What's the science behind precision baskets?

Question

There are a number of aftermarket filter baskets available that are praised on different forums, including this site (from Shiri's answer):

VST baskets are replacement baskets for your portafilter with filter holes positioned and sized to aid uniform extractions. Would recommend getting one.

What's the reasoning behind these aftermarket baskets? I get that they pay more attention to the size and positioning of the holes, but I'm wondering why that matters so much.

perfectdailygrind.com writes:

Prior to this, most baskets had serious design flaws. Hole sizes varied wildly – some were big enough to allow particles to pass through into the espresso, while others so small that they were easily blocked, all in the same basket.

I'm wondering to what extent that quote is still relevant meaning there's still a basis for aftermarket baskets. Studies or simple tests objectively comparing the operation of stock and precision baskets are appreciated.

Answers could include citing ideal margins on hole sizes, thus establishing really when a hole is too big or too small for proper extraction.

Answer 1

So, it's kind of complicated. To understand why the hole area is important, we need to consider how an espresso is made and what it takes to taste good.

When does coffee taste good?

Making coffee is the processes of extracting water solvable compounds from the coffee grounds. Some taste good, some don't. The trick is to extract the right amount of each. You can control this by how much and how long the water touches the coffee grounds. Too much water and bitter, nasty compounds are extracted. Too short and only sour and volatile compounds end up in the water. You want the right balance for that delicious espresso you have come to love.

Setting up the ideal coffee puck

Espresso is about extracting coffee flavors from the small coffee particles. We want to extract each particle exactly as much. To do so, you carefully grind the particles to a certain size. The reason why people buy expensive burr grinders is to make the particle size distribution as narrow as possible (so all coffee grounds have the same size, so water touches the same surface area). You tamp the coffee in the filter basket as level as possible. Ideally, all particles are the same size and the density of coffee is homogeneous in the whole coffee puck (achieved by proper distribution and tamping level). Then water is pressed trough the puck at 9 bars of pressure. Ignoring the filter basket, the flow rate trough the puck will be the same everywhere.

It's about flow rate

The water will take the path of least resistance (lowest density, largest holes). So if you've carefully set up your puck to have a perfect particle distribution and homogeneous density but the holes in your filterbasket vary in size, more water will flow trough the larger holes. More water passes trough that path and the coffee above, therefore extracting those parts of the coffee puck more. This is an uneven extraction and deviates from the ideal where every particle is extracted exactly as much.

To get a homogeneous water resistance, we try to give the holes the same surface area. This ensures that if you have the perfect puck, water will flow evenly throughout.

When I bought my Strada basket, it came with a histogram showing the hole size distribution, which I think is pretty cool. You can see how narrow that Gaussian is, and it was a noticable improvement from the filter baskets that came with the machine.