THE PROCESS IN MOTION

What does Lockhart's diagram measure?

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

▶ Play the process01Two numbers not to be confused02Extract a lot, dilute a lot03The panel zone, 195704Not everything comes out at the same speed05Two diagrams, two audiences

Two numbers not to be confused

The extraction yield is the share of the dry grounds that has dissolved. The concentration, or TDS for total dissolved solids, is the share of the liquid that is dissolved coffee. The 1957 diagram crosses them: each cup becomes a point there.

Read the stages without animation

1. Two numbers not to be confusedThe extraction yield is the share of the dry grounds that has dissolved. The concentration, or TDS for total dissolved solids, is the share of the liquid that is dissolved coffee. The 1957 diagram crosses them: each cup becomes a point there.
2. Extract a lot, dilute a lotThe two numbers vary separately. Adding water changes nothing about what has dissolved, but lowers the concentration: the point goes down. A cup can therefore be well extracted and weak at the same time.
3. The panel zone, 1957Lockhart's ideal zone comes from consumer panels: 18 to 22% yield, 1.15 to 1.35% concentration; the institute held 1.0 to 1.5% to be acceptable. Below 18%, the cup is under-extracted, grassy and peanutty; above 22%, it turns bitter and astringent.
4. Not everything comes out at the same speedAcids and sugars extract quickly, bitter, metallic or astringent compounds at the end: beyond 28%, one would have nearly everything that can dissolve. The average yield hides uniformity: two cups at 20% can be a regular extraction, or a mix of cells at 28 and at 15%, and they do not resemble each other.
5. Two diagrams, two audiencesIn 2023, the team at the UC Davis Coffee Center published not one diagram but two: one sensory, placing the attributes in the plane, the other of preference, for two consumer segments that do not want the same cup. The coffee/water ratio becomes an explicit axis there; espresso, for its part, is drunk at 7 to 12% concentration.

To remember: Yield and concentration are two independent axes; the preferred zone depends on who drinks the cup.

In history: Ernest Earl Lockhart directed the Coffee Brewing Institute, created in the early 1950s.

Read the full article and its sources ↓

What ends up in the cup depends on two numbers that must not be confused. The extraction yield is the share of the dry grounds that has dissolved. The concentration, or TDS for total dissolved solids, is the share of the liquid that is dissolved coffee.

The two vary separately, and that is the whole trap: one can extract a lot and dilute a lot. A cup can therefore be well extracted and weak at the same time.

Ernest Earl Lockhart crossed these two numbers in 1957. A biochemist trained at MIT, he directed the Coffee Brewing Institute, an organization created in the early 1950s by the National Coffee Association and the Pan American Coffee Bureau. His article, The Soluble Solids in Beverage Coffee as an Index to Cup Quality, drew from it the Coffee Brewing Control Chart, a plane in which each cup occupies a point.

• before 1900Coffees judged on the appearance of the bean (Ukers 1922)
• 1917The Santos exchange classifies by the cup
• 1922Ukers describes the cup test: weighed dose, crust, spittoon
• 1957Lockhart, Coffee Brewing Control Chart, 18-22%, 1.15-1.35%
• 1986Lingle, The Coffee Cupper's Handbook
• 1990Spadone et al.: rio is TCA
• 1992Blank, Sen, Grosch: 13 key odorants
• 1995First SCAA flavor wheel
• 1999First Cup of Excellence, Brazil
• 2004SCAA scoresheet and protocol; launch of the Q Grader
• 2007Hofmann and Frank: lactones and phenylindanes
• 2016WCR lexicon (110 attributes) and new wheel
• Apr. 2023CVA in beta version, Portland
• 2023New extraction diagram, UC Davis
• Nov. 5, 2024The CVA replaces the 2004 scoresheet
• Oct. 1, 2025The Q Grader passes under the SCA

sources cited in the text

The ideal zone does not come out of a laboratory but from consumer panels. The preferred filter coffee lies between 18 and 22% extraction and between 1.15 and 1.35% concentration; the institute held 1.0 to 1.5% to be acceptable. Below 18%, the cup is under-extracted, grassy and peanutty. Above 22%, it turns bitter and astringent.

The Norwegian Coffee Association, and later the SCAA, took up the diagram with slight shifts of the ideal zone.

Lockhart, Coffee Brewing Institute Publication (1957); Lingle, The Coffee Brewing Handbook, SCAA (1996); Gagné, The Physics of Filter Coffee (2021), p. 2-3; Fresh Cup, Why the New Coffee Brewing Control Chart is a Big Deal (trade press)

Gagné recalls why the average yield tells something about the profile: compounds do not extract at the same speed, acids and sugars quickly, the slow compounds, bitter, metallic or astringent, at the end, so that beyond 28% one would have nearly everything that can dissolve, and around 15% one would lack sweetness and complexity. He adds three nuances.

The average yield hides uniformity: two cups at 20% can be one a regular extraction of all the cells, the other a mix of cells at 28 and at 15%, and they do not resemble each other. The preferred zone has slid upward with today's lighter roasts, grinders and green coffees, with yields below 19% rarely described as preferred and some going beyond 22%, a figure that itself rarely exceeds 23.5%. And concentration is a matter of intensity, hence of personal taste: espresso is drunk between 7 and 12% at similar yields.

Rao noted a local optimum of preference around 15%, a hated zone between 15 and 18%, then a rise. The refractometer, which deduces TDS from the refractive index, has put measurement on every tasting table, including in training protocols.

Gagné 2021, p. 3-5; Rao, The Coffee Roaster's Companion (2014) and Everything but Espresso (2016); professional training document, tasting guide, p. 5

Lockhart's diagram was redone in 2023. Guinard, Frost, Batali, Cotter, Lim and Ristenpart, at the UC Davis Coffee Center, had filter coffees described by trained panels of twelve people. The same coffees were then rated by consumers. These coffees covered a wide range of concentration, yield and coffee/water ratio.

In the Journal of Food Science they publish not one diagram, but two. The first is sensory: it places the attributes (sour, bitter, sweet, roasty, tea) in the concentration/yield plane. The second is a preference diagram: it shows the response surfaces of two consumer segments, because these two groups do not want the same cup.

The result contradicts the idea of a single ideal zone. Attributes and preferences follow more complicated trends than the 1957 rectangle. “Good coffee” is not a single thing: it depends on the group of consumers it is served to.

The coffee/water ratio, which the original diagram showed only in outline, becomes an explicit axis there.

Guinard, Frost, Batali, Cotter, Lim, Ristenpart, Journal of Food Science 88 (2023), p. 2168-2177, 10.1111/1750-3841.16531; Fresh Cup (trade press)