Direct electrochemical appraisal of black coffee quality using cyclic voltammetry

Article Open access Published: 28 April 2026 Nature Communications volume 17, Article number: 3618 (2026) Cite this article 2726 Accesses 153 Altmetric Metrics details AbstractDespite coffee’s popularity, there are no quantitative methods to measure a chemical property of a black coffee drink in situ and relate it to a flavor experience. Here we show that cyclic voltammetry can be used without any additional sample preparation to directly measure the strength of a coffee beverage and, separately, how dark the coffee has been roasted; these two properties are implicated in the sensory profile of the beverage. We show that the current passed for the cathodic features that precede hydrogen evolution are linearly related to beverage strength. The same features are suppressed with subsequent cycling due to coffee material accumulating on the electrode. The magnitude of suppression is directly related to roast color, which dictates the ensemble chemical composition and flavor of the beverage. Together, this voltammetric analysis decouples beverage strength from roast color and offers a strategy for rapidly assessing flavor-correlated chemical properties of coffee. Similar content being viewed by others IntroductionSince the 1950s, the coffee industry has sought quantitative methods to assess beverage qualities beyond those informed by sensory panels. In the meantime, a litany of research on the topic has revealed that beverage concentration1,2, and roasted bean color3,4, are the two primary and independent factors that dictate the sensory perception of coffee. Bean color is readily determined by spectrophotometry5,6,7, while the most widely used technique to measure concentration of solvated coffee relates the refractive index of the beverage8,9, to an effective concentration through an empirically derived polynomial. The refractive index method reports an approximated total dissolved solids (wt.% TDS) ratio of the mass of dissolved coffee to the mass of the beverage and informs beverage strength10. From wt.% TDS, the efficiency of the extraction process can then be calculated by taking the mass ratio of the dissolved solids to the dose of ground coffee (i.e., extraction yield11). The industry has coalesced around the phenomenological observation that dissolution of ≈20% of the dry mass, yielding a beverage ≈1.35 wt.% TDS generally produces an enjoyable cup of filter coffee12,13. To date, measurements based on refractive index remain the industry standard.

However, the refractive indices of solutions are dependent on the identity of the solutes. Glucose, for example, will have the same refractive index at 2% w/w as a solution of ethanol that is twice as concentrated, making refractive index measurements unable to identify chemical differences in more complex mixtures (e.g., coffee), and particularly compositions result in dissimilar sensory outcomes14,15,16,17,18,19. Given the composition of roasted coffee primarily depends on roast color, the existing quantitative approach cannot discern differences between light-roasted and dark-roasted coffees that achieve the same refractive index, let alone higher-fidelity chemical differences achieved in the same coffee using modified brewing parameters or roast profiles. A rapid quantitative method that is sensitive to compositional information beyond wt.% TDS remains a major target for the industry.There exist numerous analytical techniques that provide both qualitative and quantitative information about chemical composition, with the gold standard being liquid or gas chromatography coupled with mass spectrometry for soluble and volatile compounds, respectively. Besides the obvious challenge of identifying specific compounds among the thousands observed in coffee that give rise to a measurable sensory experience, these techniques also suffer from slow run times, laborious sample preparation, and high associated costs while yielding limited predictive insights. Instead, some research groups have used electrochemical techniques to measure the concentrations of common molecules in solution20,21. Electroanalytical techniques measure the amount of current passed between electrodes immersed in the solution at voltages where solvated molecules undergo redox reactions; the measured current is directly proportional to the local concentration of the reacting molecule(s). While this approach is generally sensitive enough to accurately measure the concentration of caffeine22, various chlorogenic acids (CGAs)23, and other molecules24, previous reports have omitted beverage strength in their analyses, precluding the development of a technique which relates preparation variables (e.g., mass of coffee, mass of water, grind setting, water temperature and pressure, contact time, roast color etc.) to the resultant chemical composition of the liquid.Herein, we report an advance in coffee quality analysis that harnesses in situ changes in the protonic electrochemical response (e.g., hydrogen underpotential deposition, HUPD) in cyclic voltammetry (CV) measurements of liquid coffee.

