A new physics study shows that higher espresso pressure can worsen channeling and slow extraction, challenging the assumption that more pressure always means faster, better espresso.
Why More Espresso Pressure Doesn't Always Mean Faster Extraction — What a New Physics Study Found
Higher espresso pressure does not reliably produce faster extraction — and a newly published physics study explains precisely why, tracing the mechanism to a phenomenon called channeling that most home baristas underestimate and most espresso machines are not designed to counteract.
The finding upends a piece of received wisdom that has shaped espresso machine engineering for decades: that the 9-bar standard, codified by Italian espresso tradition, represents a kind of universal optimum. Research covered by Daily Coffee News shows the relationship between pressure and extraction rate is far more complicated — and in some conditions, pushing harder on the puck actively works against you.
Before diving into the mechanics, it helps to have a reference frame for how pressure, grind, and extraction time interact across common espresso scenarios. The table below summarizes the key variables the study's findings touch on:
| Variable | Traditional 9-bar Espresso | Low-Pressure (6–7 bar) Espresso | High-Pressure (>10 bar) Espresso |
|---|---|---|---|
| Typical extraction time | 25–30 seconds | 30–40 seconds | 18–28 seconds (highly variable) |
| Channeling risk | Moderate | Lower | Higher — pressure amplifies existing voids |
| Flow rate consistency | Moderate | More consistent | Erratic when channeling occurs |
| Grind sensitivity | High | Moderate | Very high |
| Flavor outcome | Balanced (when dialed in) | Sweeter, less bitter | Uneven — over- and under-extracted simultaneously |
| Recommended dose range | 18–21 g (double) | 18–22 g | Requires very tight puck prep |
One pattern is immediately visible: higher pressure is not a simple amplifier of extraction speed. It is a variable that interacts with grind uniformity, dose, and puck integrity in ways that can produce the opposite of the intended effect.
What exactly is channeling, and why does it matter so much?
Channeling is the formation of preferential flow paths through a coffee puck — narrow corridors where water moves far faster than through the surrounding bed of grounds. Rather than percolating evenly through every particle, pressurized water finds the path of least resistance and exploits it, often within the first few seconds of a shot.
The consequences are asymmetric extraction: grounds adjacent to a channel are over-extracted (bitter, harsh) while grounds farther away remain under-extracted (sour, thin). The shot's average extraction yield may look acceptable on paper, but the flavor profile is muddled because the two extremes are blended together in the cup.
What the new physics study contributes is a mechanistic explanation for why channeling intensifies at higher pressures rather than being washed away by brute force. As reported by Daily Coffee News, espresso grounds behave in ways consistent with a self-regulating flow system: when a channel forms, the localized pressure differential across that channel increases, which can cause the surrounding puck to compact further, narrowing the effective flow area and partially offsetting the higher input pressure. The net result is that the machine works harder without delivering meaningfully faster extraction — and in some configurations, the shot actually slows down.
This is counterintuitive because everyday intuition about pressure and flow comes from simple pipe systems, where doubling the pressure roughly doubles the flow rate (Hagen-Poiseuille's law). Coffee pucks are not pipes. They are compressible, heterogeneous, and dynamic — they change shape and permeability in real time as water moves through them.
How does the physics of a coffee puck differ from a simple fluid system?
In fluid-dynamics terms, a coffee puck is a deformable porous medium — a bed of solid particles with interconnected void spaces through which a fluid percolates under pressure. This puts espresso extraction in the same analytical family as oil recovery from sandstone, water filtration through soil, and airflow through a packed catalyst bed. The governing framework is Darcy's Law, which relates flow rate to pressure gradient, fluid viscosity, and the permeability of the medium.
The critical word is permeability. In a rigid porous medium, permeability is fixed — it's a property of the geometry. In a coffee puck, permeability is not fixed. It changes as:
- Grounds swell when they absorb water (CO₂ degassing also plays a role, especially in freshly roasted coffee)
- The puck compresses under applied pressure, reducing void space
- Fines — the very small particles produced by any grinder — migrate through the puck and lodge in pore throats, reducing permeability further
- Channels erode their walls, generating more fines that then redistribute
The study highlights that this dynamic permeability is the root cause of flow regulation. When you increase pump pressure, you simultaneously increase the driving force for flow and the compressive force on the puck. These two effects partially cancel each other. The net increase in flow rate is smaller than a naive pressure-flow calculation would predict — and if channeling is already present, the compaction around the channel can drop the effective permeability of the bulk puck enough that total flow rate actually decreases.
This is what the study means by "flow regulation" — not a deliberate engineering feature, but an emergent property of the puck's physics that acts like a natural pressure regulator, dampening the effect of changes in input pressure.
What did the study actually measure or model?
The study approached espresso extraction as a physics problem rather than a culinary one, which is what makes its findings generalizable beyond any single machine or recipe. While the full methodology warrants reading the primary research, the Daily Coffee News report indicates the researchers examined how flow rate responds to pressure changes in the presence of channeling, and found that the puck's self-regulating behavior is robust — it persists across a range of conditions rather than being an edge case.
