Confused by the change in brew pressure during extraction

Beginner and pro baristas share tips and tricks.

#1: Post by Armanius »

Hello! Thanks for the help.

I have a Breville Dual Boiler. When using a two month old Intelligentsia Black Cat (bought it from the local grocery instead of direct), I noticed the following in the third day after I opened the bag:

- after the preinfusion, pressured ramped up to about 9, with first drip at 10 seconds
- the pressure then dropped to about 5 bar, where it stayed for the remainder of the extraction
- meanwhile the espresso was coming out s-l-o-w in spite of the low pressure
- I got about a 10 gram yield at 30 seconds
- After the extraction, I examined the puck, and it looked fine. It didn't break apart or was excessively soaked or dry.

This didn't happen in the first two days of using the Black Cat. I used the same amount of beans at the same grind setting each time.

I tried it again for the next couple of days, and the same thing happened. Any ideas what is causing this? Is this because of old beans? Shouldn't low pressure equate to fast flow?

I ended up dumping the beans and switching to a bag of Nossa Familia Teodoro that I got from Amazon, which I presume was probably older than the Black Cat. The Nossa Familia didn't give me the same issue. I have also used a bag of Lavazza Super Crema from Amazon, and also didn't encounter this issue.

Thanks for the help!!


#2: Post by new2espresso »

Have you tried a bottomless portafilter to look for channeling? How does the machine do with the rubber disc in the basket? Is there any leakage from your water spout?
Kind regards,


#3: Post by Armanius »

I don't have a bottomless portafilter, but I have been looking for one. Those are hard to find for the Breville.

For the plug, do I just run a backflush cycle, or just pull a normal shot?


#4: Post by paland »

FWIW - I'm a BDB as well, I see similar things a lot of the time and I'm still trying to figure out what is causing it.
Pre-infusion for 7s, shoots up to 9-10 bars by 10 seconds and then usually drops to somewhere around 7 pretty quickly for the rest of the shot. Every once in a while though I get a shot that sits right at that 9-10 number and it almost always seems to taste just a hair better.


#5: Post by Jeff »

First, the gauge pressure is only roughly related to the extraction pressure in most machines.

If you're seeing a significant drop during extraction (and that sounds significant to me) it can be indicative of channeling or other issues with puck prep.


#6: Post by Armanius » replying to Jeff »

My puck prep is real consistent. Could coffee bean freshness be a factor?


#7: Post by JModdd »

BDB owner here. I mostly ignore the pressure gauge and stick with how the shots are pulling. While you're correct in assuming old coffee should in theory be so stale that there is no CO2 left to give it the resistance it needs, the speed of extraction isn't just dependent on that (size of grind, amount of coffee, heat of your PF).
I'd personally ignore your pressure gauge and focus on dialing in your shots with nailing down your variables and getting your coffee into a range that tastes good. Also, make sure your machine and all the components are ripping hot before you brew with them. Make sure you LEAVE your PF in the machine while it's heating, and also it wouldn't hurt to run 3-5 seconds of brew water through it just to make sure it's the same temp as your group head. Variance in heat is a huge factor in under-extraction.


#8: Post by Armanius » replying to JModdd »

Thank you. I increased the dose from 18g to 19g using the same beans, and the pressure kicked up to 10 and then dropped to about 8. Got about 29.5g yield in 30s total time. Shot came out acceptable.

How long do you wait until you start your first shot after turning BDB on? The lights indicate ready fairly fast.


#9: Post by DamianWarS »

Armanius wrote:My puck prep is real consistent. Could coffee bean freshness be a factor?
older coffee has less CO2. CO2 will create a back pressure that the bars counters enough to push the coffee out, this part contributes to things like crema. If there's no CO2 then that back pressure won't be there so the machine doesn't need to push as hard. I don't know if that's what's happening with you but it also does sound like a channeling issue.

Old coffee has different properties than new, it's more plastic and flexible which may contribute to fewer fines and more borders. Fines are needed for extraction and the more big chucks you have the less pressure is needed to push the water through. Perhaps it ramps up initially to 9 bars as it works through the puck but down to low bars because it no longer is needed. There may not be a single major channel but several small ones created by too many big chucks with several easy paths for the water to run through.

You can try things like grinding finer or dosing higher to try and counter these effects but there is a point of diminishing returns where it's just old coffee and you can't get anymore out of it.


#10: Post by Armanius » replying to DamianWarS »

Very interesting info. Thank you!!!