In modern fast-paced life, coffee has become a daily necessity for many, often paired with milk (dairy or plant-based) to balance its bitterness. A new study in the Journal of Agricultural and Food Chemistry explores how different coffee roast levels interact with milk types (including oat milk). Using sensory evaluation and real-time aroma detection, researchers from Germany's Fraunhofer Institute and Italy's Edmund Mach Foundation analyzed how these factors influence aroma release and taste perception. The findings reveal the chemical basis of coffee-milk pairings, offering practical guidance for both coffee lovers and industry professionals to enhance flavor experiences.
In the hustle and bustle of contemporary life, coffee has solidified its status as an essential daily ritual for millions, with milk—whether derived from cows or plants—serving as a timeless companion to mellow its inherent bitterness.
A groundbreaking study recently published in the Journal of Agricultural and Food Chemistry delves into the nuanced interactions between varying coffee roasting degrees and different milk types, including plant-based alternatives like oat milk.
Employing state-of-the-art sensory evaluation techniques and real-time aroma detection technologies, researchers from the Fraunhofer Institute for Process Engineering and Packaging and the Edmund Mach Foundation in Italy have unraveled how these components shape in vivo aroma release and dynamic taste perception. The findings not only decode the chemical mechanisms underlying coffee-milk pairings but also offer actionable insights for both casual coffee drinkers and industry professionals seeking to optimize flavor experiences.
The Alchemy of Roast and Dairy: A Deep Dive into Coffee's Flavor Evolution
Coffee's sensory profile is a direct reflection of its roasting trajectory, a process that transforms green beans into complex flavor matrices. Light to medium roasts are characterized by vibrant top notes of blooming flowers, sun-ripened fruits, and toasted nuts, while dark roasts develop a bold palette of caramelized sugars, robust bitterness, and smoky undertones. While milk has long been celebrated for its ability to temper coffee's astringency—largely through interactions between its proteins, fats, and coffee's polyphenolic compounds—scientific understanding of its impact on in vivo aroma release (the volatile compounds exhaled through the nasal passages during consumption) has remained fragmented. Adding to this puzzle is the rise of plant-based milk alternatives: as oat milk, almond milk, and their counterparts gain popularity, questions persist about how their unique compositions alter the flavor synergy with coffee compared to traditional cow's milk.
Roasting Depth: The Blueprint of Coffee's Flavor Identity
The study's core findings underscore the pivotal role of roasting level in defining coffee's sensory foundation.
Figure 1 Dynamic sensory dominance (TDS) curves of medium (MED) and dark (DAR) roasted coffee mixed with water, milk or oat milk
Medium-roasted Arabica coffee embarked on a flavor journey that began with a pronounced bitterness, gradually giving way to the warm, inviting aroma of freshly ground beans. This created a dynamic interplay where bitterness and roasty notes alternated as the dominant sensations, maintaining a delicate balance from the first sip to the last.
In stark contrast, dark-roasted coffee unleashed a more intense and enduring bitterness, with Burnt aroma (evoking burnt bread or smoldering charcoal) emerging as the secondary flavor protagonist. The duration of roasty notes in dark roasts was significantly curtailed, resulting in a more monolithic flavor profile dominated by deep, robust bitterness.
Proton transfer reaction mass spectrometry (PTR-MS) analysis provided molecular-level insights into these differences. Dark roasts were found to release higher concentrations of volatile compounds associated with Burnt aroma and smokiness, such as methylpyrazine and 3-methylfuran, which are formed during the advanced stages of the Maillard reaction and caramelization. Medium roasts, conversely, retained a richer array of precursors for floral and fruity aromas, including acetaldehyde and furfural, which contribute to their bright, lively character.
Milk as a Flavor Modulator: Masking, Enhancing, and Reconstructing Aromas
The addition of cow's milk introduced a dual effect of taste masking and aroma reconstruction. Notably, it reduced the initial intensity of bitterness in both roast profiles: for medium roasts, the dominance of bitterness dropped from 64% to 30.8%, while dark roasts saw a decrease from 69% to 46.2%. Concurrently, cow's milk introduced creamy, buttery notes that complemented the coffee's base flavors. In the case of dark roasts, cow's milk even provided a subtle mitigation of Burnt aroma，allowing the roasty notes to linger slightly longer on the palate.
