Reading a shot
What can be seen and measured during and after an espresso shot – flow, time, beverage weight, colour, crema, the spent puck and strength – and how much each one tells about the extraction and the cup.
Across the chain: Science
Contents
In brief
- In one 2026 study, water took about 5–10 s to soak and swell the dry bed after the pump started; the first drops fell during this phase.4
- In the same study, the first drops held about 25% TDS; after about 20 s the concentration fell fast and the brew turned lighter.4
- Crema is measured as foam volume, foam index (foam ÷ liquid volume) and persistence; a 2011 review traces all three to the carbon dioxide in the coffee.1
- The same review states that judging a shot by crema colour rests on trial and error, not on objective, reproducible measurement.1
- Extraction yield needs dose, beverage mass and a refractometer reading of total dissolved solids (TDS).5
What a shot shows#
An espresso shot lasts well under a minute, and a barista sees only a few things. The stream starts, speeds up and changes colour. A foam, the crema, forms in the cup. Afterwards there is a wet cake of spent coffee in the basket. With a scale and a timer, the barista can add the beverage weight and the shot time.
Only one measurement says directly how much coffee dissolved. That is the strength of the cup, read with a refractometer, combined with the dose and the beverage weight.5 Everything else is an indirect sign. This entry sets out what studies have measured for each sign, and where no study consulted for this entry links a sign to the cup.
How a recipe is chosen is covered in espresso recipes and ratios. How the bed is built, and how channelling starts, is covered in puck preparation.
Flow onset and flow rate#
When the pump starts, nothing reaches the cup at first. In a 2026 study, water first had to soak into the dry bed and push out the air in its pores.4 The bed swelled at the same time.4 This took about 5–10 s, and the first drops fell during it.4 A 2025 tomography study followed the same wetting front as it moved into a dry espresso bed.10
The start of brewing came a couple of seconds before the first drops.4 A shot time counted from the first drop is therefore shorter than one counted from pump start. Recipes should say which start they use.
After the first drops, the flow rate rose almost linearly in that study.4 Beyond the usual length of a shot, it levelled off or fell slightly.4 Flow rate is easiest to measure with a scale under the cup. In a 2023 study, flow rates were calculated from a scale’s time-stamped readings of beverage mass.6 A 2011 review states that flow into the cup should ideally be close to 1 mL/s.1 A café survey cited in the review found 0.40–2.73 mL/s.1
Shot time and beverage weight#
Time and weight are the two numbers a barista can record for every shot. In a 2024 study, a load cell recorded the beverage weight during each shot, and the time was read from the machine.5 The shot was stopped by hand at 40 g.5
Shot time is mostly a result of how much the bed resists the water. In the 2024 study, adding fines slowed shots, but all shots still scattered around one curve of extraction yield against time.5 For that one coffee, shot time tracked the yield, whether a finer grind or added fines slowed the bed.5
A fast shot is not always a weak one, and a slow shot is not always strong. When water channels through a few paths, the authors of the 2026 study note, the yield stays low even at high flow.4 A short time together with a low yield is therefore a reason to suspect channelling, not proof of it.4,7
Colour of the stream#
The stream changes colour during a shot. In the 2026 study, the brew grew lighter at later times, which the authors read as less dissolved material.4 The first drops held about 25% total dissolved solids (TDS).4 After about 20 s, TDS fell fast and was close to zero at about 60 s.4 A 2023 study found the same pattern for each compound: highest in the first fraction, then falling exponentially as more espresso was collected.6
A paler stream therefore shows that the liquid leaving the bed at that moment is weaker. It does not show how much has been extracted in total, or how evenly. No study or textbook consulted for this entry uses or defines the term “blonding” for this colour change. No study consulted for this entry tested whether stopping a shot when the colour changes improves the cup.
