Tech

Brewing A Great Cup Of Coffee Depends On Chemistry And Physics

REUTERS/Stefano Rellandini

The Conversation Contributor
Font Size:

By Christopher Hendon, University of Oregon

Coffee is unique among artisanal beverages in that the brewer plays a significant role in its quality at the point of consumption. In contrast, drinkers buy draft beer and wine as finished products; their only consumer-controlled variable is the temperature at which you drink them.

Why is it that coffee produced by a barista at a cafe always tastes different than the same beans brewed at home?

It may be down to their years of training, but more likely it’s their ability to harness the principles of chemistry and physics. I am a materials chemist by day, and many of the physical considerations I apply to other solids apply here. The variables of temperature, water chemistry, particle size distribution, ratio of water to coffee, time and, perhaps most importantly, the quality of the green coffee all play crucial roles in producing a tasty cup. It’s how we control these variables that allows for that cup to be reproducible.

How strong a cup of joe?

Besides the psychological and environmental contributions to why a barista-prepared cup of coffee tastes so good in the cafe, we need to consider the brew method itself.

We humans seem to like drinks that contain coffee constituents (organic acids, Maillard products, esters and heterocycles, to name a few) at 1.2 to 1.5 percent by mass (as in filter coffee), and also favor drinks containing 8 to 10 percent by mass (as in espresso). Concentrations outside of these ranges are challenging to execute. There are a limited number of technologies that achieve 8 to 10 percent concentrations, the espresso machine being the most familiar.

There are many ways, though, to achieve a drink containing 1.2 to 1.5 percent coffee. A pour-over, Turkish, Arabic, Aeropress, French press, siphon or batch brew (that is, regular drip) apparatus – each produces coffee that tastes good around these concentrations. These brew methods also boast an advantage over their espresso counterpart: They are cheap. An espresso machine can produce a beverage of this concentration: the Americano, which is just an espresso shot diluted with water to the concentration of filter coffee.

All of these methods result in roughly the same amount of coffee in the cup. So why can they taste so different?

A barman prepares Lavazza "slow coffee" blend during the new opening of Lavazza's flagship coffee store in Milan, Italy, September 19, 2017. REUTERS/Stefano Rellandini

A barman prepares Lavazza “slow coffee” blend during the new opening of Lavazza’s flagship coffee store in Milan, Italy, September 19, 2017. REUTERS/Stefano Rellandini

When coffee meets water

There are two families of brewing device within the low-concentration methods – those that fully immerse the coffee in the brew water and those that flow the water through the coffee bed.

From a physical perspective, the major difference is that the temperature of the coffee particulates is higher in the full immersion system. The slowest part of coffee extraction is not the rate at which compounds dissolve from the particulate surface. Rather, it’s the speed at which coffee flavor moves through the solid particle to the water-coffee interface, and this speed is increased with temperature.

A higher particulate temperature means that more of the tasty compounds trapped within the coffee particulates will be extracted. But higher temperature also lets more of the unwanted compounds dissolve in the water, too. The Specialty Coffee Association presents a flavor wheel to help us talk about these flavors – from green/vegetative or papery/musty through to brown sugar or dried fruit.

Pour-overs and other flow-through systems are more complex. Unlike full immersion methods where time is controlled, flow-through brew times depend on the grind size since the grounds control the flow rate.

The water-to-coffee ratio matters, too, in the brew time. Simply grinding more fine to increase extraction invariably changes the brew time, as the water seeps more slowly through finer grounds. One can increase the water-to-coffee ratio by using less coffee, but as the mass of coffee is reduced, the brew time also decreases. Optimization of filter coffee brewing is hence multidimensional and more tricky than full immersion methods.

Other variables to try to control

Even if you can optimize your brew method and apparatus to precisely mimic your favorite barista, there is still a near-certain chance that your home brew will taste different from the cafe’s. There are three subtleties that have tremendous impact on the coffee quality: water chemistry, particle size distribution produced by the grinder and coffee freshness.

First, water chemistry: Given coffee is an acidic beverage, the acidity of your brew water can have a big effect. Brew water containing low levels of both calcium ions and bicarbonate (HCO₃⁻) – that is, soft water – will result in a highly acidic cup, sometimes described as sour. Brew water containing high levels of HCO₃⁻ – typically, hard water – will produce a chalky cup, as the bicarbonate has neutralized most of the flavorsome acids in the coffee.

Ideally we want to brew coffee with water containing chemistry somewhere in the middle. But there’s a good chance you don’t know the bicarbonate concentration in your own tap water, and a small change makes a big difference. To taste the impact, try brewing coffee with Evian – one of the highest bicarbonate concentration bottled waters, at 360 mg/L.

The particle size distribution your grinder produces is critical, too.

Every coffee enthusiast will rightly tell you that blade grinders are disfavored because they produce a seemingly random particle size distribution; there can be both powder and essentially whole coffee beans coexisting. The alternative, a burr grinder, features two pieces of metal with teeth that cut the coffee into progressively smaller pieces. They allow ground particulates through an aperture only once they are small enough.

There is contention over how to optimize grind settings when using a burr grinder, though. One school of thought supports grinding the coffee as fine as possible to maximize the surface area, which lets you extract the most delicious flavors in higher concentrations. The rival school advocates grinding as coarse as possible to minimize the production of fine particles that impart negative flavors. Perhaps the most useful advice here is to determine what you like best based on your taste preference.

