Coffee scientist Sara Yeager: ‘Acids bring much more flavor impact than just sourness’

MILAN – A new study by researchers at the UC Davis Coffee Center, titled “Acids in coffee: A review of sensory measurements and meta-analysis of chemical composition,” was published recently in the journal Critical Reviews in Food Science and Nutrition. Coffee contains a variety of organic acids (OAs) and chlorogenic acids (CGAs) that contribute to overall sensory properties.

Large variations in preparation and measurement methodology across the literature complicate interpretation of general trends. The study performs a systematic review and meta-analysis of the published literature to elucidate the concentrations of OAs and CGAs in both Coffea arabica (arabica) and Coffea canephora (robusta), for both green coffee and roasted coffee at multiple roast levels.

A total of 129 publications were found to report acid concentration measurements, yielding 8,634 distinct data points. Analysis of the full data set reveals several trends. First, roasted robusta has considerably more acidic compounds than arabica with 2 to 5 times as much total OAs, and much larger amounts of formic and acetic acid.

As for CGAs, in both arabica and robusta 5-CQA is the major component, and progressive roasting decreases the concentration of all CGAs. The total amount of CGA present was more dependent on roast level than the type of coffee (arabica vs. robusta). Overall, this meta-analysis suggests that the increases in certain OAs with roast level might play more of a role in the sensory profile of dark roast coffees than previously suspected.

Peter Giuliano, Chief Research Officer of the SCA and Executive Director of the Coffee Science Foundation, interviewed lead author Sara Yeager, Coffee Scientist at Toddy, about the newly published article, authored by Sara E. Yeager, Dr. Mackenzie E. Batali, Professor Jean-Xavier Guinard, and Professor William D. Ristenpart. Here is the interview published on the Sca site.

Peter Giuliano (PG): This paper is a review and meta-analysis of the published research on acidity in coffee, for which you and your colleagues reviewed and analyzed 129 publications and over 8 thousand data points. What made you take on such an ambitious project?

Sara Yeager (SY): Originally, this project was aimed at collecting the necessary background information on acid concentrations in coffee for future investigations about cold brew coffee, to provide context for our own measurements—and it ballooned into a huge project! Knowing that acids are arguably one of the most important components in coffee, it was essential to collect any and all information out there about acids in coffee. Two fundamental questions guided the research: How does each acid affect the final sensory profile, and how much of each type of acid is present in the coffee beans? Given the complexity of the coffee food matrix, understanding how each acid affects the sensory profile is a non-trivial pursuit. However, the most daunting aspect of this research is that the published scientific literature on coffee documents the amounts of acids in many disparate ways, including for multiple coffee species, roast levels, extraction methods, and analytical techniques. Additionally, there is no universal criterion for defining roast levels in coffee, making it especially difficult to compare acid concentrations (which are heavily dependent on the roasting process) across multiple demarcations of roast. The original scope of the project increased dramatically once it became clear that such a wide array of sample types existed in coffee literature. Research on acids in coffee proved to be very disjointed, and general trends about how acid levels can vary remained unresolved. The main goal of this research is to present an exhaustive review and meta-analysis of the scientific literature aimed at identifying the acid compositions of arabica and robusta coffee, for both green and roasted coffee, to provide insight on the resulting sensory profiles. The data collected will serve as an updated, comprehensive resource that documents acid concentrations across a variety of sample types, intended to serve as a guide for future research.

PG: You separate the acids in coffee into two groups, organic acids (OAs) and chlorogenic acids (CGAs). How do these groups of acids differ from each other?

SY: While both groups of acids are important to coffee’s overall sensory profile, they are fairly different from each other. OAs are those that you typically think of when you hear the term “acid”: citric acid, malic acid, lactic acid, and so on. Some of these acids can then go on to form other breakdown products during roasting, affecting the overall sensory profile. [1] In fact, the roasting process will actually cause some of these acids to increase in concentration, in some cases causing roasted coffee to become more acidic than green coffee. [2]

Chlorogenic acids (CGAs) are a bit less well-known than OAs, and are more complex than OAs. CGAs are naturally occurring bioactive compounds that accumulate in the bean as the coffee fruit matures. [3] The term CGA covers many varieties of quinic acid and caffeic acid esters (joining quinic and caffeic acid by means of an oxygen atom), and as such, the structures of CGAs can become quite complex and large. Within the CGA category, there are many different subgroups, based on the structure and composition, and within each subgroup, there are many different isomers, amounting to quite a large class of molecules. CGAs are phenolic compounds and have been investigated for their antioxidant properties in green, roasted, and soluble coffee. [4-8] During roasting, CGAs turn into CGA lactones, which contribute to bitterness in roasted coffee. [9]

In short, OAs and CGAs are vastly different in terms of structure, role as flavor precursors, and contributions to the sensory profile of coffee.

PG: In your section on flavor, you note that acids can bring much more flavor impact than just sourness—what cuppers call “acidity.” Can you tell us a little about the diverse sensory attributes acids might trigger?

SY: Besides being just “sour,” acids in coffee can contribute many other subtle nuances to the sensory profile. Of the OAs, acetic acid will be associated with vinegar characteristics, pyruvic will be responsible for a burnt caramel flavor, and formic acid will contribute a fermented aroma. [10-12] Some acids, such as formic, quinic, succinic, and caffeic will be slightly bitter.9-10,12 Besides taste and aroma, organic acids can also influence mouthfeel and astringency, such as quinic and lactic acid. [11-13] Lastly, organic acids, like fumaric, tartaric, and oxalic acid, can simply serve as flavor enhancers, boosting other flavor compounds already present in the sensory profile. [10,13]

Unfortunately, the organoleptic properties of CGAs are less well-known. When 5-caffeoylquinic acid (5-CQA, a type of CGA) is characterized, it is less sour than free quinic acid, slightly bitter, and di-caffeoylquinic acids will add a metallic and astringent note. [14-15] This same acid, 5-CQA, can also contribute to flavor enhancement, [16] but the limited characterization of CGAs in coffee make it challenging to draw conclusions about the impact of CGAs on the sensory profile.

