30 Days of Blog - Day 29 - Thermal integrity in espresso

Welcome to day 29 of 30. Here it is, the next to last post! We got a little slow there for a bit, but I'm now getting back into a little more of the regular habit. The past few posts have been leading toward some specific points, and today's is staying in a similar area though perhaps a little different focus. A lot of what you have been reading will be revisited in my upcoming book about coffee extraction, where I will get a bit more in depth with the data.

Today I am going to be talking about controlling a vital element in coffee extraction, and it may not be exactly what you think. One of the primary reasons for this topic is the consideration of repeatability of great coffee brewing. 

I find that there are a couple of specific places we lose control of our coffee brewing in cafes (and surely elsewhere). I don't think this loss of control is from poor training, though it is possible, but rather that these specific ideas are rarely talked about. The big part of this is temperature stability in extraction and it affects espresso strongly. 

Last year at WBC we saw Berg Wu dip his portafilters in cold water for the purpose of lowering the handle temperature before each extraction. This obviously worked because he won, but what are the real details behind this technique? I have talked to more than a few people who are interested in this cold portafilter technique and my initial reaction is to say "don't do that" since it is what we have always been taught in regard to proper methodology. I don't want to be a naysayer, so keep an open mind though.

In the basic concept, chilling the portafilters will essentially lower the brewing temperature of the machine for a short time until the metal has reached equilibrium with the brewing water again. That is, if it does at all. It's essentially a poor man's temperature adjustment for the espresso machine, and while it worked in WBC last year, the rules have since been changed to allow the barista to have the machine temperature changed. With that rule change there should be little reason to chill your portafilters in competition in the future. I also have to ask how well that chilling is controlled when doing it. 

So it was a good call on Berg's part, as he affected the extraction in the way he desired and got the rules changed for future competitors. 

When it comes to the every day espresso operation, depending on how busy your cafe is, the portafilter temperature may not be much of an issue. If the cafe is busy there is little time for the handle to cool down, and you are probably going to worry more about the machine keeping up with hot water more than other thermal loss. However, what happens when your cafe has a lull, or it is the beginning of the day, or the end of the day? We always teach to keep the portafilters in the machine for thermal stability of the group and the handle, and I'm not going to tell you differently here (sorry, no coffee magic today). The problems come much more when the handle is removed from the machine for various times between insertion and extraction. It has become common for a "shot" barista to prep handles and stage them on the counter until they are needed these days. Again, depending on how busy the bar is or how efficient the barista is will determine how much heat is lost. 

The longer you sit and mess with a demitasse spoon scooping out careful 1/10 gram amounts out of the basket, the more heat you are losing. Imagine for a moment that it takes me roughly 30-45 seconds to pull/purge, knock, wipe, dose, distribute, tamp, clean and insert the portafilter handle each shot. I haven't timed myself in a while, but this should be more than enough time. The 30-45 seconds has a minimal effect in terms of heat loss in the portafilter. Now, let's look at a cafe barista who is pulling the portafilter, knocking, purging, wiping, taring the scale, dosing, scooping grinds, distributing, tamping, purging, cleaning, purging, wiping the drip tray, setting scales on the tray, inserting, taring the drip tray scale, and finally extracting coffee. 

I have seen this taking somewhere between 1 - 2 minutes per shot, and occasionally even longer. No joke. 

This can make it obvious that more heat is lost in the portafilter depending on how long it is sitting on the counter. While the temperature loss may not be massive, it does still have an impact on the flavor of the shot. The more lost, the more thermal energy is being sucked away from the extraction point. Some of you might actually need the thermal loss, but I bet most of you don't since you are probably extracting lighter roasted high density coffee. The biggest thing here is inconsistency. If you don't need to scoop any coffee, remember to purge once, and get it done quickly half of the time (let's say it took you that 1 minute), but other times it takes you an extra 30 seconds to scoop grinds because your grinder is inconsistent (*hint - it probably is), you have various amounts of thermal loss. Guess what, just a couple degrees fahrenheit will affect the flavor of your shots.

