Relation between the temperature and pressure

[sid]

I've been struggling for a long long time...This is the situation:
I've been trying single origin espresso with triple naked-portafilter. The roast degree is City+(I don't like dark roast). I tried 200F ~ 205F and the pressure ranges from 9.0bar through 11.5bar. Can any one tell me about the relation between the temperature and pressure? Thanks.

[HB]

I've been struggling for a long long time...

What are you struggling with, the question or problems with the results? In general, brew pressure doesn't garner nearly the same attention as brew temperature. I think it's partially because brew pressure is easily measured and controlled within reasonable ranges. Most accept a flat pressure profile after full pressurization as "good enough" and move onto the next question. Me, I've settled into regulating brew pressure to a happy medium of 8.5-9.5 bar; I favor the lower pressures because it delivers good results and doesn't punish barista technique faults as badly as higher pressures.

Can any one tell me about the relation between the temperature and pressure?

I think of pressure as a main contributor to body / mouthfeel and temperature primarily serving the flavor / taste balance, though it's more of a hunch than conclusions based on careful analysis. Jim Schulman and Ken Fox may have a stronger sense for this since they've conducted at least two comprehensive brew pressure related studies (e.g., How to Preinfuse; Extraction Pressure Redux).

On a related note... as I mentioned earlier, although manipulating pressure hasn't gotten nearly the same level of attention as temperature, there has been a number of interesting discussions. Chris proposed a Pump Comparison Project (unfortunately it stalled). Designing pressure profiles picked up the idea of modulating the pressure during the extraction. Andy's Schectermatic is PID controlled and delivered lusciously mousse-like mouthfeel at Gimme! Coffee booth at the USBC:



Sean Lennon and Lino Verna are also quietly researching possibilities, some of which were showcased at EspressoFest 2006 (also known as the Monolith and the Flux Capacitor). Sean has a CD packed with pictures of the event that he promises to share when he returns from a long business trip in Mexico.

[sid (original poster)]

My ideal extraction time is: 8~10 sec the first drop and 25~30 sec the ending. But it always started at 3~5 sec and ended at about 15~18 sec. The taste is sweet but acid, too. I tried to set the grinder finer...well...it started better and ended better. But it tasted worse although the acidity disappeared. Sweetness less and it came out with a little dry mouthfeel. That is why I changed the temperature and pressure. I hope to find the best temperature and pressure for it. I need help...

[jason_casale]

There is a direct relation between pump pressure and temperature. If you experiment long enough you will see this is right. If you have a pid set it at a static temp say 203 pull shots at whatever pressure gives you good tasting normal shots for a default time and volume IE 25 seconds 1 1/2 ounces. 8.5 for me is my default brew pressure. Slowly increase pump pressure and keep the temp on the pid the same and the volume and shot times the same. so stay at 203 and increase rotary pump .5 bar each time. You will find the shots keep tasting more burnt or over extracted as the pump pressure increases past your normal pressure. If you keep the temp at 203 on the pid and change nothing in your routine and go below the normal pressure you use for instance I use 8.5 bar I would go below to 8 bar then 7.5 bar you will find the shots to be sour even though the pid says 203 and you have changed nothing in your routine. This is something I figured out when doing the versalab testing. So you can see there is a direct correlation between pump pressure and temperature. There is also a direct relation ship between temperature and grind. If you are brewing at 195 on the pid and have your grind set to give you 1 1/2 ounces in 25 seconds bump the brew temp up to 200 pull another shot with the same amount of coffee and same grind ie do not adjust the grind just be consistent in your coffee dose distribute and tamp pressure. You will see you will get less coffee volume using the same parameters as the temp of the water goes up and more volume using the same parameters as the temp goes down assuming you are not changing any other factors except temp. These are relational cause and effect issues that are not apparent unless you can easily change and maintain stable brew temp and pressure like the versalab can. thought I might share some observations with you all.

[HB]

My ideal extraction time is: 8~10 sec the first drop and 25~30 sec the ending. But it always started at 3~5 sec and ended at about 15~18 sec. The taste is sweet but acid, too. I tried to set the grinder finer...well...it started better and ended better. But it tasted worse although the acidity disappeared. Sweetness less and it came out with a little dry mouthfeel. That is why I changed the temperature and pressure.

15 seconds? I don't recall sampling an enjoyable espresso that extracted that fast. With the limited information you've provided, my first thought is address the short dwell and extraction time by adjusting the grind and overdosing. Chris et al offered more specific suggestions in How do you explore the extraction space?

[sid (original poster)]

Thanks to Dan for so much information. I really admire those people like Sean Lennon and Lino Verna and you...etc. Your great jobs perfect the espresso. My machine is Synesso. The way I pack is overdosing in volume not in weight, somewhat like the Weiss Distribution Technique. But my shot volume is only 1 oz. (If I extract more, I'll always get blonding aside) I used to test with double-basket. Now I use triple-basket...no more blonding aside. But what I dislike is the acidity stays too long and too strong although it's sweet enough. Can this problem be solved without changing the roast degree or resting time? (I always roast to the degree before 2nd crack and rest for no more than 24 hours.)
And thanks to jason.
You really give me some confirming answer about what I'm not positively sure. I feel the same about the burnt and sour taste while adjusting so. And it sounds quite interesting about the relation between shot volume and temperature. I think this will give me a great many experiments to do in the future. Thanks for sharing.

[another_jim]

Jason is right about higher pressures delivering rougher tastes -- there is some research to confirm this.
http://tinyurl.com/lw9qe
However, I'm not sure one can compensate for higher pressure with lower brew temperatures, certainly the procedure Jason uses could not show this.

The Italian conventional wisdom is to set vibe pumps on home machines to 11 bar, and regular pumps to 8 to 9. I don't know how they set vibe pumps in commercial pourovers (like the Rancilio or Cimbali one groupers). My guess this has more to do with their low estimation of home barista skills than any difference between vibes and rotaries.

My SOP is to set the pressure as low as possible while still getting adequate crema. This happens to be around 8 to 8.5 bar. However, I don't have any research to show this approach is in any sense optimal.

[sid (original poster)]

I've seen some videos of espresso extractions in the forum. I noticed that some pressure settings of good extractions are around 11 bars. Does it mean a higher pressure is necessary when we use more coffee grounds in the basket? Or all we have to do is just giving a higher temperature? I need some advice.

[HB]

Does it mean a higher pressure is necessary when we use more coffee grounds in the basket?

No, I don't alter the brew pressure based on the amount of coffee. Some are proponents of higher pressure like Jon (How to make a beautiful "naked" triple espresso):

Increase the Pressure: I set my espresso machine's maximum brewing pressure up from 9 bar to 10 bar by adjusting the over-pressure valve (OPV, also sometimes called an expansion valve). Machines with rotary pumps have an adjustment on the pump. Some blends respond well to higher-pressure extraction. I especially love the Espresso Delight Blend from Mitalena Coffee as ristretto (the thicker I can draw a shot, the sweeter and more chocolaty it gets). I find that you can get a syrupy body and sweeter taste that lingers in the middle of the palate when pulling at higher pressure.

The espresso lab machines at Counter Culture are set to 9.5 bar. I favor closer to 8.5 bar as a good balance of extraction quality, crema production, and smooth flavor profile, though it's worth experimenting to find your preference.