18 grams dry coffee to yield 36 grams of espresso in approximately 30 seconds brewed at approximately 94C at 9 bars.
You can run through some of the maths included in this brief guide on the brewing calculator page, it will, if used correctly give a rough outline of brew ratios etc.
When brewing espresso, you probably don't know the initial brew water weight*, you know your beverage weight, you know your tds and you might know your absorption coefficient if you weigh the spent puck and do some maths on the beverage and tds. For arguments sake we approximate espresso to absorb roughly 1g of water per 1g of coffee. To be totally accurate you know 4 parts of the equation so a guessed absorption coefficient means the initial brew water figure maybe unreliable.
This is intended as a basic guide to get you going at home. In order to make espresso you need an espresso machine. You can not make it in an aeropress whatever you read on Reddit.
You will need:
- scales (accurate to 0.1g)
- thermometer (or digital thermostat)
You may like to have:
- refractometer (measures percentage of a liquid that is coffee)
A normal espresso machine runs at approximately 9 bars of pressure.
This can be varied if you know what you're doing but I would not recommend it unless you have a refractometer to hand or an exceptionally attuned palate. Most home machines are fairly limited in terms of adjusting temperature and pressure. If you are thinking of buying a home machine look for one with a PID (Proportional Integral Derivative) which typically gives you the option of accurately adjusting brew temperature.
You may find that lower pressures can yield higher rates of extraction and sweeter shots as they cause less channelling in the puck. This in turn will expose the whole bed of coffee to a more even flow. There is an attrition point, generally at about 6.5 bars, at which the level of extraction will dramatically drop off and will result in sour, underdeveloped espresso.
The espresso machine should be set to produce water at anywhere between 90-95C.
Brew temperature is more important than you think. If a coffee is a bit roasty, dropping it by 1C can often be enough to sweeten it up and take the edge off. Never brew over 96C no matter what anyone tells you about coffee being roasted to 200C. Brewing over 96C releases all of the oxidised roast components such as acrylamide and can make the sweetest coffee taste bitter.
Firstly let's define extraction and over extraction.
Time, Temperature and Turbulence define your coffee. Coffee is 30% soluble and 70% cellulose. You want to get the highest percentage of that potential 30% into your cup whilst retaining sweetness, body and finish. Too little is under extraction and you will get nasty, sour shots, too much is over and results in nasty, bitter shots.
Limitations in the extraction peak are defined by technique and equipment. Typical commercial grinders and linear pressure machines can extract up to 20-21% of the dry mass of a shot.
Your coffee puck must be consistently ground on a grinder that will allow you to make small adjustments to particulate size. Adjusting particulate will increase and reduce the resistance of the bed of coffee. Smaller particles fit together more tightly and produce a dense more resistant bed of coffee that will slow the flow of water, this is how you control your "time in contact" with water.
Particulate size for espresso (depending on roast darkness and density of the coffee bean) is typically anywhere between 150-300 microns. The better the grinder you have the higher the percentage of particles produced will be at the desired grind size. Cheaper grinders will produce a lot of super fine/super coarse particles either side of the desired grind fineness. These fine particles in turn usually result in over extraction and bitterness as they are exposing a higher surface area to water. This limits the peak extraction level of a given coffee.
You can easily push extraction higher by grinding finer and brewing hotter but unless you have an amazing grinder and a well calculated extraction technique you are likely to produce stewed, bitter and roasty espresso.
When you hear Scott Rao talking about how he only drinks EK43 shots, this is because an EK43 (or similar) produces a particularly tight grind size average. The high peak at the desired size allows shots to be pushed further in terms of extraction resulting in a higher percentage of dry mass of coffee in the cup, anywhere up to 28% dry mass using long low pressure shots.
Things you can not use:
- blade grinders/spice grinders will produce unpredictable results that allow no control over particle size.
- small domestic burr grinders aimed at producing filter coffee. these will generally not allow you to make fine enough adjustments to accurately dial in a coffee. My Hario Skerton for example yields a shot of 15 seconds at one setting and 2 minutes at the next increment of fineness.
Tamping coffee is about prepping a bed of coffee for optimum and even flow through the puck. This means even at every point, minimising over and under extraction. You should typically aim to apply anywhere between 10-15kg of pressure to a tamp. Even distribution prior to tamping and a perfectly level bed after tamping are key.
How hard you tamp will not change anything on a commercial machine.
A commercial basket has a 58 mm diameter giving an area of 26.42cm (Pi r2). Pressure * area = force, 9 bars * 26.42cm = 2214 newtons which results in 225.76kg at the group head.
Your tamp weight will only affect how water initially interacts with the puck before it becomes saturated. Uneven pressure or weakened areas in the puck will allow channels to form and cause a higher flow through these paths of least resistance resulting in over extraction around the channels and under extraction the rest of the puck.
Well roasted coffee should yield a 1:2 ratio at the very least. If you put 18 grams of dry coffee in the portafilter you should be able to take 36 grams of espresso out at the end. With medium and light roasted coffee we typically aim for an extraction time of 27 - 33 seconds. Less than 27 will usually under extract, over 33 will usually over extract. You can make a puck of coffee less resistant to water by grinding coarser and more resistant by grinding finer.
Coffees with a higher acidity such as single origin Kenyans will usually favour a slightly longer shot to reduce their perceived acidity, where as a darker roasted Brazilian coffee may favour a shorter ratio to hide the burnt notes.
Crema is ONLY a measure of freshness.
Crema is formed by CO2 released during the roast process rising up out of the coffee and mixing with chlorogenic acid and oil at the surface of an espresso. The equivalent in filter coffee is the bloom.
If you brew coffee 5 minutes after roasting then you will get a whole cup of crema as the coffee is packed with carbon dioxide. 12 weeks later the coffee will present as a flat brown liquid with little crema when brewed.
Carbon dioxide in solution is carbonic acid. This makes your espresso taste tart and acidic. Beans are typically rested anywhere between 1-4 weeks prior to brewing in order to present a more balanced acidity.
A refractometer will allow you to objectively assess the percentage of coffee you have in solution.
It also allows you to objectively assess whether the roaster has given you a properly roasted coffee.
Bottomless (naked) portafilters show you right away if channelling is occurring. If it is it will spray all over the back of the coffee machine.