Crema#
Crema is the foam on an espresso; its gas is mainly carbon dioxide from roasting (see espresso).1,2 According to a 2011 review, foam volume, persistence and consistence all follow from the carbon dioxide originally present in the coffee.1 Crema therefore says most about the coffee’s gas content: how fresh it is, how dark it was roasted and how it was brewed.1 How crema changes as roasted coffee loses its gas is covered in degassing.
Crema can be measured, not only looked at:
- Foam volume, read in a graduated cylinder straight after extraction.1
- Foam index, the ratio of foam to liquid volume. The review cites an expectation of at least 10% for a correct preparation.1
- Persistence, the time until the liquid below first shows through the foam as the cup cools.1
- Consistence, how long the foam bears a known weight, such as a little crystalline sugar, before it sinks through.1
- Bubble size, under a microscope, and crema height in a cylinder over time.9
In a 2022 capsule study, crema made at 19 bar was more stable and kept smaller bubbles than crema made at 15 bar.9 The two pressures came from two capsule systems that also differed in machine and dose (5 g and 7 g), so pressure was not the only difference.9 The chart shows how the bubbles grew in 10 minutes.
Data table
| µm | Source | |
|---|---|---|
| Guatemala, 19 bar | 36 → 38 µm (no significant change) | 9 |
| Brazil, 19 bar | 26 → 39 µm | 9 |
| Guatemala, 15 bar | 27 → 59 µm | 9 |
| Brazil, 15 bar | 44 → 81 µm | 9 |
The review lists colours that point to under- or over-extraction; the espresso recipes and ratios entry gives the list.1 It describes pure arabica crema as reddish-brown with a “tiger skin” pattern, and pure robusta crema as dark brown with grey tones.1 The review suggests that the pattern may point to a stabilising role of the solid particles, minute cell-wall fragments, in the foam.1 A textbook chapter calls such signs only secondary indications, after the flavour of the coffee.2
The spent puck#
The puck knocked out after a shot is not the puck that was brewed. In the 2026 study, the spent puck had cracks and had peeled away from the filter.4 The authors link this partly to backflow through the machine’s valve when the pump stops.4 Swelling turns the grounds into a firm cake that crumbles under moderate stress.4
A 2023 imaging study of capsule beds found that, after extraction, porosity always decreased from the outlet at the bottom to the inlet at the top.11 A spent bed thus carries traces of the brew, but they are changed by what happens after it. No peer-reviewed study consulted for this entry linked the look of a spent puck to extraction yield or flavour. The puck preparation entry covers what practitioner tests have found.
Strength and extraction yield#
A refractometer gives the strength of the cup as TDS. With the dose and the beverage mass, it gives the extraction yield, the share of the dry coffee that dissolved.5 Research labs filter espresso through a 0.45 µm filter before reading it.5 The arithmetic is in espresso recipes and ratios; the instrument and the brewing control chart are in extraction yield and TDS.
A single reading averages the whole cup. In the 2026 study, the shot was split into 5 s fractions; the most concentrated early fractions had to be diluted before they could be read.4 Neither a cup reading nor a fraction reading shows how evenly the bed was extracted.7,8
What appearance cannot tell#
Crema reading has a long tradition. A popular claim holds that crema reveals every fault in grinding and brewing. According to the 2011 review, this rests on almost a century of trial and error, not on reproducible measurement.1
No study consulted for this entry tested a visual cue against flavour judged by a sensory panel or against measured extraction yield. That covers stream colour, crema colour, uneven streams and spurts from the basket, and the spent puck. The 2022 capsule study scored crema and flavour with a trained panel but did not test a link between them; its authors only suggest that finer bubbles could release less aroma.9 A coffee science textbook names small adjustments, frequent tasting and objective measurement where possible as best practice.3
Three regimes of flow#
The 2026 study recorded beverage mass and pump pressure 10 times per second on a café machine with a scale under the cup.4 From the flow rate it delineated three regimes:4
- Wetting. The water front penetrates the bed and expels the trapped air while the bed swells; about 5–10 s, during which the first drops fall.4 A 2025 study tracked this front by X-ray tomography and found good agreement with a one-dimensional model of flow into an unsaturated bed.10
- Transient dissolution. The flow rate rises almost linearly while solubles leave the wetted bed.4 Typical flow was of the order of 1 g/s, the flux of dissolved solids about 0.1 g/s.4