The key quantitative insight is that the relationship between pressure and extraction rate is nonlinear and, in channeling-prone conditions, can have a negative slope in certain pressure ranges. That means there is a pressure threshold above which pushing harder produces diminishing returns or outright slower shots. Where exactly that threshold sits depends on grind size distribution, dose, tamp force, and puck preparation — which is why the finding doesn't translate to a single universal "correct" pressure number.
This is an important epistemic point: the study does not say 9 bar is wrong, or that 6 bar is right. It says pressure alone is an insufficient lever for controlling extraction, and that the internal state of the puck — particularly whether channeling is occurring — determines how pressure translates into flow.
Does this mean the 9-bar standard is obsolete?
The 9-bar standard is the pump pressure at which water is delivered to the group head in traditional Italian espresso preparation, a figure that became an industry norm through the work of manufacturers like Faema in the 1960s and was later codified by organizations including the Specialty Coffee Association. It was derived empirically — shots at roughly 9 bar tasted good to the people who set the standard — rather than from first-principles fluid dynamics.
The new study does not invalidate 9 bar as a practical starting point. What it does is explain why some baristas and machine designers have found success at lower pressures (6–7 bar is common in the specialty coffee world, and some competition baristas use pressure profiling that starts high and ramps down). The physics suggests that lower pressure may reduce the compaction effect on the puck, maintain more consistent permeability, and produce more even extraction — particularly with lighter roasts, which tend to be denser and more prone to channeling than darker roasts.
Pressure profiling — where the machine varies pressure throughout the shot rather than holding it constant — is the engineering response to exactly this problem. Machines like the La Marzocco Linea Micra and the Decent Espresso DE1 allow baristas to program pressure curves. The physics study provides a theoretical foundation for why a declining pressure profile (high at the start to wet the puck, then lower during extraction) might outperform a flat 9-bar profile: the initial high pressure saturates the puck before channeling can establish, and the subsequent pressure drop reduces compaction and channel amplification.
That said, the study's findings are most relevant to understanding why things go wrong, not as a prescription for a specific pressure number. Grind quality, distribution technique, and water temperature remain at least as important as pressure in determining extraction quality.
Why do some shots run faster at lower pressure — and is that a good thing?
This is where the study's findings become practically useful for home baristas. Conventional intuition says a faster shot means higher pressure or coarser grind. But if channeling is the dominant variable, a faster shot at high pressure may simply mean the water found a large channel and bypassed most of the puck — producing a thin, under-extracted, watery result despite the short time.
A faster shot at lower pressure, by contrast, might indicate that the puck's permeability is genuinely higher (perhaps due to a slightly coarser grind or lower dose) and that flow is more uniform. The extraction yield in the cup could be similar or even higher because more of the puck is contributing to the shot.
The research shows that shot time is a proxy variable, not a direct measure of extraction quality. It correlates with extraction under controlled conditions, but when channeling is present, that correlation breaks down. A 25-second shot with significant channeling can have a lower extraction yield than a 35-second shot with uniform flow, even though the former "looks right" by the clock.
This has practical implications for diagnosing problems. If a shot runs fast and tastes sour or thin, the instinct is often to grind finer — which increases resistance and slows the shot. But if the fast shot is caused by channeling rather than coarse grind, grinding finer may not fix the channel; it may just increase overall resistance while the channel persists, producing a slower but still unevenly extracted shot. The fix in that case is puck preparation: better distribution, a leveled tamp, or a distribution tool that breaks up clumps before tamping.
What does this mean for home espresso machine buyers?
For anyone shopping for an espresso machine, the study's findings reframe what "better pressure" means as a marketing claim. Many entry-level machines advertise 15-bar pumps — a figure that sounds impressive but is largely irrelevant, since the pump's maximum pressure is not the pressure actually delivered to the puck (a pressure regulator or OPV, over-pressure valve, typically limits it to 9 bar or thereabouts). More importantly, the study suggests that the consistency and controllability of pressure delivery matters far more than the peak number.
Features worth prioritizing, in light of the physics:
A pre-infusion stage — where the puck is wetted at low pressure before full pressure is applied — directly addresses the channeling problem by allowing grounds to swell and seal potential void spaces before high-pressure flow begins. This is the process of saturating the coffee puck at sub-extraction pressure (typically 2–4 bar) for 5–15 seconds before ramping to full brew pressure. Many mid-range machines now include this as a fixed feature; higher-end machines allow the duration and pressure to be adjusted.
Pressure profiling capability allows the barista to dial in a pressure curve matched to the coffee's density and roast level. This is currently a feature of prosumer machines in the $1,500–$4,000 range, though the technology is gradually moving downmarket.
Grinder quality matters more than machine pressure precision for most home setups. A grinder that produces a narrow particle size distribution reduces the population of fines that migrate through the puck and cause channeling. Burr geometry, burr size, and RPM all affect the distribution. This is why the specialty coffee community consistently recommends spending more on the grinder than the machine when budget is limited.
How does this connect to the broader science of coffee extraction?
