The Secret Life of Ristrettos

[Abe Carmeli]

Here is what I came across when I was doing temperature tests on the Brewtus. Ristrettos behave differently than doubles in the following manner:

On a dual boiler machine like the Brewtus, they reduce brew temperature by ~ 1F for every 1/2 oz of shot volume below 2 oz. A standard 1.5 Ristretto will show an average brew temperature which is 1F lower than a 2 oz double brewed under similar conditions. I believe that he reason for the difference is flow rate, though Greg Scace is probably the person to confirm it or explain it. Flow rate is slower with a Ristretto and thus allows the brew water more time to cool off on their way to the cup. This has been demonstrated over and over again on the Brewtus. I have not tested it on a H/X machine where the a slower flow rate affect on brew temperature may be negated by a temperature boost from the H/X .

Ristrettos exhibit a smaller temperature variation within a shot. The variation is reduced by 50%. The rule of thumb again is that every 1/2 oz reduction in brew quantity below 2 oz will reduce temperature variation by 50%. Keep in mind that all other conditions stay the same: That is, the shot length stays 25 seconds, the set temperature and flush routine stays the same. I suspect that the reason for it is that brew water spend more time in the grouphead, which in turn helps flatten temp variation.

They are perfectly suited for back to back shots on the Brewtus. Unlike doubles, you can do 10 of them back to back, without causing a meaningful rise in brew temperature.

[Walter]

The reason for the difference is flow rate. It is slower with a ristretto and thus allows the brew water more time to cool off on their way to the cup.

Hmmm, and you think that the heat loss at the beginning of the brewing due to heating up the relatively cold coffee-bed is negligible?

[Abe Carmeli]

Hmmm, and you think that the heat loss at the beginning of the brewing due to heating up the relatively cold coffee-bed is negligible?

Doubles and ristrettos go through the same cold coffee puck. So the puck initial temperature should not be the reason for their different temperature profile.

[Walter]

Doubles and ristrettos go through the same cold coffee puck. So the puck initial temperature should not be the reason for their different temperature profile.

The energy spent on heating up the puck cools the water. And since heat transfer is always proportional to the temperature difference, that happens mostly at the beginning of the flow. Which would make for a lower temperature of the outgoing flow until the puck is heated to some 200F...

Given an identical flow-rate the profiles should be very similar (or identical), but since the temperature of the outgoing coffee is somewhat lower at first (with ristrettos and doubles) the first - say - 1/2 oz flowing into the cup should be the coolest.

I am not disputing this effect per se, which I consider a fact, I am just wondering about its magnitude, since it always seems to be neglected in the considerations regarding temperature stability. (And I wonder if this could be the reason why sometimes a HX-hump yields better results, but that's a different story)

[HB]

While my comment isn't directly related to this thread, lately I've been experimenting with extraction rates as part of the Vetrano wrapup. That, plus Ken and Jim's The Impact of Preinfusion on the Taste of Espresso Shots has me thinking about extended preinfusion for E61s, despite their native capability.

Combining the two ideas, I increased the water inlet pressure to three bar. When the lever is up and the machine is off, water flows readily through the grouphead. The "rebound" time for the Vetrano is 30 seconds after the flush, so I reserved the last 10 seconds for additional preinfusion. Specifically:

• Flush as usual; start preparing the portafilter (or prepare basket separately and drop it in later)
• Lock in the portafilter
• The heating element will click off 15-20 seconds after the flush; once it does, turn off the machine
• At the 20 second mark, lift the lever up. The mains pressure will preinfuse the puck ever-so-gently
• After 30 seconds, turn on the machine. Since the lever is still up, the pump begins immediately.

So what does this have to do with ristrettos? Getting the grind perfect for ristrettos on heat exchanger espresso machines can be tricky. Too slow a flow and the HX will overheat, producing a doubly-bitter shot (partially from high temperature, partially from overextraction). Too fast a flow and the rich, sweet, punchiness of a ristretto can be lost. The extra preinfusion appears to increase the margin of error.

As a rule of thumb, ten seconds of (extra) preinfusion equals a grinder adjustment of two millimeters coarser on the Mazzer Mini, or a reduction of coffee equal to approximately 1.5 grams (*). Running the mega-sized preinfusion "softens" the puck and opens a wider window for good extractions. My results so far for ristrettos are improved, though the opposite occurred for regular doubles, where the flavors became muddy and flatter.

(*) Don't take these numbers too literally, they are meant to convey that the effect is very small.

[another_jim]

Interesting results. I've always thought that the lower flow rate of ristrettos tended to reduce the amount of solids extracted from the puck (since the liquid is more concentrated, it absorbs slightly less, so a 1 ounce ristretto isn't twice as concentrated as a 2 ounce normale). Dan's flavor with the preinfusion on normales speaks to this (some LM users dislike preinfusion for this reason); in ristrettos the same preinfusion may clarify some flavors underrepresented in a regular ristretto.

But I never considered the temperature implications.