- Steady flow. Past the usual shot length, the flow saturates or falls slightly towards a stable value.4
The flux of dissolved solids peaked at about 30 s.4 Early in the shot it was limited by the low flow, later by how much soluble material was left in the puck.4 At long times, flow rose roughly linearly with pressure only below about 5 bar and saturated in the usual brewing range; puck preparation covers the physics.4
These regimes come from one coffee, one grinder setting and one machine. The study’s grind was set for 37 g in the cup at 32 s from an 18.5 g dose, and the authors note natural variation around it.4
Concentration over the shot#
The 2026 study fitted its fraction data with an S-shaped breakthrough curve. The fitted TDS starts at about 25.6% and falls to half that at about 20.9 s, with a decay time of about 8.8 s (our reading of the published equation).4 By the authors’ estimate, the total mass removed over the whole prolonged shot came to about 10% of the 18.5 g dose.4 Their fitted total soluble mass, 2.26 g, is about 12% of the dose (our calculation: 2.257 g ÷ 18.5 g).4 Either way, this is well below the “around 20%” that another study gives for a typical espresso (see espresso).5 Shots, coffees and methods differ: one is an estimate from a fitted flux, the other refractometer readings of whole cups.4,5
The 2023 flow-controlled study gives the order in which compounds thin out. TDS and trigonelline fell fastest, then 5-caffeoylquinic acid, then caffeine.6 The recipes entry shows what this means for ratio and yield.
For reading a shot, two points follow. First, the colour of the stream reports the present concentration, not the cumulative one.4 Second, the late fractions are weak, so they mainly dilute the cup; the recipes entry puts numbers on this.6
Channelling: what can be measured#
Channelling is flow focused in one or a few channels instead of passing evenly through the bed.4 Three lines of evidence show it indirectly.
- Yield against grind. In a 2020 study, measured extraction yield peaked at an intermediate grind setting.7 A model assuming even flow had predicted yield to keep rising as the grind got finer; the authors attribute the gap to uneven flow.7
- A feedback in the bed. A 2023 model shows that dissolving coffee raises porosity and permeability.8 A path that carries more water therefore opens further, so small differences grow during the shot.8
- Flow without yield. In the 2026 study, restarting a brew several times raised the flow without dissolving more.4 The authors link this to the bed reorganising each time pressure returns.4
What a barista can measure follows from these: shot time, beverage mass and yield, and how much they vary between shots of the same recipe.4,5,7 The 2026 authors note that they had no direct view of the bed reorganising during brewing.4 No study consulted for this entry related spurts or uneven streams seen under a bottomless portafilter to measured channelling.
Measuring crema#
The 2011 review compiles methods and values from earlier studies.1
| Measure | Method | Values reported | Source |
|---|---|---|---|
| Foam volume | Graduated cylinder or beaker, straight after extraction | 1.2–3.9 mL in a 1997 roast study (50 mL cylinder); 5.1–6.9 mL with rising water pressure in another | Illy & Navarini1 |
| Foam index | Foam volume ÷ liquid volume | At least 10% expected | Illy & Navarini, citing Illy & Viani1 |
| Persistence | Time until the liquid shows through as the cup cools at room temperature | 6–40 min in the first study; 24.7–30.0 min at 7–11 atm (≈7.1–11.1 bar; arabica, 7.5 g, 40 mL) | Illy & Navarini1 |
| Consistence | Time the foam bears a known weight, such as 1.5 g of crystalline sugar | 0.5–14 s in one study; correlated with persistence | Illy & Navarini1 |
| Crema height | 25 mL graduated cylinder, read every minute for 10 min | 19 bar (Guatemala): ≈40% of the initial height left at 10 min; 15 bar: gone | Lomolino et al.9 |
| Bubble size | Stereomicroscope, Sauter mean diameter | 26–44 µm fresh; 38–81 µm after 10 min | Lomolino et al.9 |
What changes the foam. In a study described in the review, holding ground coffee for 24 h in ambient air cut foam volume from 10 to 6.5 mL.1 In a 1997 study described there, foam volume rose linearly with roast degree for both arabica and robusta.1 Persistence instead peaked at a medium roast.1 Water temperature and dose seemed to have no significant effect on persistence, while water pressure did.1
Volume against stability. The review reports that conditions giving more foam, such as a short time since roasting, gave less persistent foam.1 It suggests that too much carbon dioxide destabilises a fresh foam.1 As the cup cools, the liquid between the bubbles becomes more viscous, from about 0.50 mPa·s at 70 °C to 1.40 mPa·s at 25 °C.1 A thick crema is therefore not the same as a lasting one.
