of acidity present relative to a neutral pH (see page 21). To better illustrate this concept, I would like you to think about the relationship between “temperature” and “heat stored in an object.”
Take, for example, a wooden bench and a stone bench situated along the same path in a sunny park. The wooden bench does not feel too hot, even when the sun has been shining on it all day long, but in contrast, the stone bench – which has been in the same sun just as long – would be an unbearably hot place to sit. Measuring both benches using an infrared thermometer might show them as being the exact same temperature, but the stone bench has a large amount of heat stored (mostly due to its large mass) that it passes to you, once you decide (perhaps unwisely) to sit on its surface.
Illustration by Ashely Elander Strandquist.
The stone bench – for this same reason – will stay warm for much longer after the sun has set. In short, the stone bench has a higher amount of heat stored (heat capacity) at a given temperature compared to the wooden bench. Therefore, the stone bench reacts with more resistance to temperature change – it has a high inertia.
Let’s return to water – pH is an indicator of the current state of the water but it does not contain any information about how easily pH will vary when coffee, gases, or metal boilers come into play – like the thermometer and the benches, pH does not tell you all the details. With the benches, it was helpful to understand the heat capacity before deciding where to sit. When determining water recipes, alkalinity is that extra bit of information you need – it outlines the water’s resistance to change (or lack thereof). Therefore, pH is to alkalinity as temperature is to the amount of heat stored.
A low pH means the water is acidic, but it does not tell you how much acidity is present, or – to put it another way – how much acidity has gone in to make the pH < 7. From a sensory point of view, this is what counts the most: the perception of acidity correlates directly with the amount of titratable acidity present in a beverage. Effectively, the perceived acidity of a given coffee beverage corresponds to the amount of acids extracted from the coffee minus the amount of alkalinity from the water.
What does this mean for coffee professionals? It is far more important to keep alkalinity within a certain range than pH when it comes to choosing a water for coffee extraction.
Adapting Water Recipes for Different Extraction Methods
As it turns out, understanding alkalinity and pH gives you tools to understand how your water and coffee interact to form the acidity of your cup. The biggest factor in this equation, besides alkalinity, is the beverage ratio of your coffee extraction. The spectrum of coffee extraction methods shows differences of up to a factor of 10 in their beverage ratio (ristretto at 1.5 vs. filter at 15). This means that for the most concentrated forms of coffee (espresso preparations) only 10% of the water faces the same amount of coffee compared to the most dilute preparations (filter preparations).
This also means that for espresso preparations only 10% of the alkalinity is available to buffer coffee acids when compared to the amount of alkalinity available to buffer the same amount of coffee acids present in filter coffee preparations. It follows that water’s alkalinity can be much higher for espresso brewing before it makes a significant impact on the cup’s acidity. The same logic also applies to total hardness, but the difference here is that there are no clear cut answers to the exact effect of total hardness on extraction. Observations from different sources cite a tendency for “overextracted flavor” at high levels of total hardness (> 250 ppm CaCO3) and conversely a trend for “underextracted flavor” at low levels of total hardness (< 40 ppm CaCO3). However, experimental measurements suggest that the impact of total hardness on the overall extraction efficiency is insignificant within reasonable variations in total hardness levels (20–250 CaCO3).
Continuing to follow this logic suggests that existing water recommendations, based on filter coffee or cupping experiments, should be scaled up for espresso preparations according to the beverage ratio used. This approach, however, leads to a conflict with real-world technical complications – scaling water recipes in this way would seriously scale up your espresso machine.
As a unified organization, the SCA’s Standards Committee aspires to build a unified water standard that both harmonizes the heritage SCAA standard and SCAE core zones and offers a scaled-up version that allows for differences in brewing ratios from filter to espresso preparations. While this discussion is still currently underway, this research suggests a possible approach (Fig. 1).
Fig. 1: Adopting water recommendations for espresso beverage ratios. Left: Simply scaling up the SCA standard would lead to heavy scale formation. Right: Safe zone for espresso machines and water boilers.
To scale up the standard for espresso preparations, the filter preparation standard was multiplied by a factor of 7.3, which corresponds to the factor between an espresso (beverage ratio 2) compared to a filter coffee (beverage ratio 14.6). This however puts the new espresso standard into a zone of extremely high scale formation (0.25–0.45 g for ever liter of water), limiting the effectively useful range by avoiding water compositions that lead to heavy scale formation or potential corrosion. What is left within the “technically safe” bounds can be seen in Fig. 1.
Compared to the SCA standard for filter preparations, this leads to an extension of the recommendation at lower total hardness (as low as 20 ppm CaCO3) and especially much higher values of alkalinity of up to 150 ppm CaCO3. But remember, the lower limit regarding total hardness is the limit set for sensory reason only, and not for technical considerations.
So, if you are interested in exploring water recipes for extraction, you will find the new standard is very restricted – but there is room to play if you are interested in trying out a higher alkalinity to soften the acidity of a particular coffee. If you do not want to start mixing water from scratch by adding salts, the easiest way to try this (for most) is to use a softening cartridge (only lowering total hardness) to reach a water high in alkalinity but low in total hardness that will not cause scale build-up in your boiler.
DR. MARCO WELLINGER is a coffee researcher in the field of chemistry, technology, and sensory analysis at the Institute of Chemistry and Biotechnology at ZHAW Wädenswil.
 “Neutralizing” the water means bringing its pH up to seven. Here is how it is possible: rainwater has an alkalinity of zero, so the small amount of gases that form acids in water leads to a large pH drop. The gases present when acidic rain forms are CO2, NO2, and SO4 – these dissolve and form acids: carbonic, nitric, and sulfuric acid, respectively.
The average US tap water has 150 ppm CaCO3 alkalinity and a total hardness of around 170 ppm CaCO3, making it “medium hard.” Even still, this represents a hundredfold more buffer than the amount of acid in the acidic rain. And that is why only one liter of tap water can neutralize a hundred liters of acidic rain!
Talking About Water: Key Principles
Beverage Ratio vs. Brewing Ratio
Currently there are different concepts of “brewing ratio or “brew ratio in use. I propose we introduce the name “beverage ratio to clearly separate the two concepts.
- Standard use for drip and immersion coffee where the amount of brewing water (volume or weight) is given per weight of ground coffee
- Most commonly written as ground coffee per volume of water 60 g/L – can also be given as a ratio of water per coffee 16.7 (1,000 g of water divided by 60 g of coffee)
- Gives handy numbers to prepare a brew or adapt recipes to different sizes
- Predominantly used for espresso, formed by the weight of the beverage and the weight of ground coffee
- For example, 18 g of ground coffee resulting in a beverage of 36 g corresponds to beverage ratio 2
- Focuses on the beverage, and is handy to calculate extraction percentage
Scientifically, a pH – or power of hydrogen” – measurement gives a measure of the proton concentration (to be more specific, hydronium concentration). Moreover, it is a logarithmic scale, or based on orders of magnitude, so a decrease by one unit (-1) corresponds to a tenfold increase of proton concentration.
Carbonate hardness is another way of saying the common minimum of total hardness and alkalinity. An easy metaphor for this involves dancing partners, where the number of pairs you can form is limited by whatever count is lower the number of people who want to “take the lead” and those who want to follow through. So, if you want to make sure there is at least 40 mg of protective layer per liter of water able to form in your boiler, neither total hardness nor alkalinity may be below 40 mg of CaCO3 per liter (40 ppm CaCO3).
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