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So, apart from the millrock competition, I've always used a non pre-infused machine. Whether it be my Isomac at home, or the Linea(s) at work. I'm really wondering what everyone thinks on this issue, and why so many new machines advertise it as though it makes a huge difference. I have had many of the best shots ever come from the non-infused Lineas, and same for my home machine. Are synessos pre-infused? Is there a difference between just turning the pump on for a second, letting it sit, then running the shot? Thats what the Frankes do, and getting even palatable espresso shots out of those things is not easy. Love to hear what everyone thinks. Any solid evidence of benefits/downfalls for either side?

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Kevin,
At the risk of diving to deeply into a seemingly "simple" yet infinitely complex question, there are a lot of different questions you might find come along with this. If a soft preinfusion is what you are referring to with the synessos, then you are close. They have both a manual preinfusion cycle that you can engage by moving the paddle half way or so preinfusing the coffee at about 5 bar, but the group head also has a flow restrictor rendering the manual preinfusion to be a confusing option.
The pressure profile and length of the preinfusion cycle paired with any specific coffee can render hundreds of different results based on the variations in pressure and cycle length, but, in general, preinfusion helps fuse the puck of coffee together where there might be some weak spots due to inconsistent density, rendering a more even extraction.
A hard preinfusion, in my opinion, seems less effective in that it slams the puck with water at 9 bar and then lets off, still leaving the chance of puck fissures and channelling. Having said all this, you can still get a great shot of espresso without preinfusion. However, there are variables like temperature stability that also come into play when you are comparing a Franke to a LM machine (apples and oranges really).

The best tool for seeing a difference is to have a naked portafilter set up for a machine to machine test (maybe multiple nakeds) with the same grinder as a constant and, hopefully, if the temperature is consistent between machines you will see the difference. You can also check out what the guys at slayer espresso are doing with pressure profiling on www.slayerespresso.com

Beware, this is a rabbit hole that just gets deeper the farther you go.
The way I think about pre-infusion, is that it is a sort of "helper". Basically, it allows water into the chamber at line pressure (i.e. the pump does not turn on, the water simply flows into the group chamber at whatever pressure the actual water line is delivering), saturating the puck, causing it to swell and expand--thus creating a more even density throughout, and minimizing any cracks or gaps in the puck. However, if everything is done perfectly prior to locking the portafilter into the group, pre-infusion won't really do much in the way of improvement. It helps when there are gaps, cracks, and density issues; and it doesn't harm when there are no issues. Hope this helps.
Which Isomac do you have at home? If it's one of their prosumer models with E61 group like the Tea or Rituale or Mondiale or Zaffiro etc. you DO have preinfusion at home. E61 groups automatically have preinfusion built in to the group design. Preinfusion was one of the main patent features of the Faema E61 group.
Just using the Venus, single boiler, no pre-infuse. Basically the Isomac "silvia".

miKe mcKoffee aka Mike McGinness said:
Which Isomac do you have at home? If it's one of their prosumer models with E61 group like the Tea or Rituale or Mondiale or Zaffiro etc. you DO have preinfusion at home. E61 groups automatically have preinfusion built in to the group design. Preinfusion was one of the main patent features of the Faema E61 group.
I would think a "hard" pre-infusion would be detrimental when you consider that the 3-way solenoid valve would disrupt the coffee bed, should there be even marginal pressure to equalize.
In Carlo Grenci's recent excellent article in Cafe Europa, he explained that preinfusion has always been standard in Lever Espresso machines- ever since the conception of the spring/piston system. When the Italian manufacturers made the great leap forward, with semiautomatic pump delivery, preinfusion was one of the thins leftout initially left out. I grewup using older semi-auto's and had no problem wth the quality of shot delivered without pre-infusion. Of course most new semi and auto machines now do have pre-infusion- but I guess the quality I can get from the lever we use makes me think preinfusion is quite important for getting a perfect shot.

I am not sure the best way of dealing with semi's with the older groups without pre-infusion- we just use these machines without tryng to play around too much. I am sure some of the more experienced barista (and compared to me thats nearly everyone!) can add much more....
I seriously thought about this all night:

As I understand it, pre-infusion on the new crop of machines featuring "just" (re: Slayer) line pressure pre-infusion that have come out is dependent upon toggling the 3-way solenoid open and closed.

Being that line pressure is introduced with solenoid open, pause, solenoid is closed, pump is engaged and pressure is brought up to 8.5-9 bars. There is no risk of the puck being disturbed by the solenoid dumping pressure then hitting it again full blast like "hard" pre-infusion suggests. I've seen this suggested for home machines on other forums like CG and it just strikes me in a way that's irksome, in that it's counter-intuitive.

Why would anyone want to potentially undue the great effort given to dosing, distributing and tamping so carefully? I can see the appeal, potentially, of doing this with a non-solenoid, pump-driven, home machine; but, at that level of equipment, even given the small edge pre-infusion offers in forgiveness of puck defects, how much difference are you seeing in the cup that isn't offset by things like temperature instability, etc?

Jeremy Conley said:
I would think a "hard" pre-infusion would be detrimental when you consider that the 3-way solenoid valve would disrupt the coffee bed, should there be even marginal pressure to equalize.
The thing to remember about pre-infusion and soft infusion is that it is not meant to improve the quality of the coffee, it is theorized that it will decrease your chances of getting poor quality.

When the machines evolved from pure steam pressure to lever activated piston pressure the quality of coffee was immediately improved by an astounding amount. When the lever is pulled down the piston rises. This allows water to enter the piston chamber, where it comes in contact with the coffee puck before the piston applies it's full force of pressure. The next significant mechanical advancement was the introduction of pump pressure. The pre-infusion chamber of the E-61 is meant to simulate, not duplicate, the conditions of the lever machine.

It very well may be that any flavor advantage comes from the fact that the first batch of water to hit the coffee will have a certain amount of temperature pulled by the coffee itself, making it unable to dissolve certain soluble solids at the beginning. Once the puck has acclimated, it is ready to give up the goods. That's when it needs full pressure. Then towards the end of the extraction, (where you begin to extract more and more bitterness) if you lower the pressure again you stand a better chance of keeping the flavors and after-taste very clean. This is something that lever machines do naturally: low pressure at the start, heavy pressure with the spring fully loaded, then decreasing pressure as the spring looses energy. While it is admittedly speculation on my part that pre-infusion helps to carve out a better flavor profile, I believe that the theory of "sealing off the puck" is just as much speculation.

One theory that disputes the "sealing off" theory states that under pressure, the particles of coffee are not allowed to physically expand, and that ultimately the expansion of the puck only happens after the pump is deactivated. Imagine a balloon in a pressure chamber, as the pressure rises, the balloon shrinks.

The premise of pressure profiling is essentially to controll how "deep" you extract at particular times during the extraction, thereby targeting certain flavor compounds and ignoring others. Pre-infusion is just a crude and rudimentary way of controlling the pressure.

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