How minerals modulate taste and their role in extraction

Dr. Monika Fekete explains the role of bicarbonates and cations in coffee extraction, and how minerals modulate taste.
coffee science

A truly outstanding brew depends on many factors falling into place at once. Carefully chosen and roasted beans need to work harmoniously with matching water and a strict brew method to extract and highlight desired flavours.

Increasingly, brewers are focused on the impact water has on taste outcomes. According to 2017 World Brewer’s Cup Runner-up Sam Corra, one of the most important factors to brewing an amazing coffee experience is having a brew water that will best represent a coffee’s attributes. He notes, however, that one water formulation will not necessarily suit other coffee varieties, processes, and origins.

Dr. Monika Fekete is the Founder of Coffee Science Lab.

His observation is illustrated by the very different mineral make-up of 2018 Australian Brewer’s Cup Champion Heath Dalziel’s brew water. 

Both brews were outstanding in their own right, but they needed careful adjustment of the minerals in the water to showcase the coffees to their full potential. But how does this work on a physical/chemical level? If we knew how minerals modulate taste, we could harness this knowledge to deliberately tailor brew water to coffee, or even coffee to water available in a particular location.

Do minerals affect extraction? Or is it all in our tastebuds?

A plausible answer would be that the three main mineral ingredients in brew water – calcium, magnesium, and bicarbonate – all help to extract different flavours from the coffee bean. Adjusting their levels in the brew water will result in different amounts of the various flavour compounds being extracted. If this is the case, changing the concentration of salts in the brew water should lead to a measurable difference in the chemical composition of the brewed coffee.

An alternative explanation could be that, instead of changing the extraction process itself, minerals act on our taste receptors, affecting the way we perceive the final brew. Or could it be a bit of both? 

Digging into scientific literature, it quickly becomes apparent that cations (calcium Ca2+ and magnesium Mg2+) and anions (mainly bicarbonate, HCO3– ) play fundamentally different roles in extraction. Let’s tackle anions first.

The role of bicarbonates in coffee extraction

Brew water contains three main minerals to extract different flavours, including calcium, magnesium and bicarbonate.

Carbonate hardness (KH) has been identified as a main culprit for producing flat, bitter tasting coffee for a long time. In a study published in 1971, filter coffee brewed with 750 parts per million (ppm) bicarbonate was characterised as bitter by the panel, but, interestingly, had a somewhat higher level of total dissolved solids (TDS). 

In contrast, coffee brewed with distilled water had lower TDS and a sour taste. When studying the effects of hard water on espresso coffee, scientists Navarini and Rivetti in their 2010 paper titled Water Quality for Espresso Coffee observed a similar effect, along with a prolonged brew time for bicarbonate- rich water. 

It makes sense that distilled water produces a sour brew compared to bicarbonate-rich water. The natural acidity of coffee is neutralised by the alkaline bicarbonate solution, which explains a flat taste. 

The increased presence of bitter notes is trickier. Bitterness is often due to over-extraction, but why would bicarbonates make coffee over-extracted? According to Navarini’s explanation, the pH of the brew water drops from around 7 to 5 during extraction due to the buffering effect of coffee itself. This significant drop in pH leads to the liberation of carbon dioxidegas from bicarbonate. 

Carbon dioxide is normally present in roasted coffee and helps build resistance in the coffee bed to slow water flow. Excess carbon dioxide from the brew water increases this effect, and it can lengthen brew time to a point that the coffee becomes over-extracted. When 1000ppm bicarbonate was added to the brew water in Navarini’s experiment, espresso extraction time was prolonged by up to 55 per cent. The extra carbon dioxidebuild-up also destroyed espresso crema. 

Excessive bubbling led to a higher volume of low-quality foam, which collapsed much quicker than foam produced by distilled water. Note that 1000ppm bicarbonate is extreme for practical coffee brewing. Detailed studies are needed to find out the lowest bicarbonate level where this effect becomes noticeable.

The effect of cations

As we have learned from master brewers, the effect of Ca2+ and Mg2+ ions is paramount on the final coffee experience. At the same time, the role of cations in extraction remains poorly understood. 

In a 2014 published paper, The Role of Dissolved Cations in Coffee Extraction, researchers used a computational chemistry approach to investigate how common coffee flavour compounds bind to dissolved cations such as Mg2+ and Ca2+. The relative binding energies of magnesium were found to be the highest, followed by calcium. The authors concluded that cation-rich water, especially water rich in Mg2+, has the potential to extract most coffee constituents. 

These theoretical findings, however, have not yet been backed up by practical experiments. If cations indeed increase extraction levels by binding to flavour compounds and pulling them into the brew, we should be able to measure an increase in TDS compared to coffee extracted with pure water.

Some studies showed no increase in TDS when 750ppm calcium chlorideor magnesium chloride was added to the brew water, compared to distilled water. Another experiment carried out by Five Senses Coffee investigated the effect of 0-500ppm magnesium chloride on the TDS percentage of Clever Coffee Dripper immersion brews. The results were inconclusive: they showed a marginal increase in TDS up to 400ppm Mg2+, but these remained within experimental error.

To date, I’m unaware of any evidence showing that varied levels of cations could lead to equal TDS levels with a different chemical make-up.

Salts and tastebuds

The interaction of salts and flavour compounds on our taste receptors is extremely complex. In 2003 research, the tastes of calcium and magnesium salts in water were characterised as “bitter and salty” by themselves. When larger organic anions, such as lactate (from lactic acid, also found in coffee) were introduced, they acted to suppress the inherent bitterness of Ca2+ and Mg2+. Imagine what could happen when we add the hundreds of coffee flavour compounds to the mixture?

A proposed experiment

If you’re feeling confused about the effect of minerals on coffee flavour, I’m with you. As we dig deeper into the chemistry, more questions than answers seem to emerge. Here’s one thought that might help us understand the effect of minerals a little better.

Do minerals affect coffee extraction itself, or do they mainly affect how we perceive the taste of coffee? If the answer is in extraction, the minerals would have to be added to the brew water. If the effect is largely taste-related, they could be added afterwards to no perceptible difference in the final outcome.  

In a randomised sensory experiment, I would like test if a panel of coffee experts can taste a significant difference between coffee brewed with minerals in the water compared to coffee sprinkled afterwards with the same salts. If you’d like to volunteer to be on this panel email me at monika@coffeesciencelab.com.au and hopefully I can report back with an interesting outcome. 

For further reading on the subject:

  • Pangborn R.M et al, 1971, Journal of Food Science
  • Navarini L & Rivetti D. 2010, Food Chemistry
  • Hendon, C. H et al, 2014, Journal of Agricultural and Food Chemistry
  • Maxwell Colonna-Dashwood, 2014 WBC performance 
  • www.fivesenses.com.au (blog post titled ‘Experimenting with the effect of water quality on water’)
  • Lawless, H. T et al, 2003, Food Quality and Preference