Tamp lighter or reduce dose even more. Looks like that particular coffee doesn’t want to take water in during pre-infusion with ease, so you start the shot with partially soaked puck.
Grinding coarser could fix it but then it would run too fast on the second half. I would drop the dose even more, and tamp lighter. When I would tamp like crazy like 18kg or more, the puck would not flow from the center, when I dropped to 10 kg or so it went much better. The puck in the 53mm group is much taller than in a 58mm. That and the fact that you have a huge temperature drop within the puck from the top to the bottom because the top puck absorbs a lot of temp from the water hitting the puck in pi (meaning the viscosity of the liquid is decreased massively meaning the water wont flow as easy as from the top) it will result in a low flow/restricted at the bottom center (just because water finds it’s path to the easiest points - at the baskets edge walls).
Dose is related to grinder, as in what type oe grind your grinder does. If your grinder is something like a niche with packed grinds of finer and coarser particles, the temp drop will be higher in the puck vertically, meaning a smaller/not tall dose will suit you better with the increased temp of course. So with a traditional grinder a smaller puck wins in our case I suspect 15-16 g max for 53mm and 17-18g max on 58mm.
With a modern grinder/burr like ssp multi-purpose or ssp 98mm high uniformity or kinda any 98mm burrs, the water penetration is much faster during preinfusion, giving you a more even and faster soak, this means that the temp drop on the vertical of the puck is less of a drop, so you are using lower temps (ssp mp vs niche) you are having a much faster pi exit at the bottom of the basket- because viscosity of fluid wont drop cause temps doesnt drop as much, and then for this type of grinders you are better with a bit higher doses- like 17-18g on 53mm and 19-20g on 58mm.
They did a test on decent espresso with vst baskets (58mm) 15 vs 17 vs 20g same coffee/grinder, doing tasting tests and Ey% taste, with 98mm ssp hu grinder, and the 20g doses always won.
Here is the “research”
https://pocketsciencecoffee.com/2022/09/13/temperature-gradient-across-an-espresso-puck/
Have a great day.
