Outside a café, a chalkboard sign makes a joke: “Not to get technical… but according to chemistry coffee is a solution!!” It’s a good line, and it’s partly true. Whether it’s right depends on which coffee you’re holding.
A cup of filter coffee is very close to a true solution. An espresso is a mix of several things a chemist would file under different headings, and that mix is exactly what gives it crema and body.
Solutions, Colloids and Suspensions
A solution is one uniform liquid. Whatever’s dissolved in it has broken down to single molecules or ions, like sugar stirred into water, and nothing settles out. It can be dark and still be clear.
A colloid has something spread through it in pieces too big to count as dissolved but too small to settle, roughly between a nanometer and a micrometer across. Milk is one. Colloids scatter light, which is why a laser beam shows up as a bright line when you shine it through a glass of milk. Try the same thing with black filter coffee and you’ll barely see the beam.
Then there are a few special cases. A suspension carries solid particles big enough to sink eventually, like the grounds that settle at the bottom of a cup of Turkish coffee. An emulsion is droplets of one liquid held in another, such as the fat in milk. A foam is gas bubbles held in a liquid.
Filter Coffee Is About 98.7% Water
The Specialty Coffee Association’s classic target for filter coffee, the Golden Cup, is 1.15 to 1.35% dissolved solids, extracted from 18 to 22% of the ground coffee’s weight. Everything else is water, so a well-made cup is roughly 98.7% water. Nearly all of the rest is dissolved: acids, sugars, caffeine and the brown compounds formed in roasting.
A paper filter does a lot of the tidying up. It traps the coffee’s oils and nearly all the fine particles. One study found under 7 mg of fat in a 150 ml cup of paper-filtered coffee, against about 50 mg through a metal screen and 60 to 160 mg in boiled coffee or espresso. That’s why filter coffee looks clear when you hold it to the light and a cup from a French press looks cloudy.
Some of the big brown molecules formed in roasting, the melanoidins, are large enough to count as colloidal, so even filter coffee isn’t a perfect solution. It’s close, though. For a mug of drip coffee, the sign is about right.
Espresso Is Several Things at Once
Espresso is far stronger, typically 8 to 12% dissolved solids in a modern café, and it’s brewed at about 9 bar of pressure, which pushes out things that filter coffee leaves behind. Food scientists describe it as a polyphasic colloid, meaning it’s several of those categories at the same time:
- A solution. Most of what’s in it is still dissolved, the same acids, sugars and caffeine as in filter coffee, only far more of them.
- An emulsion. Tiny droplets of coffee oil, 90% of them under 10 micrometers, are spread through the liquid. In one reference shot they made up 0.2 to 0.3% of the volume. They carry aroma and give espresso its creamy weight in the mouth.
- A suspension. Fragments of coffee cell wall, mostly 2 to 5 micrometers across, about 150 mg in a single shot. They add body, and together with the dark melanoidins they make espresso opaque.
- A foam. That’s the crema.
What Crema Is
Roasting leaves carbon dioxide trapped in the beans, somewhere between 4 and 8.6 mg for every gram of freshly ground coffee. Under pressure, extra gas dissolves into the brewing water. When the espresso reaches the cup at normal pressure, the gas comes back out as bubbles between 10 and 150 micrometers across. Carbohydrates and the brown melanoidins coat the bubbles and hold them in place, and that’s the crema. Oil works against it.
A good crema should last at least a couple of minutes. In one study, crema from a medium roast lasted about 34 minutes for arabica and 49 for robusta, which is part of why robusta blends look so good in the cup, as we explained in arabica vs robusta. Stale coffee makes less crema, because much of the gas has already escaped.
The Oils and Your Cholesterol
The difference between filtered and unfiltered coffee matters for more than looks. Coffee oil contains two compounds, cafestol and kahweol, that raise LDL cholesterol. A paper filter keeps almost all of them out of the cup. A metal filter, or none at all, lets them through: one study measured about 3.5 mg of cafestol in a cup of French press coffee, 3.9 mg in Turkish coffee and 1.5 mg in an espresso, with negligible amounts in drip coffee. The effect grows with how much you drink, so it’s worth knowing if you have several cups of unfiltered coffee a day and keep an eye on your cholesterol.
So Is the Sign Right?
For a mug of filter coffee, mostly yes. It’s a solution with a small colloidal fraction, clear enough that a laser beam hardly shows. For an espresso the sign is only partly right. There’s a strong solution in there, carrying an emulsion of oil, a suspension of fine particles and a foam on top, which is why it looks and feels so different from the same coffee brewed through paper.
The joke still works. It just needs a footnote for espresso.