Scientists reveal secret to perfect cup of coffee

Scientists have uncovered the surprising secret behind the perfect cup of coffee — reasonably hard tap water.

Experts used computational chemistry methods to examine the impact of different compositions of water on the flavour of coffee.

They found water composition made a “dramatic difference” to coffee from the same bean — with certain types of hardness boosting the taste.

High magnesium ion levels were found to increase the extraction of coffee into water, though high bicarbonate levels were bad for flavour.

Sodium-rich water — such as that produced by water-softeners — was also not beneficial to taste, according to chemist Christopher Hendon of the University of Bath.

Mr Hendon embarked on the project with his friend, coffee shop owner Maxwell Colonna-Dashwood, who was crowned Britain’s Barista Champion in April.

Armed with their new knowledge, the pair are heading to represent Britain in the World Barista Championships in Rimini, Italy.

“Coffee beans contain hundreds of chemicals; the precise composition depends on the type of bean and how it is roasted,” said Mr Hendon, a PhD student at the University of Bath’s Centre for Sustainable Chemical Technologies.

“The flavour of the resulting coffee is determined by how much of these chemicals are extracted by the water, which is influenced by roast profile, grind, temperature, pressure, and brew time. We’ve found that the water composition is key to the proportions of sugars, starches, bases, and acids extracted from a particular roast.”

Mr Hendon used computational chemistry methods in the study, published in the Journal of Agricultural Food Chemistry.

These methods were used to examine how different compositions of water affect the extraction of six chemicals that contribute to the flavour of coffee, along with caffeine.

The coffee industry currently uses guidelines on the ideal water for coffee extraction from the Speciality Coffee Association of Europe, which puts emphasis on measuring ionic conductivity to quantify the total dissolved solids.

However, the research found it was in fact the proportions of these ions that affected the extraction and, therefore, the taste of the coffee.

“Hard water is generally considered to be bad for coffee, but we found it was the type of hardness that mattered — while high bicarbonate levels are bad, high magnesium ion levels increase the extraction of coffee into water and improve the taste,” said Mr Hendon.

Mr Colonna-Dashwood, who owns Colonna and Small’s coffee shop in Bath, Somerset, said: “Unfortunately, most of the time you are limited by the source water available. Water from the tap varies regionally and from day to day depending on how much it rains.”

“Bottled water can offer a consistent water source but none seem to offer the type of water we really want. The best results are reached through utilising different filtration methods and manipulating a reasonably hard tap water.

“For the championships, we test the local water and then select the roast that is most suitable for that particular water. For example, you could use a heavy roast with a soft water as it doesn’t extract very much, but with hard water it would extract too much and give a bitter taste, so it would be better to use a lighter roast.

“Traditionally, the coffee- making industry is most concerned about using water that doesn’t scale up their machines. But we argue that more value should be placed on the flavour of the coffee and want to use chemistry to help people make the best coffee they can with the water they have available.”