Brewing a great cup of Depends on chemistry and physics

January 1–7, 2018
Samira Bouaou/the Epoch Times
Coffee provides not only caffeine to kick off the morning, but also a chance for a break later in the day.
Brewing a great cup of
Depends on chemistry and physics
offee 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 café 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 café, 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 per cent by mass (as in filter
coffee), and also favour drinks containing 8 to
10 per cent 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 per cent concentrations, the espresso machine being the
most familiar.
There are many ways, though, to achieve a
drink containing 1.2 to 1.5 per cent 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.
immersion methods.
Other variables to try to control
Even if you can optimize your brew method
and apparatus to precisely mimic your favourite barista, there is still a near-certain chance
that your home-brew will taste different from
the café’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.
All of these methods result in roughly the
same amount of coffee in the cup. So why can
they taste so different?
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 flavour 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 flavour wheel to help us talk about these flavours – 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 in which time is controlled, flowthrough 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 finely 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 trickier than full-
Given coffee is an acidic
the acidity of your brew
water can have
a big effect.
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 flavoursome 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 (0.0030043456 pound/
The particle size distribution your grinder produces is critical, too.
Every coffee enthusiast will rightly tell you
that blade grinders are disfavoured 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 flavours in higher concentrations. The rival school advocates grinding
as coarse as possible to minimize the production of fine particles that impart negative flavours. 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 flavourful cup of coffee. Most cafés will not serve
coffee more than 4 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 H. Hendon is an assistant
professor of computational materials and
chemistry at the University of Oregon. This
article was originally published on The
Examples of coffee
brewing methods
French press
Cold brew
Moka pot