PG: You have another section on quality and consumer liking. What did you learn about the relationship between acids and quality?

SY: As mentioned above, OAs and CGAs can serve as precursors to flavor compounds, ultimately affecting coffee quality. In green coffee, differing levels of some OAs, such as lactic, malic, and citric acids, can lead to different sensory perceptions, but have little effect on cupping quality scores. [17] Conversely, cupping scores can be affected by the fermentation process. Increases in citric, lactic, malic, acetic, and fumaric acid as a result of microbial inoculation has been linked to overall higher cupping scores. [18-20] CGAs have been studied more extensively for their role as flavor precursors in coffee, however many conflicting reports make it difficult to correlate CGA concentrations in green coffee to coffee cupping quality.

Instead, a more direct approach is to analyze brewed coffee to compare with quality measurements. Acetic and propionic acid have been associated with lower consumer liking scores. [21] Other acids, such as citric, malic, caffeic, 5-CQA, nicotinic, and tartaric can increase flavor intensity, but have little effect on cupping score. [22] Higher CGA concentrations have been linked to higher cupping scores, but also higher bitterness and astringency levels. [23-24] However, there is also contradictory evidence that it is not CGAs that impact coffee quality, but rather the derivatives of CGAs. [25-26] There is much room for targeted and systematic research concerning the link between CGA concentrations and their contribution to the flavor, quality, and consumer liking of coffee.

PG: You richly detail the specific differences in acids composition between two coffee species, Coffea arabica and C. canephora (robusta). What are the main differences between these species in terms of acids?

SY: Arabica and robusta coffee have the most differences in the OA category. OA concentration depends heavily on the type of coffee (arabica vs. robusta). Arabica coffee tends to have lower amounts of OAs, minimizing at a medium roast level. In Robusta, progressive roasting yields to a large increase in OAs, most notably acetic acid and formic acid. The OA profile present at each roast level will differ depending on coffee type, meaning that roasting profiles can be optimized based on type of coffee.

Interestingly, the difference in CGAs between arabica and robusta is less pronounced. Both types of coffee tend to have similar levels of CGAs, with the exception of total diCQA. It has been previously reported that robusta has higher levels of CGAs than arabica [14, 27-28], but the differences are not statistically significant over this much larger sample size. In terms of CGA concentrations, roast level plays a much larger role than type of coffee.

PG: You also explore how roasting changes acid composition. How does roasting change acids?

SY: The roasting process has a profound effect on both OAs and CGAs. As the coffee is roasted, the acid profile present will transform. In terms of OAs, the most abundant acids present in green coffee (citric and malic acid) will break down into other acids. Conversely, acetic acid will increase in concentration during roasting because the sucrose (sugar) naturally present in the green bean fragments into many molecules, including acetic acid. In robusta coffee, there is an increase in formic acid, most likely due to the Maillard reaction that occurs during roasting. There is no general correlation between roast level and specific OA concentrations; the roasting process affects individual OAs differently, depending on coffee type. CGAs, however, decrease similarly with roast in both arabica and robusta coffee, as they break down into CGA lactones, aromatic compounds, and other products during the roasting process.

PG: What was the most surprising thing you learned doing this work?

SY: The most surprising thing is that such a large but scattered array of coffee research is out there. While collecting all of the publications, we came across so many creative approaches to analyzing coffee. Dating from 1959 until present, many bright minds have dedicated their time and effort into truly understanding what makes coffee so incredibly complex, but also so delicious. We’re very proud that we were able to collate all of that data into one resource, because it also highlighted huge holes in the literature. For example, specialty coffee experts know that acetic acid and tartaric acid both contribute to coffee flavor—but in the open literature, there are only three reported measurements ever of acetic acid concentration in dark roast arabica coffee, and zero measurements ever of tartaric acid in light roast coffee. In general, only 22% of the reported acid concentrations were for OAs (the rest for CGAs), and very few of these directly examined links to sensory attributes. Much remains to be learned about the role of acids in coffee.

PG: How do you hope this work will be used?

SY: This work was created with the goal of serving as an updated, comprehensive resource for future research on acid concentrations in coffee. Additionally, this research highlights current gaps in coffee scientific literature—especially for less abundant acids and light or dark roast coffee. Lastly, the general trends illustrated by this project will hopefully help further understanding of how roast level and type of coffee can impact the acid concentrations, and thus the sensory profile, in coffee. Aside from our manuscript, published open access in Critical Reviews in Food Science and Nutrition, our compilation of the raw data is also freely available in a large database posted on the Dryad data repository for use by interested researchers.

PG: Anything else?

SY: None of this research would have been possible without support from the Coffee Science Foundation, especially with underwriting provided by Toddy, LLC. We thank them for their support of coffee science!

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Lead author SARA YEAGER is a coffee scientist at Toddy, LLC.

“Acids in coffee: A review of sensory measurements and meta-analysis of chemical composition,” an open-access article published in Critical Reviews in Food Science and Nutrition, was authored by Sara E. Yeager, Mackenzie E. Batali, Jean-Xavier Guinard, and William D. Ristenpart.

This project was made possible thanks to generous underwriting from Toddy, LLC.