An espresso machine requires ground coffee, compressed into a ‘puck’ inside a basket by using a tamper. This is then put into the group head, and is exposed to hot, high-pressure water (9-15 bars).
There are several variants that will decide whether the result will be a good coffee or bad coffee, and an experienced barista will be able to tell if an extraction is good or not, as soon as it starts pouring.
What to look for:
A shot of coffee should be 30mls and take approximately 25 seconds to complete.
A golden brown crema, with ‘tiger stripes’ or uniform texture.
Fragrant, with a full bodied flavour
A ‘mouse tail’ where the flow of coffee forms a solid stream and breaks about halfway
The variants that will determine your results are:
The grind of your coffee
Ground coffee particles must be the correct size to allow the high pressure water to flow through them at the right speed to dissolve enough of the coffee oils, without burning and tarnishing. The grind of your coffee will change each day and sometimes during the day, it is dependant on the type of bean, the blend of the beans and the humidity as the beans will absorb moisture from the air. Experience will allow you to adjust your grind quickly as you season your machine, or after your first shot of the day.
Too coarse, water will flow through too quickly and your coffee will look pale, and taste watery. There is not enough resistance for the water
Too fine, your coffee will take longer to start pouring as there is too much resistance, look dark and taste burnt.
Find the middle ground where the texture is like damp sand, this allows enough space for water to flow, yet enough resistance so that it absorbs the oils from the grind.
The correct amount of coffee in your basket
This will vary from blend to blend, machine to machine, and basket to basket.
Baskets vary in size depending on the model and preference of the barista. A smaller basket lets full bodied and intense coffees shine, without being over bearing or single origin coffees that are delicate in flavour. A large basket provides oomph to a more common every day blend
Over full basket will not fit in the group head properly, and will restrict water flow, over extracting the coffee
Too little coffee in the basket and there will not be enough resistance, causing under extraction.
N.B. Ideally you don't want your coffee to sit lower than 7-10mm from the rim of the basket.
Pressure applied when tamping
Tamping coffee too hard or too lightly is the same as having incorrect grind, the water will flow to quickly or too slowly if you do not have this correct and will result in a poorly extracted coffee.
Each barista is different, has a differing strength and stands a different way so find which way is comfortable and ergonomic for you. Apply even pressure, do not exert yourself, it should not require a lot of effort to tamp correctly. Make sure your tamper base sits flat inside the basket and is not on an angle, it is helpful to use your thumb an forefinger on each side of the tamper to make sure you have this level. This reduces the chance of “channeling” where the water finds the easiest way to escape the pressure and flows through the spot with least resistance.
Once tamped, release the pressure and turn slightly to “polish” the top of the coffee.
Tampers have taken many styles and shapes over the years, some being attachments on the front of the grinder or machine, others with plunging or spring mechanisms. Nonetheless, they all have the same function. Most modern tampers comprise of a stainless steel base in varying sizes to fit differing machines, with a handle of any sort (wood, plastic, metal).
Recent developments allow customisation of most parts of your tamper, and even add attachments to increase your accuracy, such as depth adjusters or spring-loaded systems. The tamper you choose depends on your machine, budget and personal preference. Choose a tamper with the correct size base to fit your basket, reaching as close to the edge before it is impractical. This ensures you tamp as much of the surface area as possible in one go. And pick a handle that both feels comfortable during use, and suits the style you desire.