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I found it was very humid in Brisvegas yesterday, I was consistently tamping and adding the same amount of coffee to each shot but the extraction was a lot faster... could the Gee's please tell me some science behind what is going on and offer some advice about what to change... I could add more coffee or change the grind but which would be most effective to making a splendid espresso?

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Hi Daniel,

We used to experience a huge change in the extraction time of the shots in Brisbane as well when we used to run our cafes there and this was largely due to the constantly changing humidity.

The grind should be the thing you change to alter the quality (or more specifically the extraction) of the shot. Keep the dosage and tamping constant but the particle size of the grind will the factor that needs changing with the changing level of humidity and here's why:

With increasing humidity, there is more water in the atmosphere. This water finds its way in between the coffee particles in your doser (coffee is hygroscopic, meaning it absorbs moisture readily). This water "pushes out" air particles, making it harder for the water from the group to filter its way through your compacted puck of coffee during the extraction process, therefore making the shot pour slower. So there is a direct correlation between rising humidity and a slower extraction rate. Hence in times of rising humidity, you need to make the grind coarser, so that there are more air holes in between the coffee particles so that the water finds them and runs through the puck faster. Conversely in times of lowering humidity you will need to make the grind finer to slow up the shot.

Ideally, your 30ml shots coming from your double group handle should run out at about 25-30 seconds. Less than 25 seconds means that the grind needs to be made finer, greater than 30 seconds means that the grind needs to be made coarser.

In your case yesterday, you should have seen the extraction time getting slower with increasing humidity.

Hope this helps! We have some podcasts on www.baristabrothers.com that explain this in great detail. It is the cornerstone of making espresso and adjustment of the grind will need to happen many times a day in a busy cafe, especially in Brisbane.

At the end of the day you need to focus on the richness of the shot and you want to maximise the honeycomb-coloured crema sitting on top of the espresso. This just happens to be - in the case of a commercial espresso machine - when the extraction from a double group handle with 14g of compacted coffee compacted at 18kg coffee, is around 25 seconds. Use taste as the ultimate guide though and try to achieve an espresso pouring out like "spaghetti in the wind" or looking like an inverted rat's tail after about 3-4 seconds of nothing once you have activated the pump.
Daniel, good question.Matthew,good answer!the problen becomes even worse in a mobile situation(imagine an outdoor coffee shop),I found myself having to adjust the grind several times by up to 10/12 mls.extremes being,dry windy to afternoon storms,rain and humidity!All makes for a fun day!!
Thanks Matt, really enjoyed your reply and the level of detail :)
This was very interesting to see, as I have actually just moved from Rockhampton (about 8hours north of Bris) and we used to have the same trouble all the time! Matt everything you said made a lot of sense, and its great to see a detailed explanation of whats actually happening. Hope the weather up that way has settled down now! I'm now living in Melbourne and our weather has been rather great (not very normal I know).

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