Comparing studies. Methods differ: cylinders of 25, 50 or 100 mL, cups of 4–40 mL, and different doses.1,9 Many studies in the review did not report roast degree or water quality.1 Crema values should be compared within one study.
Reference values#
| Parameter | Value reported | Context | Source |
|---|---|---|---|
| Wetting phase, first drops | First drops during a wetting phase of about 5–10 s | 18.5 g, 58 mm basket, café machine | Waszkiewicz et al.4 |
| Pump start to first drops | A couple of seconds | Same study, scale and pressure logged at 10 Hz | Waszkiewicz et al.4 |
| TDS of first drops | ≈25% (fit 25.6%) | Stable for ≈20 s, near zero at ≈60 s; 5 s fractions | Waszkiewicz et al.4 |
| Peak flux of dissolved solids | ≈30 s | Flow of the order of 1 g/s, solids ≈0.1 g/s | Waszkiewicz et al.4 |
| Flow into the cup | Ideally close to 1 mL/s; 0.40–2.73 mL/s observed | Review; survey of Italian cafés | Illy & Navarini1 |
| Flow rate from a scale | 0.96, 1.9, 2.8 mL/s achieved | Set to 1, 2 and 3 mL/s; flow-controlled machine | Schmieder et al.6 |
| Concentration over the shot | Highest in the first fraction, then exponential fall | All compounds measured | Schmieder et al.6 |
| Yield against time | One curve, with or without added fines | 20 g → 40 g, 9 bar, one coffee | Smrke et al.5 |
| Refractometer sample | Filtered, 0.45 µm | Espresso, lab practice | Smrke et al.5 |
| Foam index | At least 10% expected | As cited in a review | Illy & Navarini1 |
| Foam persistence | 6–40 min | First study on the subject | Illy & Navarini1 |
| Crema bubbles, 10 min | 36 → 38 µm (19 bar) to 44 → 81 µm (15 bar) | Capsule espresso, four samples | Lomolino et al.9 |
Open questions#
- Visual cues and the cup. No study consulted for this entry tested stream colour, crema colour, uneven streams or the spent puck against sensory scores or measured extraction yield.
- Blonding. The colour change is documented with refractometer data in one study.4 Whether its timing is a useful stopping signal was not tested in any study consulted for this entry.
- Crema and flavour. The crema chapter treats crema as secondary to flavour2; the capsule study scored both without testing a link.9
- Measuring evenness. Uneven flow is inferred from yield, models and imaging.4,7,8 No study consulted gives a café method to measure how evenly a puck was extracted.
- Standard crema methods. Studies use different cylinders, cup sizes and doses, and many omit roast degree and water.1,9
Key studies#
Waszkiewicz et al., 2026 – Flow and dissolution in espresso. Logged beverage mass and pressure on a café machine for shots at 1–12 bar, split one shot into 5 s fractions for TDS, and imaged spent pucks. The first drops followed a wetting phase, concentration fell after about 20 s, and spent pucks showed cracks linked to backflow.
