ResearchPhysics of Fluids· Jun 23, 2026
A University of Warsaw soft-matter group (Waszkiewicz, Lisicki and colleagues) instrumented a café-grade espresso machine and measured flow through pucks across a range of pump pressures. Up to about 5 bar the bed behaved like an ordinary porous medium (flow proportional to pressure), but above that the flow rate flattened and then fell as pressure rose: once the soluble fraction has dissolved (roughly 30-40 s in), the remaining bed is a poroelastic solid that compacts under load, and its permeability drops faster than the extra pressure can compensate. A minimal model reproduces the non-linear pressure-flow curve and the time evolution of flow during a shot. Caveat: one light-to-medium roast Brazilian coffee, one machine setup; the paper models flow and soluble concentration, not sensory outcome.
Why it matters: If your shots choke or run slow, cranking pressure above 6 bar is unlikely to help and may make the bed compact further; grind, dose and lower pressure profiles are the levers, which supports the trend toward 6-bar and declining-pressure espresso.
ResearchJournal of Food Engineering· Jun 1, 2026
Francisco Trujillo's group at UNSW Sydney turned a portafilter basket into an ultrasonic sonoreactor, coupling high-intensity ultrasound directly into a loosely packed coffee bed while water percolates through it. Cavitation bubbles collapsing against the grounds pit and fracture particle surfaces, so espresso-strength concentrations are reached in 2-3 minutes without heat or 9 bar; the paper reports the ultrasonic brew used 24.3% of the energy of a conventional espresso machine (the university's press material rounds this to 'up to 75% less'). Brew ratio, grind size and sonication time were tuned, and beverages were characterised for colour, pH, caffeine and chlorogenic acids. In a blind test, 100 regular coffee drinkers could not distinguish the room-temperature ultrasonic shot from a conventional espresso, and an ultrasound-brewed filter coffee was preferred on bitterness. Caveats: a lab prototype with no consumer device; untrained consumer panel rather than descriptive analysis; the energy comparison depends on the reference machine's standby and heating losses.
Why it matters: Nothing to buy yet, but it is the clearest demonstration so far that heat and pressure are means, not ends: strength and flavour compounds can be pulled out of coffee at room temperature if you supply the mechanical energy another way, which reframes how we think about cold and low-temperature extraction.
ResearchJournal of Food Process Engineering
Mahima Bora and Heiko Briesen at the Technical University of Munich measured laser-diffraction particle size distributions of coffee ground on different grinders at several grind levels and from beans of different roast degrees, then compared ten two-component (five-parameter) distribution models for how well they describe the characteristic fines-plus-coarse bimodal shape. Lognormal/Weibull, Weibull/Betaprime and Gamma/Weibull fitted best across the dataset; fit quality depended strongly on grinder type and grind level and only minimally on roast degree. The point is methodological: reducing a full PSD to five parameters lets future extraction and sensory models use the whole distribution instead of a single median particle size. Caveat: no brewing or sensory measurements; the study characterises the powder, not the cup.
Why it matters: Nothing to change today, but it reinforces that a grinder's 'setting' is a poor description of what water sees; the fines-to-coarse shape differs by grinder even at the same median, which is why two grinders at the 'same' size brew differently.