By sampling features in the electrochemical response that are affected by the ensemble chemical composition of the coffee rather than measuring the concentrations of individual molecules, this approach captures quantitative information about both roast color and beverage strength. These two properties drive the sensory profile of the beverage, thereby allowing this analytical technique to exceed the insights provided by refractive index measurements and provide additional quality information that correlate with flavor.ResultsInitially, we sought to directly measure the concentration of caffeine (Eox = ≈1.4 V vs. Ag/AgCl25), CGAs (Eox = ≈0.2–0.5 V vs. Ag/AgCl, depending on the isomer23), and other redox-active species in undiluted coffee samples to study their dependence on conventional brew parameters. However, caffeine and CGAs form an aggregate at concentrations typical of brewed filter coffee26, terminally impacting their redox activity—they only become electrochemically well-resolved in acidified dilute solutions with added electrolyte. Literature examples dilute to below 0.02 wt.% TDS27, nearly two orders of magnitude weaker than filter coffee. Other groups have shown that by adulterating coffee with either CGA or caffeine, both boron-doped diamond (BDD) and glassy carbon (GC) electrodes can provide molecule-specific information28,29,30,31,32,33,34,35,36. Yet, these approaches required laborious sample preparations, in opposition of both our and the industry's goal of measuring features of as-consumed coffee. Ideally, ensemble chemical measureables map to sensory experiences and relate the electrochemical features to total beverage strength, rather than measuring the concentration of specific molecules contained within. Thus, we first ascertained the redox landscape in filter-strength coffee by making CV measurements using BDD, GC, and Pt working electrodes across the electrochemical window of the beverage (Fig. 1a).Fig. 1: Assessing the voltametric features present in coffee extracts.The alternative text for this image may have been generated using AI.Full size imagea Cyclic voltammetry performed at 200 mV s−1 using boron-doped diamond, glassy carbon, and platinum working electrodes. Caffeine and chlorogenic acid potential ranges studied by other groups are highlighted. Here, we focus on the protonic features (HUPD and acid redox) region.

b These features are suppressed with subsequent CV cycling due to deposition of coffee material on the surface of the electrode, and the total charge can be extracted by subtracting the hydrogen evolution reaction (HER) background. c The current depends on wt.% TDS of the brewed coffee because the protonic and organic concentration scales with wt.% TDS. d Scanning anodically results in mass accumulation on the working electrode, leading to electrode fouling. e The mechanism of mass accumulation is likely proton-assisted, given that appreciable mass does not deposit until the surface has accumulated a critical H-atom concentration. Source data are provided as a Source data file.As-consumed filter coffee extracts are sufficiently conductive for direct electrochemical analysis without the addition of a supporting electrolyte (ranging from 2-3 mS cm−1, see Supplementary Fig. 1). Additionally, coffee extracts are self-buffered to pH of ≈4.8–5.9 depending on the distribution of compounds in the coffee and brew water composition37,38,39. Even after performing bulk electrolysis at oxygen evolution potentials for two minutes, the pH of a typical filter coffee remains numerically identical, reinforcing the significant buffering capacity of brewed coffee. Yet, despite the plethora of molecules in coffee extracts, the CV response of a Pt working electrode in 1.56 wt.% TDS coffee is consistent with that of dirty acidic water (Fig. 1a)40,41,42,43,44,45.The cathodic Faradaic features map to the response expected for protonic reactions with the Pt surface (e.g., HUPD), followed by H2 evolution at more negative potentials. At positive potentials, OH adsorption and eventual O2 evolution are also evident. To ascertain whether the anodic feature at –0.6 V is linked with the cathodic features in a reversible redox couple, we probed the scan rate dependence, Supplementary Fig. 2. The linear dependence of peak current on the square root of the scan rate for both features demonstrates the diffusion-controlled nature of the redox events, Supplementary Fig. 3, and the lack of an increase in the peak potential separation with increasing scan rate indicates Nernstian behavior, Supplementary Fig.

4. However, the large peak separation of ≈200 mV at all scan rates suggests that the reversibility of the redox couple is obfuscated by sluggish kinetics.The same Pt surface sites that adsorb H+ and OH– are also able to adsorb other molecules in solution. In the case of oxidative cycling, some impurities in water compete for the Pt surface, resulting in reduced current with subsequent cycling due to a decrease in the accessible surface area. Given that Pt is known to interact with caffeine and other molecules in coffee46,47, we expected to see a decrease in exchange current density with sequential cycling. When scanning from 0 to –1.0 V, the HUPD and protonic features (Epc = –0.4 and –0.7 V, respectively) smear together and current decreases by ≈34% from CV scan 1 to 2 and ≈18% from scan 2 to 3 (Fig. 1b)48. In pH 7 water purified by reverse osmosis, the same features are not observed (Supplementary Fig. 5), suggesting that the response is due to protonic chemical steps associated with the coffee and not the water.Further experiments were run to ensure that these features mapped to HUPD/weak acid reduction and its suppression by coffee molecules, rather than fluctuations in dissolved O2 and other spurious effects, Supplementary Fig. 6. Since the integral of the current density depends on the activity of H+, the HUPD and acid features indirectly provide ensemble insights into the families of molecules in coffee that function as H+ donors and acceptors, the concentrations of which should depend on roast color, brewing parameters, brew water composition, and so forth. Some data in support of this hypothesis is that HUPD/acid reduction current density decreases with decreasing coffee concentration due to the diminished concentration of available H+ (Fig. 1c). Because the feature is concentration dependent, there are also fewer organic molecules competing to bind to the surface of the electrode. As we will show later, the integral of the charge current density of this feature linearly maps to wt.% TDS. Perhaps this is a surprising result, given that a single acidic feature should not necessarily depend on ensemble concentration.