Hi Experts,
I used two different roast levels of beans to extract espresso in the same conditions(dose, grind, tamped pressure...).
I found the flowing of light roast is faster than the flowing of dark roast.
I would like to know why I got the result.
Can anyone help me to explain that?
Thanks a lot.

I am certainly not an expert, but I did a lot of searching/reading on this particular question, so I will try to summarize.

To answer your question I will start with little explanation of physics behind espresso extraction.

Grinder setting is in fact distance between burrs, which determines maximal size of the particles. But particles are not all equal in size, in reality there is a mix of different sizes, bigger and smaller. Variation in size and quantity of differently sized particles is shown well on so called histogram graphs, search this forum for ""particle size distribution".

Smaller particles (called fines) fill gaps between bigger particles, together they work as "bricks and mortar". Correct ratio and even mix of bigger and smaller particles is very important for correct flow of espresso and correct extraction of flavors.

Different grinders produce different "particle size distribution", there are endless discussions on this topic.

Different coffees are ground differently and also produce different "particle size distribution". There are a lot of things in play, coffee variety, altitude, ripeness, type of processing, style and level of roasting, age post roast, etc. All these aspects determine structure of particular bean, its hardness, brittleness, moisture content etc.
That's why different beans react differently to crushing forces between burrs and produce different particle distribution.

Most of the times (but not always), espresso from lighter coffee will flow faster at the same grind setting. Possibly there are less fines (small particles = mortar) produced, so resulting powder will not have sufficient resistance to water. Then you need to compensate by finer grinder setting.

So called "forgiveness" of particular coffee, may be related to how "good" particle size distribution those beans produce.
Some coffees, namely single origin, high gown, hard bean Arabica varietals roasted lightly may produce very flavorful brew with a lot of acidity and fruitiness, but such coffees are often very difficult and even almost impossible to "dial in" without proper high quality grinder.

I'd be interested to know if your coffees were of the same freshness. My own experience is the opposite of yours. That is, a lighter coffee always requires a coarser grind than a dark roast coffee. In fact, very dark roast coffees - like what Starbucks would call an espresso roast - tend to run too fast unless I set my grinder to a very fine grind.

My experience is lighter coffee needs a finer grind. Darker roasts create more fines than lighter roasts, I believe, because they break more easily.

linusch wrote:I used two different roast levels of beans to extract espresso in the same conditions(dose, grind, tamped pressure...).
I found the flowing of light roast is faster than the flowing of dark roast.

Same coffee? Sumatras flow faster; as do some lower grown coffees.

In general, here are the causes we know:

• the flow rate depends on fines; fewer fines, faster flow.
• the more brittle the beans, the more fines they create when ground, the slower the flow
• beans go through a "glass phase" during roasting, expanding rapidly, at the end of the first crack. Once this is done, the surface is brittle, and fines are produced. Roasts stopped very light, during the first crack, like cupping roasts, can have soft bean surfaces and be challenging to extract as espresso
• very dark roasted beans have softer, oil soaked surfaces, so produce fewer fines and require finer grinds.
• some beans, like Sumatras, have very soft surfaces even at medium roasts

If your case isn't covered by one of these; then we learn something new.

IME lighter meant coarser, darker meant finer. Dark roasts compact easier due to coffee oils in the grinds. A good example would be making a snowball with dry snow compared to wet. Darker roast = higher moisture content in the grinds = more solid puck making the water harder to penetrate.

linusch wrote:Hi Experts,
I used two different roast levels of beans to extract espresso in the same conditions(dose, grind, tamped pressure,freshness...).
I found the flowing of light roast is faster than the flowing of dark roast.
I would like to know why I got the result.
Can anyone help me to explain that?
Thanks a lot.

--> Both kinds of beans are 6th roasted days (fresh)
Kenya AA -> In the middle of second crack (Vienna Roast)
Peru -> the end of first crack + 30 ses (City Roast)
Can you predict if I can get the same result after 1 month?
I need to roast new beans to experiment since the beans were gone.

Very appreciate your reply !!

In this case it's the difference in beans; you wouldn't expect the same bean's grind level to change going fro the start to the end of the 2nd crack

Thanks all.
I got an important thought about "particle size distribution" here.

Is there other probability about exuding carbon dioxide.
The dark roasted beans exude more carbon dioxide.
It will provide higher resistance to get slower flowing.

I've never found that time after roasting (= CO2 outgassing) has much effect on brew time, at least within a week. Months old stale coffee, definitely. It helps if you think of roasting coffee as being similar to making candy. At lighter roasts the beans are more flexible, more plastic, and tend to shear rather than shatter when ground, producing less fines.

Darker roasts are more brittle, and you get a lot more dust (fines) when you grind them. The analogy breaks down with really oily dark roasts, where the volume of oil can produce some really odd effects. Note that really stale beans coated in half polymerized oils are even less predictable.

Alan