1 Anatomy of the Espresso

There are dozens of methods of brewing coffee, from the humble filter, plunger, aeropress and cold brew. But here in Australia, when we hear "coffee", we think espresso. There is a lot of mysticism around coffee culture and the espresso, so as a coffee enthusiast and a nerd, I figure it is my responsibility to help out.

There are two factors which set Espresso apart from other methods of coffee brewing:

1. Constant water flow
2. High-Pressure extraction.

Let's explore what these two things mean for our coffee below.

1.1 Flow

Unlike immersion methods like the plunger, where the coffee is immersed in a fixed batch of hot water, espresso cycles fresh water through the coffee at a constant rate. This results in more efficient extraction. Imagine trying to dissolve your milo, the more milo is already in solution the harder it is to dissolve any more. So by cycling fresh water through the coffee grinds, we make it easier for that water to dissolve the flavour compounds in the water.

1.2 Pressure

While plenty of other methods use flow (e.g. filter coffee), Espresso stands alone in using high pressure to maximise extraction. Using high pressure extraction further maximises the ability of the water to dissolve the flavour compounds.

The combination of these two factors is what allows the incredibly rich and concentrated nature of the espresso, and it is these factors we keep in mind when exploring how espresso works.

2 Flavour

2.1 Taste and Aromatics

Like many natural products (e.g. beer and wine) it is important to consider taste and flavour differently. Taste is the 5 basic tastes, sweet, salt, acid, umami, and bitter. While flavour is the complex interaction between taste, aroma and texture. Think of it like the difference between citric acid and a juicy lemon.

2.1.1 Taste

The primary taste profile of Espresso is salt and a balance of sour and bitter, with just a small amount of sweetness. Taste in an Espresso is optimised by balancing its extraction while maximising its strength.

2.1.2 Aromatics

Aromatics are the parts of the flavour that aren't the sense of taste, or texture. When a coffee says it tastes like stone fruit they are talking about the Aromatics.

Aromatics are experienced through our sense of smell. The pharynx is the tube that connects our mouth to our nose. It is what got filled up that time you laughed so hard that spaghetti came out of your nose, but it is also what lets us experience the smell of food while we are eating.

In coffee, the compounds responsible for aromatic expression are Esters, alcohols and Phenols. These highly volatile compounds freely evaporate allowing us to smell them, but are also very sensitive to changes in their environment.

2.2 Extraction and Strength

The difference between a good espresso and a bad espresso is the balance of its extraction and its strength.

2.2.1 Extraction

During an espresso's extraction, the flavour profile is not even throughout the shot. The most soluble products are extracted first, while the less soluble are extracted after.

Typically the early fraction is acidic, tasting sour, brightly coloured, a warm honeycomb, and carries the oils and oil soluble Aromatics.

The later fractions are the near inverse. They are alkaline, tasting bitter, very darkly coloured like dark chocolate, and carries out the water soluble salts and aromatics.

An "over-extracted" espresso is characterised by a burnt and ashy taste. When people complain that their coffee is "burned" it is normally over-extracted.

Therefore, a "balanced" espresso is one which balances the bitter alkaline with the sour acid to reach a point where they approximately cancels out and is not too bitter or sour.

2.2.2 Strength

While a balanced espresso is one where the right amount of soluble compounds are dissolved, a strong espresso is one which achieves this in as little water as possible (or more accurately, maximises the concentration of flavour per mL)

A strong Espresso is more viscous than water, owing to the significant fraction of oils suspended in the solution. This gives an Espresso its texture (or as the weirdos call it "Mouthfeel")

3 Parameters Baristas Measure and Control

An espresso "recipe" is three numbers: 1. Dose 2. Yield 3. Time. When a barista wants to replicate the taste of espresso days apart, they attempt to replicate these numbers.

3.1.1 Dose

Dose is quite simply the amount of coffee which goes into the basket before you extract.

Increasing the dose does two things. 1. Increase the strength 2. Increase the resistance to flow, increasing extraction

Point 1 is quite simple to understand, the higher the dose-to-yield ratio, the more concentrated the Espresso will be. Point number 2 is a little more complicated though. By adding more coffee to the basket, it increases the resistance to the pressure applied by the machine, resulting in a longer extraction time. Read on to see how this affects things.

3.1.2 Yield

While dose is how much coffee grind you put in, yield is how much Espresso we pull out. Increasing the yield will change the coffee by. 1. Increasing the extraction 2. Decreasing the strength

Since dissolving things into solution, is a chemical equilibrium process, sometimes the barista might add extra water to squeeze out that last bit of flavour.

3.1.3 Time

The time is quite simply how long it takes to pull the shot. This final variable is where the art of espresso is revealed. Because while the barista controls the dose and yield, they do not have any control over how long the espresso takes to extract. This is something they can only measure.

Generally speaking, for a fixed dose and yield, an increased time will result in higher extraction, since the water stays in contact with the coffee grinds for longer.

The time it takes to extract a shot is driven by the machine pressure and the coffee's resistance. Since the pressure is not something the barista changes during service, this leaves resistance as the variable.

Coffee's resistance itself varies with a number of factors including:

• Grinder Settings
• Grind Dose
• Environmental Temperature
• Humidity
• Grinder Condition
• Coffee's Age
• Basket Geometry
• And More!

Of all the factors that affect resistance, the only ones under the barista's control are dose and coarseness. Since the other factors change over the course of the day, so will the coarseness have to change to account for these differences.

4 Dialing in the espresso

The process of making an espresso taste good, is the process of exploring the parameter space in order balance extraction and maximise strength, by adjusting dose, yield and grinder settings. .

5 Conclusion

And that's it! That is all the information you need to know to understand what the hell those weird coffee nerds are talking about, or maybe level up your own coffee :)