Time Is the Symptom, Not the Disease
2025-10-23 • Jesús E. Rodríguez López
Rethinking the Role of Contact Time in Coffee Extraction
In modern barismo, few elements are measured with such precision as time; thirty seconds for an espresso, four minutes for a French press—everything seems to revolve around the timer, as if time itself were the exact boundary between a perfect cup and a disastrous one. But let’s pause for a moment—what if I told you that time isn’t the main character, not even the supporting actor? Maybe it’s just the visible layer of something deeper, a kind of reflection of what’s happening inside the coffee bed. What if those seconds don’t dictate flavor at all, but only reveal the effects of a physical imbalance that was already there? Let me explain.
The Scientific Base: Dissolution and Diffusion
Every extraction can be understood as the interaction between two inseparable processes: dissolution and diffusion. First comes dissolution—the water (our solvent) comes into contact with the ground coffee particles, breaking bonds and releasing soluble compounds. Then diffusion takes place, the movement of those dissolved molecules from inside the particle toward the surrounding water. This flow occurs due to a difference in concentration, following what Fick’s Law describes. In other words, while dissolution happens on the surface, diffusion carries the flavors from the inside out. They move together, which means that any change in one directly affects the other.
Solvent Saturation and Gradient Renewal
As the water fills with solutes, its ability to keep dissolving decreases—it becomes saturated. When that happens, diffusion slows down. If the water flow is continuous (as in a pour-over or espresso), the concentration gradient is constantly renewed, allowing the process to remain active. That’s why brewing methods with steady flow tend to produce more stable extractions. It’s not magic—it’s applied physics. The incoming water flushes away already dissolved compounds, restoring balance and giving new molecules a chance to leave the coffee.
The Disease and the Symptom
When a coffee “runs fast” (underextracted, sour taste) or “takes too long” (overextracted, bitter taste), the clock isn’t the problem—it’s just the sign that something inside the system has lost its balance. Time only points to the symptom of a deeper issue, which almost always has to do with grind size, resistance, or the energy applied.
a) Grind Size and Resistance
A finer grind increases the surface area of contact between water and coffee, but it also increases resistance to water flow. If that resistance exceeds the system’s capacity, the flow slows down and the time stretches out. Not because the coffee is “extracting more,” but because equilibrium has been disrupted. Now, picture something simple: two glasses of water—one has a single large ice cube, the other has the exact same amount of ice, but divided into five smaller cubes. Which glass do you think has the greater contact surface between water and ice? If you thought of the second one, you’re right. That means we’re beginning to understand how grind size affects surface area and extraction.
b) Flow and Gradient
Water flow acts as a regulator—if it’s too low, the area around the particles becomes saturated and diffusion stagnates. If it’s too high, water passes so quickly that it doesn’t dissolve efficiently. In both cases, the time changes, but for different reasons.
c) Solvent Energy
Water temperature determines the kinetic energy with which molecules move: at lower temperatures, dissolution slows down; at very high ones, sensitive compounds degrade. So yes, time changes—but only as a consequence of altered chemical velocity, not as a direct cause of flavor.
Time as a Diagnostic Variable
In reality, time isn’t a control tool—it’s an interpretive one. It helps us read what’s happening inside the system, not direct it. It’s a diagnostic variable, useful only if we understand what it’s telling us. The most precise baristas don’t depend on the timer—they use it as a mirror. Their attention is on how the flow behaves, on the texture of the bed, on the water temperature, on the consistency of flavor. Time only confirms what experience already perceives.
Conclusion: The Physics Behind Flavor
Barismo has turned time into a symbol of control. How many of us feel reassured watching the timer tick according to the “right” parameters—feeling that we’re mastering the process—when the true source of flavor lies in the invisible: in friction, in gradient, in diffusion, in solvent saturation. Time doesn’t extract—it only measures the consequence of the balance (or imbalance) between Resistance and Diffusion within our system. Understanding that changes everything: we stop following recipes and start diagnosing systems. Because in the end, time is the symptom, not the disease. And a good barista doesn’t treat symptoms—they adjust physical causes until they reach sensory balance.
