Tipsheet: GH temp control is key!

by Steven Ornish

Monitoring the grouphead temperature will increase the chances of pulling a better shot. In general, if the water temperature inside the grouphead is too hot, the shot will be over-extracted and bitter. If it is too low, the shot will be under-extracted and sour. Too much heat can also lead to choking when the puck over-expands and the pull becomes too difficult to continue. Here is some theory behind it. The temperature of the water in the boiler is going to be related to the boiler pressure (Guy-Lussac’s law), so pulling a shot at a boiler pressure around 0.7-0.9 bars seems to be about right. When the LP reads 1.0 bar, in reality it is actually 2 bars, since sea level is 1.0 bar and we don't live in a vacuum. The boiling point of water at 1.0 bar of atmospheric pressure is 100C, and at 1.0 bar of pressure gauge pressure (2 bars in reality) it is approximately 120C. Therefore, the water in the boiler is superheated, not unlike a pressure cooker. However, as soon as you lift up the lever to pre-infuse the puck, the pressure (and thus temperature) of the superheated water pre-infusing the puck immediately drops, since it is no longer in a closed system. During this transition phase, the temperature differential between the temperature of the superheated water infusing the grouphead, and the temperature of the grouphead just prior to the lever being lifted, is critical. In other words, if the superheated water is too hot, due to the pressure gauge, say, reading 1.5, or if the grouphead is too hot or too cold at rest just prior to lifting the lever, these temperature differentials will not be optimal and will dramatically affect the extraction and the quality of the shot. Why that is seems somewhat counter-intuitive, but there have been scientific studies correlating degree of extraction with temperature, time, and pressure.

Moreover, superheated (120C) water will extract more rapidly and efficiently than non-superheated water that one might use, say, in a pour-over (90C). The reason for this is because superheated water behaves more like a water/methanol mixture, since disruption of hydrogen bonding at temperatures above 100C occurs and allows molecules to diffuse more freely due to a decrease in viscosity and surface tension. Organic molecules (which are what gives coffee its flavor) show a dramatic increase in solubility with increased temperature, in part due to polarity changes, and in part because the increased enthalpy of solution with the increased temperature found in a superheated solution leads to increased solubility of organic molecules.

Extraction using superheated water also tends to be much faster due to increased diffusion rates, so small differentials in boiler temperature and grouphead temperature in the La Pavoni will lead to significant differences in extraction, especially since all of the extraction has to occur over an approximately 30 second period (10 seconds for preinfusion and 20 seconds during the shot pull) as compared to the longer brew times of 1.5-4 minutes one uses with other coffee makers such as an AeroPress, a French press, or a pour-over.

What the LC (liquid crystal) strips adhered to the grouphead permit you to do is to control one of the important variables -- the grouphead temperature, and the boiler pressure gauge permit you to control another important variable -- preinfusion water temperature. Empirically, I have found that a grouphead external temperature of 90C is optimal for my pre-millennium La Pavoni (although some claim 85C is better, while others like to pull at a higher temperature of 92-95C). The Pre-millennium models groupheads tend to run hotter than the post-millennium LPs, which is why the company added a teflon sleeve inside the grouphead in the post-millennium models to better control the temperature. If the grouphead liquid crystal temperature strip is reading 90C, I don't know what the internal temperature of the grouphead actually is, but in some ways it is irrelevant since the liquid crystal external grouphead temperature should well-correlate with internal grouphead temperature multiplied by some unknown constant.

To better control the temperature differential between the superheated water, the grouphead, and the portafilter, I have found I get more consistent results if I preheat the portafilter and basket in a hot water bath prior to tamping and engaging the portafilter in the grouphead (first dry off the basket before adding coffee and tamping).

If the grouphead temperature is < 90C, before engaging the portafilter, lift the lever half-way just before water begins to flow through the grouphead until the temperature reads 90C. However, after the first shot is pulled, the grouphead temperature will shoot up to around 100C. If you want to pull a 2nd shot immediately after the 1st shot, you will need to cool the grouphead down with an ice water bath, and this can be done by swirling a ramekin or bowl of ice water under the grouphead until it again reads 90C. If you attempt to pull the 2nd shot immediately after the 1st shot, without first cooling down the grouphead to 90C, it will often either "choke" due to the increased expansion of the puck from the extra heat from the grouphead, or it will be bitter from over-extraction, or some combination thereof.

Another technique that I more recently have been using which has permitted me to achieve even greater consistency between the first and subsequent shots, which I now prefer over the one described in the above paragraph, is to engage the empty portafilter and lift the lever all the way up, which flushes hot water through the portafilter and preheats the portafilter and basket. I then disengage the portafilter, add coffee to the basket, and tamp. The grouphead temperate at this point is approximately 100-105C. I hold a ramekin of cold tap water with a couple of small ice cubes in it under the grouphead until it reads approximately 90C (gently swirling the ramekin will permit the grouphead to cool down faster). I reengage the portafilter and pull my shot. I clean the grouphead of residual grounds with a brush, back-flush it with the empty portafilter engaged which further flushes out the residual grounds, and repeat the process with a ramekin to cool down the grouphead for the next shot.

Hope how I think about the physics and chemistry of making espresso is useful, and I'm sure others in this group may have their own approaches that work for them.

Created 25-JUL-2017 | updated 03-FEB-2023 | BCP