Given my long history at Macquarie Bank some of you might be a little puzzled about the subject line of this email. No, I won’t be discussing the quirky interactions I had with some of the most senior members of the Macquarie network! Instead I’ll be discussing the characteristics of the coffee machine group head - the junction point between the machine and the portafilter holding the ground coffee which will magically be extracted to become espresso elixir.
The main characteristic of the group head that I wanted to discuss is that of thermal management (honestly, how many of you thought that group head temperature management would be the subject of today’s newsletter?). It will be useful to consider the E61 group head as a starting point.
The E61 was invented in 1961 and was named after the total solar eclipse which occurred that year. It is composed of a large amount of metal which provides a heap of thermal mass which is exposed to the external environment and therefore acts as a heat sink, slowly bleeding heat away to the external environment. A thermosyphon effectively moves water from the brew chamber through the group head and keeps the group head at optimum temperature. Given the heat sink characteristics it’s clear that the brew chamber water is at a higher temperature than the target group head temperature… and herein lies the potential problem. If you were to extract a lot of shots of coffee close together, then the temperature of your E61 group head would rise. On a more granular level, even through the process of extracting a shot, the brew temperature will rise slightly as the extraction progresses. This is generally not a problem as E61 group heads have so much thermal mass and most of us (thankfully) do not have the tasting acuity to sense the difference in any case nor do we make enough coffee that group head temperature would be an issue.
Of course the coffee industry caters to those of us who are total control freaks and they do it via the use of the “gruppo saturo”, Italian for saturated group head (everything sounds better in Italian). The saturated group head is designed such that water from the brew chamber surrounds the group head allowing for the whole system to be controllable and consistent.
You won’t be surprised to hear that saturated group machines tend to be more expensive! Do you use a saturated group coffee machine?