I have been trying to improve the control over the extraction parameters of my Gaggia Color after reading a lot around the net about making better espresso. In preparation for an Arduino-based control add-on, I have spend the weekend taking temperature measurements for different scenarios. (A thermocouple was attached right below the filter basket inside the portafilter and a Ad22100 sensor awaited the coffee in the cup.) More about the results further down, but first a general question:

Question - For all "reasonable" brewing protocols the temperature in the portafilter (right were the extracted coffee comes out of the filter basket) hovers between 80C-85C (176F-185F). That is a fat 10C below every recommendation I was able to find. The Gaggia spec talkes about 177F, so I guess this is not a defect, but intended. At the same time the water pressure is 15bar (spec, not measured), while everybody talks about 9bar as being recommended. Does higher pressure allow to work with a lower temperature? How does that effect the expectable espresso quality?

Insights - Here are a few things I learned from my experiments:

1. Absolutely do prime the machine by drawing a double shot without coffee in the filter, even if it had plenty of time to warm up. It takes a full 22sec into the extraction (water coming through the filter) for the temperature to reach 80C. In all subsequent extractions it takes only 2sec to reach the nominal brewing spec again.

2. In some posts about making better Latte I have seen recommendations to reverse the natural order of things: First prepare the frothed milk, then pull the espresso. This way you don't have to wait for the boiler to reach steam temperature while the espresso cools down. That is true, but during the steam extraction through the wand the filter basket reached 98C (208F) for most of the time. When you switch to pulling espresso right after the froth is prepared, the basket temperature is at 95C and drops to 85C (203F-185F) during the double shot extraction. That is both twice the temperature gradient (10C vs. typically 5C) you have in the regular way of operation, and the temperature is overall much higher than desired (well, what means desired is part of my question above).

3. Finally a word about using frozen coffee powder: Don't. I buy very good roast (or so I think) and grind myself (another weekend and a pound of coffee went into adjustig the grinder...). Since grinding is messy and makes a lot of noise, both unappreciated by my wife, I grind once a week and keep the powder in the freezer. Outch, you say. Well, I hear you - now. When I redid the measurements above with a basket loaded with powder fresh out of the freezer I almost fell of my bar chair. The brewing temperature went from a creepy 55C (131F) to 75C(167F) during the full extraction - arctic overall and with a huge spread. The second espresso right after that was the same, just 5C higher overall. No wonder my coffee tastes like dog poop - which is why I started the whole project in the first place