The E61 is heated by the hot water from the boiler. If an HX machine's balance is just right, the extra cooling of the head at ristretto volumes may be compensated by the extra heating of the HX. But since it's about balance, it could logically go either way, depending on the the HX. In practice who knows. On my machine single, ristretto and double volumes register within 1f (can't measure much closer). On an brew boiler/E61 combo, the temp drop during the shot could be more dramatic with lowered volumes. Since the brewtus temps tend to climb at normal volumes, ristrettos could be more stable.

Based on all this, I'm guessing that this may vary machine to machine, even within the "e61 box" tribe.

[HB]

Based on all this, I'm guessing that this may vary machine to machine, even within the "e61 box" tribe.

That's true. Although the HX Love article suggests they behave similarly, it's a bit of a simplification. For example, the Expobar Lever's temperature rebounded like a rocket compared to just about any other E61-based machine. I'd expect the shape of the profile to be affected by your choice of rebound or flush-n-go technique too. But in all honesty, I was inferring there was a temperature rise for very slowly pulled ristrettos (I'll allow up to 40 seconds) versus the regular pace of a double (I'm getting itchy around 30 seconds) by taste, not direct measurement. I could be attributing the bitters of overextraction for those of over-temperature.

That said, I'm not sure what to make of this:


Red = regular, blue = preceded by 10 seconds of passive preinfusion (using thermofilter as a flow restrictor)

Since adding extra seconds of passive preinfusion, I've noticed the peak temperature reported by the TC adapter was lower, but I never bothered recording it until tonight. In the chart above, the first 10 seconds of the blue line are the passive preinfusion. I expected the temperature to spike quickly like it does when the pump engages, but it barely rises (the first 10 seconds of the red line are idle temperatures I included for completeness).

I didn't check if the thermofilter picked up this difference too. My wild guess is that the pump's much higher flow rate is increasing the "reach" of the HX injector, essentially pushing the hotter water at the bottom of the HX further toward the group sooner than the more gentle line pressure would. If that's the case, the puck would get a hotter initial shock from the non-passive preinfusion case.

(Or maybe I need to take a break from all this thermofilter / TC adapter business... and switch full-time to a lever machine for a few months)

PS: For those reading along looking for a bit more detail, the diagram below shows the HX and boiler. The bottom portion of the HX (immersed in the super-heated water) will warm more quickly than the top portion. The HX injector (not shown) is a short tube that enters the HX from the pump. Manufacturer's "tune" the HX temperature mix by trimming the HX injector length.


The HX injector, if shown, would enter the HX from the bottom (dotted lines)

[another_jim]

Passive preinfusion on an e61 may be tricky -- Micheal will need to confirm this, but my understanding is that while the preinfusion cylinder ramps closed, one is getting tempered water, that is, flow of cold water from the pump through the thermosyphon return into the group. I know this happens for a little bit by design, but I'm not clear when the valving inside the group shuts this flow path down.

[mikep]

PS: For those reading along looking for a bit more detail, the diagram below shows the HX and boiler. The bottom portion of the HX (immersed in the super-heated water) will warm more quickly than the top portion. The HX injector (not shown) is a short tube that enters the HX from the pump. Manufacturer's "tune" the HX temperature mix by trimming the HX injector length.

I apologize if this is veering off, but I am not understanding what the injector is. Is it an additional line from the pump of water not mixed with the return from the thermosiphon loop?

Also, does the water in the return side of the thermosiphon loop get pushed back into the group when the pump is engaged, or does the 3-way solenoid valve prevent this?

[HB]

I am not understanding what the injector is...

The injector is nothing more than an inlet tube from the pump within another tube, namely the HX. I don't have any pictures of one matching the diagram above for a straight top-to-bottom HX tube (pointers welcome), but the injector of the Cimbali Junior's cigar-shaped HX is clearly shown in its owner's manual:



Owen Egan documented his close encounter with one in the Cimbali / Pasquini Junior resurrection project, specifically on this page. What began as a simple replacement of a busted HX evolves into a funny man versus machine story. But I digress.

does the water in the return side of the thermosiphon loop get pushed back into the group...

Looking at Lino's E61 schematics, I don't see any valving in the group to prevent backflow from the return section of the thermosyphon loop.

[mikep]

Thanks Dan! I had previously thought the heat exchanger was just a simple tube. I wonder if my expobar has a relatively short injector, since it picks up heat very efficiently (long flush required, low rebound time).

[HB]

I used a friend's Expobar Lever for a couple months; you aren't kidding - it requires an olympic-sized flush! A shorter injector may be part of the reason, plus other factors like the delivery tube length and diameter. I've not compared them carefully, but they look longer and larger in diameter than other E61s. That said, I had no trouble pulling good shots on it with appropriate adjustments to the flush amount (long) and rebound time (short).

From Odd Expobar problem - no thermosiphon

[jrtatl]

I used a friend's Expobar Lever for a couple months; you aren't kidding - it requires an olympic-sized flush!

No joke here. With the top of my deadband somewhere around 1.14bar, I have to flush 8oz and immediately lock in (or within about 10 sec) for any beans darker than City roast. If I do a few pseudo-shots as I prep my basket, I can sometimes lower the flush volume to 6oz, but this really works better with the lighter coffees.