Precision baskets such as those from IMS and VST have been on the market for some time and are widely acknowledged amongst baristas and coffee enthusiasts as being essential to achieving high quality espresso, due to their high precision circular hole geometry.
What Are The Optimum Extraction Levels For The Best Espresso?
Before we delve too deeply into the mechanics of basket holes, its worth clarifying what is actually going on when we extract espresso.
Extraction is where we dissolve the desired flavour compounds from the grounds into a volume of water. There is an optimum extraction somewhere between 18% and 22% of the mass of grounds, before you start extracting things from the beans that don't taste nice. However, not all compounds extract at the same rate which means that even coffees produced within this range can still be 'over-extracted'.
By increasing the flow rate, we can extract all those delicious compounds without developing unwanted flavours in our brew. This is where precision baskets come in.
The Role of Espresso Basket Geometry
The basket's role is simple; it is a filter that allows ultra-fine coffee under high pressures to be extracted.
There are three particularly important characteristics espresso baskets that achieve the best results:
- Hole size - This impacts the amount of particles that pass through the filter.
- Flow resistance - The basket holes can get clogged by grounds, leading to uneven flow and uneven extraction.
- Hole Distribution - If the holes are distributed evenly, right up to the edge of the basket, it will improve extraction uniformity.
Ideally, the best precision baskets will filter out grounds effectively, whilst being able to reduce uneven flow and extraction caused by blockages.
Finding a design which achieves this is not very straight forward. Ground coffee can never be perfectly consistent, meaning that these tiny particles come in all sorts of shapes and sizes when looked at under a microscope.
Slightly larger particles can 'clog' up holes in your basket, resulting in pockets of under-extracted coffee which will have a knock on effect on the overall taste of our espresso. Misshapen, sharp and rough holes will only increase the chances of this happening during your brew.
The Mechanics of Precision Basket Flow
As their name suggests, these baskets have been manufactured to fine tolerances, meaning that each hole in the basket will be pretty much identical. They are commonly electro-chemical-polished to remove microscopic burs and shape edges from the basket holes, and are either pressed and ground to shape, or in some cases laser drilled.
To illustrate the difference that hole geometry can make, we took a non-precision 53mm basket and a IMS precision basket, and placed them both over a jug on a set of scales with the scales set to measure flow rate.
Whilst keeping the basket constantly full, water was poured into each basket and the corresponding flow rate was measured. For the non-precision baskets, this was between 9 and 12 ml/second. The precision IMS basket on the other hand was between 25 and 30 ml/second. Over double the flow rate! The water pressure in this test was only 0.0025 bar, thousands of times lower then the brew pressure of the 9Barista, yet the water flow rate was 20 times higher then you would get when brewing an espresso.
From this single experiment, it can be concluded that the pressure drop from the basket itself is negligible, compared to the pressure drop across the ground coffee.
However, these measurements neglect any flow resistance that is incurred if ground coffee particles start to particularly ‘plug’ the perfectly circular holes in the basket, which does have the the potential to increase flow resistance. It’s for this reason that the new generation of slotted baskets have a slight advantage over circular holed precision baskets.
Where Do We Go From Here?
Safe to say, the mechanics of filter baskets is complicated, but what's clear is that new precision baskets do make an appreciable difference to extraction levels and uniformity. They're a great way to not only improve your extraction, but also the overall taste of your espresso.
Looking to upgrade your 9Barista? You can purchase IMS baskets on our website here!
What are your thoughts on precision baskets? We'd love to hear from you!
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