The previous blog post discussed tamping pressure. We learned that the absolute pressure is almost irrelevant as long as it remains consistent from shot to shot. The firstshot tamper and other dynamometric tampers guarantee this through a pressure regulation system. With the firstshot tamper, the pressure can be adjusted to personal taste or preference by changing the spring. If the pressure is of lesser importance, what is crucial when tamping? What determines whether the espresso flows evenly or is subject to channeling from the bottomless portafilter?
The answer is simple: it depends on the pressure distribution during tamping. The pressure distribution during tamping influences the density distribution of the coffee puck. If the pressure is greater on one side of the coffee puck than the other, it will be more compressed on that side. This results in a coffee puck with uneven density. When the water in the machine then encounters the espresso puck, it simply takes the path of least resistance and flows through the coffee puck unevenly. The water distributes itself evenly at a constant pressure in the portafilter, but flows more esily through the less compressed part of the coffee puck. In the best-case scenario, some of the espresso grounds are over-extracted, while another part barely passes through and is therefore under-extracted. In the worst-case scenario, the water creates a channel in the less compressed coffee grounds and flows unimpeded through this channel, bypassing the coffee grounds – a phenomenon known as channeling. This is visualized in the following diagrams (Figs. 1-3).
Fig. 1: Uniform flow rate with a uniformly compressed espresso puck
Fig. 2: Uneven flow rate due to uneven pressure distribution during tamping
Fig. 3: Channeling due to uneven pressure distribution during tamping
But how does uneven pressure distribution occur during tamping, and how can it be effectively prevented? The most important cause is tilting the tamper base during tamping. This is illustrated in Figures 4 and 5. To prevent this, there are various recommendations regarding hand and arm positioning, as well as visual or tactile control of the tamper base's alignment with the portafilter basket's edge, for example, by placing the fingertips on the basket's rim. These approaches require a great deal of practice to master and remain quite prone to error. The firstshot tamper offers a much simpler and more precise solution. It features a mechanism that attaches to the portafilter and guides the tamper base precisely during tamping. Furthermore, the spherical handle decouples the tamper's movement from the hand position. This guarantees 100% level tamping and ensures even compaction of the espresso grounds.
Fig. 4: Even pressure distribution during tamping
Fig. 5: Uneven pressure distribution during tamping
A second cause of uneven pressure distribution when tamping is the use of a tamper that is too small for the portafilter basket. Some tampers have a smaller diameter compared to the basket. This creates a gap at the edge between the tamper base and the basket, where no pressure is applied to the espresso grounds. This can lead to the espresso puck not being properly compressed at the edges. The base of the firstshot tamper, at 58.4 mm, is slightly larger than the nominal size of a 58 mm basket. This minimizes the gap around the edge in most baskets and results in more even compaction at the edges. While ensuring compatibility with most baskets.
The third reason lies in the even distribution of the espresso grounds before tamping. These effects will be discussed in a separate blog post.
More information on the firstshot tamper and visit the firstshot tamper in the shop!