Illy & Navarini, 2011 – The espresso foam. Reviewed how crema is measured and what controls it. Foam volume, persistence and consistence trace back to the carbon dioxide in the coffee; the review warns that reading crema by eye rests on trial and error.
Lomolino et al., 2022 – Crema of capsule espresso. Measured crema height over 10 minutes and bubble size under a microscope for two arabica origins in capsules brewed at 19 and 15 bar. Crema made at the higher pressure was more stable with smaller bubbles.
Smrke et al., 2024 – Fines and shot time. Weighed shots during extraction and measured yield at several grind settings with added fines. Yield followed shot time along one curve, so fines acted by slowing the flow.
Cameron et al., 2020 – Uneven flow. Compared an even-flow model of espresso with measured shots. Measured yield peaked at an intermediate grind, which the authors attribute to uneven flow at fine settings.
- Crema as a sign of extraction: a 2011 review lists crema colours that point to under- or over-extraction, and argues that foam volume, persistence and consistence follow from the carbon dioxide in the coffee1; a textbook chapter calls the dark brown colour, “tiger skin” and fine bubbles signs of a good extraction.2 The same review states that the popular reading of crema rests on almost a century of trial and error rather than objective characterisation1, and the chapter ranks crema only as a secondary indication, after flavour.2
Coffearium (2026). Reading a shot. Coffearium – The open coffee compendium. Retrieved 4 November 2026, from https://coffearium.org/entries/reading-a-shot/
@misc{coffearium2026-reading-a-shot,
author = {{Coffearium}},
title = {{Reading a shot}},
year = {2026},
howpublished = {Coffearium -- The open coffee compendium},
url = {https://coffearium.org/entries/reading-a-shot/},
urldate = {2026-11-04},
note = {Retrieved 4 November 2026}
}Entries are updated when the evidence changes; the retrieval date tells your readers which version you used. Changes are listed in the Change log.
Change log
- – First-drop timing restated as falling during a 5–10 s wetting phase (brief, quick facts, reference table, timeline), not 5–10 s after pump start Reason: The study gives 5–10 s as the length of the wetting phase and says brewing start precedes the first drops by a couple of seconds
- – Capsule crema study described as untested for a crema–flavour link (not "no link reported"); pressure comparison noted as confounded with capsule system and dose; tiger-skin attribution softened to the review's suggestion Reason: Lomolino et al. 2022 and Illy & Navarini 2011 as read in context
- – Unverifiable dose details removed from the foam-volume row and the method comparison; fitted total soluble mass (≈12%) added beside the authors' ≈10% Reason: Doses sit only in a review table that could not be read; Waszkiewicz et al. fit gives 2.257 g
Sources
- AIlly, E.; Navarini, L. (2011). Neglected Food Bubbles: The Espresso Coffee Foam. Food Biophysics 6(3), 335–348. doi:10.1007/s11483-011-9220-5 · other copiesQuoted passage: a result cited from earlier work; Review of espresso foam studies (arabica and robusta, lab espressos); compiles foam volume, persistence and consistence data from earlier work.
- BFolmer, B.; Blank, I.; Hofmann, T. (2017). Crema – Formation, Stabilization, and Sensation. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 17, pp. 399–417. doi:10.1016/B978-0-12-803520-7.00017-7 · other copiesQuoted passage: a description; Textbook chapter on crema formation and stability; no numbers carried in this entry.
- BMestdagh, F.; Glabasnia, A.; Giuliano, P. (2017). The Brew – Extracting for Excellence. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 15, pp. 355–380. doi:10.1016/B978-0-12-803520-7.00015-3 · other copiesQuoted passage: a recommendation; Textbook chapter on brewing and extraction; general advice for baristas.
