THE PROCESS IN MOTION

What does ground coffee give up to water?

Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.

▶ Play the process01The current carries off the surface02Diffusion, from the core to the water03A ceiling: 30 to 32%04Lockhart's window05An average that hides everything

The current carries off the surface

As soon as water touches a coffee particle, it carries off what is already dissolved at its surface: this is advection, transport by the current. What is taken from the bean this way is called the extraction yield, the dissolved mass divided by the dry dose. The concentration of the beverage, written TDS, also depends on the quantity of water.

Read the stages without animation

1. The current carries off the surfaceAs soon as water touches a coffee particle, it carries off what is already dissolved at its surface: this is advection, transport by the current. What is taken from the bean this way is called the extraction yield, the dissolved mass divided by the dry dose. The concentration of the beverage, written TDS, also depends on the quantity of water.
2. Diffusion, from the core to the waterThe rest must first travel from the core of the particle to its surface: this is diffusion, slow, and faster the hotter the water and the smaller the particle. Not all compounds move at the same pace: caffeine and trigonelline come out quickly, even from large particles. A fine, made of broken cells, gives everything at once.
3. A ceiling: 30 to 32%A roasted bean yields only 30 to 32% of its mass to water. The rest, cell-wall cellulose first of all, does not dissolve, and the oils stay in the large particles. The spent grounds also keep water, more than twice their weight: that is why the yield is calculated on the weight of beverage, not on the water poured.
4. Lockhart's windowIn 1957, Ernest E. Lockhart crosses yield, concentration and the preferences of a panel: 18 to 22% yield, 1.15 to 1.35% concentration. Below that, the cup tastes of grass and peanut; above it, bitterness and astringency. The SCA 310-2021 standard turns this into three instructions: 55 g of coffee per liter, water at 93 ± 3 °C, 4 to 8 minutes of contact.
5. An average that hides everythingTwo cups with the same average yield can differ completely: one comes from a uniform extraction at 20%, the other from a mixture of exhausted cells at 28% and barely touched cells at 15%. The Davis diagram, published in December 2024, replaces Lockhart's square with more than a hundred descriptors: it says what the cup is like.

To remember: What water takes from coffee is limited to 30 to 32% of its mass; the grind, the heat and the uniformity decide the share it takes, and its taste.

In history: In 1957, Lockhart directed the Coffee Brewing Institute; the SCA made his window the Golden Cup Standard.

Read the full article and its sources ↓

A roasted bean yields only 30 to 32% of its mass to water; the rest, cell-wall cellulose first of all, is insoluble. This ceiling limits every preparation. What one takes of it is called the extraction yield (dissolved mass divided by the dry dose); the concentration of the beverage (TDS, total dissolved solids, in % of dissolved solids) also depends on the quantity of water.

Two cups with the same average yield can differ completely if one results from a uniform extraction at 20% and the other from a mixture of exhausted cells at 28% and barely touched cells at 15%.

Gagné 2021, pp. 1-4 (de Maria et al. 1994; Ellero and Navarini 2019)

Yield and concentration are two laboratory measurements, and neither figure says whether the cup is good. In 1957, Ernest E. Lockhart, first director of the Coffee Brewing Institute, made the link that was missing. He crossed these two quantities with the preferences of an American panel, in The Soluble Solids in Beverage Coffee as an Index to Cup Quality.

The panel draws a window. Yield of 18 to 22%, concentration of 1.15 to 1.35%. The European grid, that of the European Coffee Brewing Centre (ECBC), keeps the same yield but places the concentration higher, from 1.30 to 1.55%, according to the joint SCA and ECBC procedure on extraction uniformity. What falls outside has a recognizable taste, and that is where the measurement becomes useful.

Below 18%, not enough has been taken from the bean: grass and peanut. Above 22%, what should have been left has been taken, hence the bitterness and the astringency.

The SCA made this window the Golden Cup Standard. It then translated it into verifiable settings in standard 310-2021, the one that certifies home coffee makers. One does not certify a taste, one certifies instructions, and there are three. 55 g of coffee per liter of water, to within ± 10%. Water at 93 ± 3 °C at the moment it touches the ground coffee. And 4 to 8 minutes of contact.

The window, for its part, has moved. Gagné notes that with light roasts and recent grinders, preference has shifted upward. Less than 19% rarely passes for good, and a few professionals claim more than 22%. Lockhart's lower bound has become a floor, and his upper bound is no longer a prohibition.

SCA, Towards a New Brewing Chart (25, issue 13); SCA Standard 310-2021, Home Coffee Brewers (PDF hosted by CoffeeGeek, Oct. 2023); Gagné 2021, pp. 2-3 (Lingle 1996; Rao 2014, 2016, 2018)

Lockhart's diagram has been redone, and what changes lies less in the figures than in the question asked. The “ideal” square answered yes or no. The new diagram says what the cup is going to be like.

The work began in 2017 at the Coffee Center of the University of California, Davis, around Ristenpart and Guinard. It was funded by Breville, with the SCA and the Coffee Science Foundation, and lasted seven years. The new Coffee Brewing Control Chart appeared in the Journal of Food Science on December 18, 2024.

It exists in four versions. In place of the single square, more than a hundred sensory descriptors cover the plane: one reads what a point yields, and not only whether it is allowed. Brew ratio lines cross it diagonally. The relationship between dose and water can therefore be read off the diagram, instead of being calculated separately.

The main conclusion lies elsewhere. The “good coffee” is not a single thing: it depends on the group of consumers surveyed. The authors themselves acknowledge the limit of their measurement. The tasting panels had twelve volunteers, and the preference tests were carried out mostly on Davis students.

Fresh Cup, Why the New Coffee Brewing Control Chart is a Big Deal (2025); UC Davis College of Engineering, press release; Guinard et al., J. Food Science, 2024

The physics of extraction comes down to two words. Advection carries off with the current what is already dissolved at the surface of the particles; diffusion, slow, makes the solubles migrate from the core of the particle to its surface, the faster as the water is hot (the diffusion coefficient grows with temperature, Einstein-Smoluchowski relation) and the particle small. Compounds do not diffuse at the same speed: caffeine and trigonelline come out quickly, even from large particles; the oils stay in the large ones.

The fines, the smallest particles, made of broken cells, give everything at once. Changing the grind therefore changes the aromatic profile even at equal yield, and not only the flow time.

Gagné 2021, pp. 5-11 (Moroney et al. 2019; Smith 2015)

Immersion and percolation are not equivalent. In immersion, without agitation, only diffusion moves the solubles away from the particle; the water loads up, extraction slows by itself: the method forgives timing errors, extracts less efficiently, lets fines through and gives more body. In percolation, clean water constantly sweeps the particles; the coffee bed filters itself and gives the cleanest cup, but any flow defect has to be paid for.

One detail matters for the calculations: the spent coffee grounds retain more than twice their weight in water (a liquid retention ratio of about 2.2 for a V60 at 1:17), and the yield is calculated from the weight of beverage, not from the weight of water poured.

Gagné 2021, pp. 12-17