Now this might also be bringing up in your head the idea that we need more accurate technology to keep from slowing down the barista and I won't dispute that. Just this past week at HOST in Milan we saw a slew of new grinders that are meant to grind by weight rather than time, and we have espresso machines which are integrating scales to allow the shots to be weighed easily. The newer technology will certainly make following a recipe easier, but it will have another effect that no one is willing to talk about.

New baristas trained with this technology will no longer have intuition on how to make amazing espresso. They will hit the base line, and that is mostly it. They will become the new common denominator in barista skill, which won't blow anyone away.

Interestingly, the new direction of technology is leading to take espresso machines and grinders back full circle. In enough time it will all become "Super Auto" machines again. When the dose is done for the barista, the machine pulls to the specs, and perhaps the milk is automatically perfectly textured, why even have a barista? Just hire a staff, put them in green aprons, and have them push the button. 

Joking aside, super auto technology is already actually quite good these days depending on the company. I see this technology rift as a new separation point for "big business" vs "artisans". It's probably the same separation point, just new versions of the technology to be honest. 

Ok, back to the point on thermal loss.

Another big point of thermal loss that I see in espresso is "bottomless" portafilters. If you are a fan of these handles we could probably debate their usefulness and effectiveness for hours and never come to an agreement. I personally loath the things, though I know and respect plenty of coffee pros who use them with good results. I will leave the fact that I almost never want a full double espresso (especially the high acid kind) at the door. To be fair, spouted portafilters tend to get dirty and are hard to clean well. But anyway, this is about thermal integrity.

The bottomless portafilter has a limited amount of metal on the exterior of the basket, which is necessary for it to lock into the group head and to hold the basket. Otherwise, room temperature air is free around the entirety of the basket exterior. In comparison with a spouted portafilter, the air immediately surrounding the exterior of the basket is heated and insulated by the very hot metal at the base. So the environment of the air surrounding the basket in a spouted handle is significantly hotter. I see this as important for one very specific reason:

The higher temperature insulation of a spouted handle means that there is a lot less heat loss to the air when brewing.

I have heard the argument that the hot water hits the grinds first, and will extract properly before the heat can be lost to the air. This is clearly false to me though, as we would not be seeing the impact on the coffee extraction that Berg had at WBC if this were true. The water does in fact hit the grinds first, and the basket itself is quite thin and not terribly good at retaining heat. So what happens?

When brewing, the @200F water hits the grinds and basket at the same time essentially. When the puck has been completely saturated and the basket becomes equally as hot as the water and grinds, you are hitting full extractability in terms of what the water can pull out. When the basket also loses heat to the surrounding air that equilibrium takes longer. When there is cold metal surrounding the basket it takes even longer. When it is insulated by a bunch of hot and heavy metal it happens faster. Basically, the thermal energy wants to reach equilibrium with the surrounding air, and different materials are more conductive than others.

Thick, heavy metal retains thermal energy. Air pulls heat because it moves as it heats! We all know that heat rises and cold falls. When the air surrounding a portafilter basket heats, it becomes more active and moves a bit. Since the basket it sealed at the top into the group head this is mitigated quickly, but it still happens. The cooler air at the base of the basket may never be at equilibrium.

If you want to see these principles in action, you should try a hot portafilter with spouts, a portafilter in ice water, and a bottomless portafilter next to each other. Try to make them taste the same with the exact same dose/distribution, extraction, etc. Not only will the flavor change, but the shot flow and speed will likely be different as well.

My recommendation is to increase work efficiency any time the portafilter is out of the machine, use spouted handles (clean them often please), and to keep the same methodology for all shots, every time. No problem, right?

Well, that's it for now. Tomorrow will be the last post of my 30 days blog (ok, 60 days-ish?), and it will be another topic that is all about extraction. 

If you are intrigued by extraction be sure to catch my book when I get it finished, as it will cover even more!

See you soon,

Pete

Posted on October 26, 2017 and filed under Blog.