- AWaszkiewicz, R.; Myck, F.; Białas, Ł.; Puciata-Mroczyńska, M.; Dzikowski, M.; Szymczak, P.; Lisicki, M. (2026). Under pressure: Poroelastic regulation of flow in espresso brewing. Physics of Fluids 38(6), 063113. doi:10.1063/5.0319611 · other copiesQuoted passage: the authors’ own result; Lab espresso, one Brazilian specialty coffee (light to medium-light roast), 18.5 g in a 58 mm basket, café machine with scale and pressure sensor; 60 shots at 11 pressures, TDS in 5 s fractions (n = 3).
- ASmrke, S.; Eiermann, A.; Yeretzian, C. (2024). The role of fines in espresso extraction dynamics. Scientific Reports 14, 5612. doi:10.1038/s41598-024-55831-x · other copiesQuoted passage: the authors’ own result; Lab espresso, one pulped natural arabica from Costa Rica, 20 g to 40 g at 9 bar, several grind settings with added fines; one Q grader for sensory.
- ASchmieder, B. K. L.; Pannusch, V. B.; Vannieuwenhuyse, L.; Briesen, H.; Minceva, M. (2023). Influence of Flow Rate, Particle Size, and Temperature on Espresso Extraction Kinetics. Foods 12(15), 2871. doi:10.3390/foods12152871 · other copiesQuoted passage: the authors’ own result; Lab espresso on a flow-controlled machine, 20 g of one washed Colombian arabica, flows 1–3 mL/s, 80–98 °C, ten fractions per shot.
- ACameron, M. I.; Morisco, D.; Hofstetter, D.; Uman, E.; Wilkinson, J.; Kennedy, Z. C.; Fontenot, S. A.; Lee, W. T.; Hendon, C. H.; Foster, J. M. (2020). Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment. Matter 2(3), 631–648. doi:10.1016/j.matt.2019.12.019 · other copiesQuoted passage: the authors’ own result; Extraction model compared with measured espresso shots across grind settings; abstract consulted.
- ALee, W. T.; Smith, A.; Arshad, A. (2023). Uneven extraction in coffee brewing. Physics of Fluids (AIP Publishing). doi:10.1063/5.0138998 · other copiesQuoted passage: a hypothesis; Mathematical model of two flow paths through an espresso bed; no new measurements.
- ALomolino, G.; Dal Zotto, V.; Zannoni, S.; De Iseppi, A. (2022). Foam Characteristics and Sensory Analysis of Arabica Coffee, Extracted by Espresso Capsule and Moka Methods. Beverages 8(2), 28. doi:10.3390/beverages8020028 · other copiesQuoted passage: the authors’ own result; Capsule espresso from two arabica origins (Guatemala, Brazil), 5 g at 19 bar and 7 g at 15 bar; crema height each minute for 10 min, bubble size by microscopy, trained sensory panel.
- AFoster, J. M.; Lee, W. T.; Moroney, K. M.; Prjamkov, D.; Salamon, M.; Smith, A.; Petrassem-de-Sousa, J.; Vynnycky, M. (2025). Dynamics of liquid infiltration into an espresso bed using time-resolved micro-computed tomography: Insights from experiment and modeling. Physics of Fluids 37(1), 013383. doi:10.1063/5.0245167 · other copiesQuoted passage: the authors’ own result; Time-resolved X-ray tomography of water entering a dry espresso bed, with a one-dimensional model; abstract consulted.
- AMo, C.; Johnston, R.; Navarini, L.; Suggi Liverani, F.; Ellero, M. (2023). Exploring the link between coffee matrix microstructure and flow properties using combined X-ray microtomography and smoothed particle hydrodynamics simulations. Scientific Reports 13, 16374. doi:10.1038/s41598-023-42380-y · other copiesQuoted passage: the authors’ own result; X-ray microtomography of coffee beds in capsules before and after extraction, with flow simulations.
Tier A · authoritative · B · trade and expert · C · general. Supporting quotes for every source are in the data version of this entry. If a publisher’s site refuses access, try the doi link or “other copies”.
The literature for this entry was last searched on 27 Oct 2026. A statement that no source consulted for this entry shows something refers to the sources above; newer or missing work may exist.
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