Finally, the freshness of the coffee itself is crucial. Roasted coffee contains a significant amount of CO₂ and other volatiles trapped within the solid coffee matrix: Over time these gaseous organic molecules will escape the bean. Fewer volatiles means a less flavorful cup of coffee. Most cafes will not serve coffee more than four weeks out from the roast date, emphasizing the importance of using freshly roasted beans.

One can mitigate the rate of staling by cooling the coffee (as described by the Arrhenius equation). While you shouldn’t chill your coffee in an open vessel (unless you want fish finger brews), storing coffee in an airtight container in the freezer will significantly prolong freshness.

So don’t feel bad that your carefully brewed cup of coffee at home never stacks up to what you buy at the café. There are a lot of variables – scientific and otherwise – that must be wrangled to produce a single superlative cup. Take comfort that most of these variables are not optimized by some mathematical algorithm, but rather by somebody’s tongue. What’s most important is that your coffee tastes good to you… brew after brew.

Christopher Hendon, Assistant Professor of Computational Materials and Chemistry, University of Oregon

This article was originally published on The Conversation. Read the original article.

PREMIUM ARTICLE: Subscribe To Keep Reading

Sign up

By subscribing you agree to our Terms of Use

You're signed up!

Sign up

By subscribing you agree to our Terms of Use

You're signed up!
Sign up

By subscribing you agree to our Terms of Use

You're signed up!

Sign up

By subscribing you agree to our Terms of Use

You're signed up!
Sign up

By subscribing you agree to our Terms of Use

You're signed up!

Sign Up

By subscribing you agree to our Terms of Use

You're signed up!
Sign up

By subscribing you agree to our Terms of Use

You're signed up!
Sign up

By subscribing you agree to our Terms of Use

You're signed up!
BENEFITS READERS PASS PATRIOTS FOUNDERS
Daily and Breaking Newsletters
Daily Caller Shows
Ad Free Experience
Exclusive Articles
Custom Newsletters
Editor Daily Rundown
Behind The Scenes Coverage
Award Winning Documentaries
Patriot War Room
Patriot Live Chat
Exclusive Events
Gold Membership Card
Tucker Mug

What does Founders Club include?

Tucker Mug and Membership Card
Founders

Readers,

Instead of sucking up to the political and corporate powers that dominate America, The Daily Caller is fighting for you — our readers. We humbly ask you to consider joining us in this fight.

Now that millions of readers are rejecting the increasingly biased and even corrupt corporate media and joining us daily, there are powerful forces lined up to stop us: the old guard of the news media hopes to marginalize us; the big corporate ad agencies want to deprive us of revenue and put us out of business; senators threaten to have our reporters arrested for asking simple questions; the big tech platforms want to limit our ability to communicate with you; and the political party establishments feel threatened by our independence.

We don't complain -- we can't stand complainers -- but we do call it how we see it. We have a fight on our hands, and it's intense. We need your help to smash through the big tech, big media and big government blockade.

We're the insurgent outsiders for a reason: our deep-dive investigations hold the powerful to account. Our original videos undermine their narratives on a daily basis. Even our insistence on having fun infuriates them -- because we won’t bend the knee to political correctness.

One reason we stand apart is because we are not afraid to say we love America. We love her with every fiber of our being, and we think she's worth saving from today’s craziness.

Help us save her.

A second reason we stand out is the sheer number of honest responsible reporters we have helped train. We have trained so many solid reporters that they now hold prominent positions at publications across the political spectrum. Hear a rare reasonable voice at a place like CNN? There’s a good chance they were trained at Daily Caller. Same goes for the numerous Daily Caller alumni dominating the news coverage at outlets such as Fox News, Newsmax, Daily Wire and many others.

Simply put, America needs solid reporters fighting to tell the truth or we will never have honest elections or a fair system. We are working tirelessly to make that happen and we are making a difference.

Since 2010, The Daily Caller has grown immensely. We're in the halls of Congress. We're in the Oval Office. And we're in up to 20 million homes every single month. That's 20 million Americans like you who are impossible to ignore.

We can overcome the forces lined up against all of us. This is an important mission but we can’t do it unless you — the everyday Americans forgotten by the establishment — have our back.

Please consider becoming a Daily Caller Patriot today, and help us keep doing work that holds politicians, corporations and other leaders accountable. Help us thumb our noses at political correctness. Help us train a new generation of news reporters who will actually tell the truth. And help us remind Americans everywhere that there are millions of us who remain clear-eyed about our country's greatness.

In return for membership, Daily Caller Patriots will be able to read The Daily Caller without any of the ads that we have long used to support our mission. We know the ads drive you crazy. They drive us crazy too. But we need revenue to keep the fight going. If you join us, we will cut out the ads for you and put every Lincoln-headed cent we earn into amplifying our voice, training even more solid reporters, and giving you the ad-free experience and lightning fast website you deserve.

Patriots will also be eligible for Patriots Only content, newsletters, chats and live events with our reporters and editors. It's simple: welcome us into your lives, and we'll welcome you into ours.

We can save America together.

Become a Daily Caller Patriot today.

Signature

Neil Patel