There have been entire books dedicated to the process of making espresso. The subject is as complex and rich as the beverage itself with many variables and possibilities to explore. We're going to keep things relatively simple here first by outlining what espresso even is and then we will dive into the three variables you need to understand in order to create successful espresso recipes. We'll finish with a few tips to help you get started.
First let us establish what we're referring to when we say espresso. Espresso is a method of brewing coffee in which a relatively small amount (10-25 grams) of finely ground coffee is brewed under intense pressure (around 9 bar) and yields a strong and flavorful concentrate in a short amount of time.
Before we get into specific tips it is helpful to have some understanding of what is happening during the brew process. Roasted coffee beans contain various chemical compounds that are able to be dissolved in water. Among these compounds are caffeine, acids, lipids, melanoidins, sugars and carbohydrates. These compounds aka "solubles" are extracted from the coffee grounds by water at various stages during the brewing process. They influence the taste, aroma, and overall sensory experience of the final cup. The balance and interaction of these solubles plays a crucial role in determining the unique flavor profile of different coffee varieties, roasts and brews.
Scientists have identified the order at which these different compounds extract into solution and can be thought of as follows. Fruit acids and organic salts are the first to extract into the cup, followed by simple and complex sugars and finally bitter agents. Our goal is to find the point at which the coffee tastes sweet, bright, and transparent with a long pleasant finish. We accomplish this through adjusting dose, yield and time.
Matt Perger of Barista Hustle breaks down espresso brewing into three variables: dose, yield and time. By simply adjusting these parameters we can greatly influence the overall quality and consistency of our espresso. Water temperature and pressure are also important variables but let's assume a brew temperature of 201F and max pressure of 9 bars. We will rely on the variables of dose, yield and time with taste as our guide to "dial in" our espresso.
Dose - Think of dose as adjusting the overall strength of the final cup. More coffee = stronger coffee. If we increase the amount of coffee in the brew basket, we are going to have a higher concentration of those soluble compounds in our cup for a given volume. This can be a good or a bad thing depending on how it tastes. If the coffee tastes overly acidic, savory, salty, and a quick finish, the coffee is likely "under-extracted" meaning we were not able to pull enough of the soluble compounds out of the coffee to create a more balanced cup. Lowering the dose might help in this case.
Yield - This refers to the total volume of liquid in your final cup. Increasing the yield can also help to boost extraction. By passing more water through the bed of coffee you are able to extract more of those solubles. Keep in mind that too much water can create astringency thanks to the increased number of bitter compounds that are pulled into the cup towards the end of the shot. There is also a tradeoff between more water and body. Body generally refers to the perceived thickness, heaviness or viscosity of the coffee in the mouth. A gain in the overall extraction may result in a lower body but it may be worth giving up some of the body for more balance and sweetness.
Time - Once you have locked in your dose and yield. You can make further refinements to your shot by adjusting the grind size. Grinding finer will increase the time it takes to reach your desired volume output and will typically increase extraction to a point. Grinding too fine will actually result in a decrease in extraction most likely due to a phenomenon known as channeling.
It will take some time to understand the relationship of these three variables and how they influence the taste of your coffee. It can be maddening trying to get a coffee dialed in and sometimes feels like you are chasing the dragon. But if you enjoy the process of continual refinement then you will likely find brewing espresso to be an extremely rewarding process.
Now for a few tips! If you are just getting started with brewing espresso I would recommend using a medium roasted coffee and preferably a blend since they tend to be a bit more forgiving and easy to extract. Now a few tips to get started:
1. Use "soft" clean water that is free of impurities. Some minerals are necessary for proper extraction but not too much. Avoid using Reverse Osmosis or Distilled water for this reason unless you are going to be adding your own minerals with either Third Wave Water packets or Lotus drops. An affordable store bought option is Crystal Geyser brand water from their Olancha water plant. This water is quite soft but is recommended by La Marzocco's Home Espresso department and considered safe to use in your machine. If you want to take care of your machine and get the best flavor out of your coffee, I can't stress enough the importance of beginning with tasty water.
2. Use a good burr grinder if possible. A good grinder is arguably the most important tool in your setup. Evenness of grind is the name of the game and will greatly improve your overall extractions and produce more clarity in the cup. Blade grinders create too wide of a distribution in grind size which "muddies" the cup due to a blend of over and under extracted particles.
3. Grind just before brewing, and tamp your coffee evenly. There are various tools on the market to help you accomplish this but a simple and affordable option is to just use the palm of your hand to bump against the portafilter several times to level out your grounds.
4. A good starting ratio of coffee to water is 1:2 with a total brew time around 25-30 seconds. For example, using this ratio, a dose of 18 grams of coffee would yield 36 grams of liquid in your cup. The appropriate dose will vary according to the size of your espresso baskets. Your user manual will usually recommend an appropriate amount.
5. Taste lots of coffee. Visit your local specialty coffee roaster and order a plain espresso so you know what a properly dialed in shot tastes like.
I'm going to link to a video that helped me enormously in understanding espresso extraction theory. I highly recommend giving this a watch.