Espresso is one of the most physically complex brewing methods in common use. Water at 90–96°C is forced through a 20-gram bed of ground coffee at 9 bar of pressure in under 30 seconds, dissolving hundreds of chemical compounds at different rates depending on particle size, temperature, and contact time. The extraction is simultaneously a fluid dynamics problem, a heat transfer problem, and a mass transfer problem — and all three interact.
The new study adds to a growing body of physics and engineering research on espresso that has accelerated over the past decade. Earlier landmark work includes a 2020 study in Matter by researchers at the University of Huddersfield and University of Oregon, which found that using less coffee ground more coarsely could produce more consistent and sometimes higher-extraction shots — a result that also challenged conventional wisdom and was similarly rooted in the fluid dynamics of puck permeability.
What these studies collectively suggest is that espresso optimization is not a matter of finding the single correct parameter set, but of understanding how the variables interact as a system. Pressure, grind, dose, temperature, and puck preparation are coupled — changing one changes the effect of all the others. The channeling study contributes the specific insight that pressure's effect on flow rate is mediated by the puck's internal state in a way that can be self-regulating and even self-defeating at high pressures.
For the wellness-minded coffee drinker, there is also a practical angle here. Channeling doesn't just affect flavor — it affects the consistency of caffeine extraction. A channeled shot may deliver significantly less caffeine than a well-extracted shot from the same dose, because a large fraction of the grounds never fully contacted the water. If you're calibrating your caffeine intake — for sleep, anxiety management, or general health reasons — shot-to-shot consistency matters. Better puck preparation and appropriate pressure are part of achieving that consistency.
If you're exploring lower-caffeine or caffeine-free alternatives for evenings or sensitive days, our guides to caffeine-free Asian herbal teas for sleep and herbal teas for digestion offer well-researched options that sidestep the extraction complexity of espresso entirely.
What should baristas actually do differently based on this research?
The study's practical implications cluster around puck preparation and pressure management rather than any single number to dial in.
Invest in distribution before tamping. Channeling most often originates from uneven coffee distribution in the basket — clumps, voids, or density gradients that give water a preferential path before pressure is even applied. A distribution tool (sometimes called a WDT tool, for Weiss Distribution Technique) that breaks up clumps and homogenizes the bed before tamping is one of the highest-leverage interventions available at any price point.
Use pre-infusion if your machine supports it. The physics of puck swelling during pre-infusion directly counteracts the void spaces that seed channels. Even a 5-second pre-infusion at 2–3 bar can measurably improve shot consistency.
Consider whether your pressure is actually 9 bar at the puck. Many machines, especially entry-level ones, deliver pressure inconsistently or have OPV settings that drift over time. A portafilter-mounted pressure gauge (available for under $30) can reveal whether your machine is actually operating where you think it is. Some machines run at 11–12 bar at the puck despite nominal 9-bar claims — which the study's findings suggest could be actively counterproductive.
Treat shot time as a diagnostic, not a target. If a shot runs in 25 seconds but tastes sour and thin, the problem is likely channeling, not grind size. Look at the bottom of the portafilter basket after the shot — uneven saturation patterns or a dry spot in the puck are visual evidence of channeling.
Be skeptical of pressure as a quality signal when buying machines. A 15-bar pump rating is a specification of the pump's maximum capability, not the pressure delivered to the coffee. What matters is whether the machine has a well-calibrated OPV, a pre-infusion stage, and — for serious home baristas — some form of pressure profiling.
What are the limits of this research?
Intellectual honesty requires noting where the data is thin. The study, as reported, is a physics modeling and analysis paper — it characterizes the fluid dynamics of channeling and flow regulation in theoretical and experimental terms, but it does not conduct large-scale sensory trials correlating pressure profiles with cup quality across many coffees and baristas. The gap between "this is what the physics predicts" and "this is what you will taste in your cup" is real, and filling it requires the kind of messy, variable-rich sensory science that is harder to publish and harder to generalize.
The findings are also most directly applicable to traditional espresso — a 1:2 ratio, 20-gram dose, 40-gram yield, roughly 25–30 seconds. The specialty coffee world has expanded well beyond this into lungo, ristretto, and various "bypass" and "turbo" shot formats that operate under different pressure and flow regimes. How the channeling-flow-regulation dynamic plays out in, say, a 1:4 turbo shot at 6 bar is an open question.
The study also addresses pressure as the primary variable. In real espresso, temperature is equally dynamic — the puck temperature changes throughout the shot, affecting both extraction kinetics and the viscosity of the water, which in turn affects flow rate. A complete model of espresso extraction would need to couple the fluid dynamics findings with heat transfer, and that work is ongoing in the research community.
What the study firmly establishes is that the simple mental model — more pressure equals faster, more complete extraction — is wrong in a specific and mechanistically explained way. That is a meaningful contribution, even if it raises as many questions as it answers.
For coffee drinkers whose interest in brewing intersects with health and wellness, the takeaway is this: the quality and consistency of your espresso depends less on the pressure number your machine advertises and more on the uniformity of your puck preparation and the stability of your process. The physics, it turns out, rewards care and consistency over raw force.