Pre-heating your TWIST -- some like it hot
Posted by Stephen O'Brien on December 02, 2012 0 Comments
We receive a lot of questions regarding the best pre-heating procedure for the TWIST. The answer is actually quite easy. Feel free to skip to the end of this article if that's all you need. However, if you're interested in a bit more background concerning how we approached the issue (it was a little more complicated than the cup of noodles approach of "just fill with boiling water"), then please read on.
During the design of the TWIST I set a clear goal: the taste of the shot could not be compromised. It had to stay true to the pressure and temperature requirements of espresso. We weren't distracted by the pressure stats of systems that claim they work at 16 or 19 bars. More bars does NOT equal better espresso. If your car's tyres are supposed to run at 28psi then you won't get a better ride filling them to 50psi, although it may be somewhat more exciting. Nine bars is the standard for every professional espresso machine that works with ground coffee; as is extraction temperatures in the range of 196 to 204 F. If you have coffee that was pre-ground months before and is packed in tiny sealed containers then there's a good chance you'll need to use a higher pressure to try to drive some latent life from the those beans when it comes time to use them, but this is absolutely not required or even desired for a decent espresso made from fresh ground coffee.
Well, anyway, that was our goal. Temperature management was a little challenging when we first showed prototypes at SCAA in 2009. For one, there was no fill lid on the water bowl. You'd need to first remove the waterbowl, pull out the shower head, turn the bowl upside down, fill with freshly boiling water, replace the showerhead hoping you hadn't overfilled the bowl as this would cause scalding water to squirt out the top of the showerhead, and then gingerly reposition the waterbowl back on the handle while trying not to cook the prints off your fingers. It was a delicate and somewhat fraught procedure. Oliver Strand from the NYT Diner's Blog wrote a review on the TWIST at the time. I met Oliver at the wonderful Zibetto espresso bar in Manhattan where the friendly barista let us take over the grinder. We pulled a sequence of really rather good shots and Oliver seemed thrilled--his reaction is probably best summed up by his review's title: "A Magic Wand for Espresso". His only negative comment: the fill procedure is really quite awkward. We took it to heart and redesigned the waterbowl so it could be filled from the top. Given we were already in the midst of tooling for production making that one change caused a few months delays, but was definitely worth it.)
Our system did already have excellent thermal stability. All of that metal in the waterbowl takes quite a bit to heat up, but once it's there it doesn't want to cool back down again in a hurry. That's the key to the TWIST's extraction temps. Bring it up to temperature and it will stay there. Pull shots back to back and it will stay there for as long as you keep pulling shots. Throw in some water without pre-heating and you'll just get a cold shot.
So anyway, back to v1. Back then the waterbowls were covered with a plastic insulation material which we learned after a while was prone to cracking. (No structural danger--just a risk of becoming unsightly). Obviously this had to be fixed, but there ensued a somewhat lengthy internal debate on exactly how. Was it just the plastic composition that should be changed. Could it be changed to a removable rubber material like neoprene, or maybe more of a slick skin like an iPhone case? Our engineers had one opinion, the factory had another.
In any case, one thing was certain: the tooling would need to change...again.
The good news is that we had already started working on v2 and had bedded down some key improvements such as a better locking system for the top cap, as well as a number of other tweaks, so if tooling was going to change then we could at least incorporate the other improvements at the same time.
After some testing we settled on a type of rubberized plastic that had just enough flexibility to not crack--ever, but was sufficiently rigid to not ever want to slip off the waterbowl even when everything was super hot. A further advantage is that this material provides much better insulation. The bowl became comfortable to handle even when the metal was extremely hot, it retained temperature for longer, had a nice soft-touch feel, and was as non-slip as a set of wet weather Pirellis in the Pyrennes. Why didn't we think of it to begin with? Well, that's just part of product development. Iterate fast, iterate often. We liked the bowl so much we implemented it on top of v1 frames as a sort of v1.9. We didn't make many, but you can recognize them because the waterbowl has a rubberized outershell and the shot counter marks, but doesn't have the protruding metal alignment tab at the base that is so prominent on v2 units.)
Anyway, back to the original question. One of the comments we heard most often with our v1 unit was that it was difficult to pull sufficiently hot shots. This wasn't a case of tongue-scalding hot, but at least with enough temperature to achieve a satisfactory extraction that avoided the sourness that can result from too low a temperature extraction.
The answer is actually quite simple. Fill the bowl with freshly boiling water, to the brim please. The heat from the water will start to transfer to the metal. The metal has a much higher thermal mass than the water. As it heats it draws energy from the water cooling the water down quite quickly while the metal heats slowly. Therefore put your kettle back on the boil, or flick its on-switch again. After about 20 seconds you reach a point of diminishing returns where the temperature of the water and the metal has equalized at about 192 F, so there is no more to be gained from the system. Empty the waterbowl and fill again with boiling water. The additional heat in the water will start flowing into the metal again. After about 20 seconds they will have equalized once more, this time at around 198 F. (Your mileage may vary, by the way, depending on everything from the design of your kettle--eg how much does the water cool down while traveling along its spout and then falling through the air into the waterbowl, and also according to your altitude as the boiling point of water diminishes the higher you go). This second pre-heat is a good time to pull a shot. Most beans are perfectly forgiving at this temperature. However if you want to go hotter still a final preheat, refilled with boiling water, and left for 20 seconds, then discarded, will pull the actual extraction temperature (the temperature of the espresso as it exits the coffee basket) up to 202 to 204 F, or basically the maximum temperature we've heard anyone ever wanting to use. Going higher than this actually ends up scalding the coffee oils, creating a bad taste all around.
So, just to coalesce that into a few quick instructions: always pre-heat your mypressi TWIST. Use freshly boiling water, fill to the brim, leave for 20 seconds, empty, and repeat. Then empty the bowl and fill with the water for your shot. If you want to go superhot, just do the pre-heat one more time.
The easiest way to do all of this is by sitting the waterbowl on the handle, filling and emptying it using the handle to control the process.
There is no need to pre-heat the handle by soaking it in boiling water as the handle does not add any thermal energy to the extraction.