Temperature in the grouphead affects the extraction of the coffee. In general, if the water temperature inside the grouphead is too hot, the shot will be over-extracted and bitter. If it’s too cold, the shot will be under-extracted and sour. Also, too much heat can also lead to choking, when the puck over-expands and the pull becomes to difficult to continue.
If pressed through choking, I think, it can cause channeling and thus under-extraction.
The theory behind it is that according to the Clausius-Clapeyron relation1, the water temperature is going to be related to the pressure is exposed at in the boiler. When you have 1.0 bar in the LP boiler (actually 2, 1 bar plus sea level pressure), the boiling point of the water becomes 120°C instead of 100°C at 1 bar. So the water in the boiler is superheated, as soon as you lift the piston and pre-infuse the puck, the temperature immediately drops. During the transition phase, the temperature differential between the temperature of the superheated water and the temperature of the grouphead is going to be critical. If the water temperature is not optimal it may effect extraction.
Moreover, superheated water will extract more rapidly and efficiently than lower temperature water. The reason for this is because superheated water behaves more like a water/methanol mixture, since disruption of hydrogen bonding at temperatures above 100°C 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 pre-infusion 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.
Outside grouphead temperature, read for example with Liquid Crystal strips adhered to the group head, can help controlling the grouphead temperature variable, while the boiler pressure gauge permits you to control the pre-infusion water temperature. Empirically, I have found that a grouphead external temperature of 90°C is optimal for my pre-millennium La Pavoni (although some claim 85°C is better, while others like to pull at a higher temperature of 92-95°C). The pre-millennium model’s grouphead tends to run hotter than the post-millennium La Pavoni, 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 90°C, 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 < 90°C, before engaging the portafilter, lift the lever half-way just before water begins to flow through the grouphead until the temperature reads 90°C. However, after the first shot is pulled, the grouphead temperature will shoot up to around 100°C. 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 90°C.
Most of the information here reported here are from a document posted by an user on the La Pavoni Lever Machine Owners Facebook group. I’ve re-organized them, summarized them and fact-checked them to be useful to me.
Footnotes
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The Clausius–Clapeyron relation describes how the boiling point of a liquid increases with increasing pressure. In simple terms, as pressure rises, more heat (higher temperature) is required for the liquid to boil. ↩