Aroma analysis revealed that cow's milk did not systematically suppress or enhance the release of coffee-derived volatile compounds. Instead, it contributed its own lipid-derived aromatics, such as 2-heptanone, which may influence overall perception through olfactory masking. This suggests that cow's milk acts primarily as a flavor moderator, softening harsh notes without drastically altering the coffee's aromatic identity.
Oat milk, however, demonstrated a more pronounced and sustained effect on bitterness suppression. For medium roasts, it reduced bitterness dominance to 36%, and for dark roasts, to an impressive 31%. Beyond bitterness reduction, oat milk introduced a distinct caramel-vanilla aroma—borne from Maillard reaction products formed during its processing—that became a dominant note in the mid-drinking phase. Consumers also reported a pronounced "fatty" or "creamy" mouthfeel when drinking coffee with oat milk, a sensation not typically associated with plant-based alternatives.
Figure 2 Comparison of aroma release curves of representative compounds when drinking medium (MED) and dark (DAR) roasted coffee (with water, milk or oat milk)
PTR-MS analysis of oat milk-coffee mixtures detected elevated levels of aldehydes and terpenes, including benzaldehyde (which imparts an almond-like aroma) and hexanal (contributing grassy, green notes). These compounds not only define oat milk's unique flavor signature but may also interact with coffee's phenolic compounds, creating a chemical synergy that further diminishes perceived bitterness.
The Molecular Basis of Creaminess: Comparing Dairy and Plant-Based Milks
The divergent effects of cow's milk and oat milk on coffee flavor can be traced to their fundamental chemical compositions. Cow's milk is rich in casein proteins and milk fat (containing 3.6% total fat, 2.3% of which is saturated). These components form a complex protein-lipid network that hydrophobically adsorbs bitter-tasting compounds in coffee, effectively reducing their perception. While cow's milk's impact on aroma release is relatively subtle, its fatty acids and phospholipids contribute to the smooth, creamy mouthfeel that has made it a staple in coffee for centuries.
Oat milk, with its lower protein content (0.7%) and unique composition of β-glucans (a type of soluble fiber), operates through a different mechanism. The β-glucans increase the viscosity of the coffee-milk mixture, which may slow the release of bitter compounds, leading to more prolonged bitterness suppression. Additionally, oat milk's lactose-free nature eliminates the potential for acidic notes that can sometimes arise from lactose degradation during coffee preparation, making it particularly well-suited to balance the intense Burnt aroma of dark roasts.
The processing of oat milk also plays a role: the Maillard reactions that occur during its production generate the caramel-vanilla notes that pair so harmoniously with the sweet undertones of medium-roasted coffee. This has practical implications for coffee enthusiasts: medium roasts are best complemented by oat milk, whose caramel notes enhance the coffee's natural sweetness, while dark roasts find their ideal partner in cow's milk, whose rich fat content softens harsh bitterness while preserving the coffee's robust character.
Implications for Coffee Culture and Future Innovations
This research transcends academic curiosity, offering tangible insights for both consumers and the coffee industry. At a sensory level, it reveals that the experience of drinking coffee with milk is not a static phenomenon but a dynamic interplay of volatile compounds releasing over time in the oral and nasal cavities. The "fatty" sensation associated with oat milk, for example, is not merely a textural perception but a complex olfactory-gustatory interaction driven by the collaboration release of hexanal and benzaldehyde.
For the food science industry, these findings open new avenues for innovation in plant-based milk formulation. By precisely tailoring the composition of alternative milks—such as increasing ester content or optimizing β-glucan concentration—researchers may be able to create products that not only mimic the sensory properties of cow's milk but also enhance specific aspects of coffee flavor. This could lead to a new generation of coffee-milk pairings designed to cater to diverse consumer preferences, from those seeking bold, intense flavors to those desiring delicate, nuanced profiles.
On a broader scale, this study reminds us that even the most mundane daily rituals—like enjoying a cup of coffee—are steeped in scientific complexity. Every choice, from the roasting degree of the beans to the type of milk added, influences a intricate dance of chemicals and sensory receptors. The next time you savor a latte or an oat milk cappuccino, take a moment to appreciate not just the taste on your tongue, but the invisible orchestra of aromas swirling in your nose and the complex chemical reactions unfolding in your cup. It is this marriage of science and tradition that continues to make coffee one of the most fascinating and beloved beverages on the planet.
References
Cleve N, Gonzalez-Estanol K, Khomenko I, et al. Effects of Roasting Level and Milk Addition on In Vivo Aroma Release and Perception of Coffee. Journal of Agricultural and Food Chemistry. 2025;73(22):13792-13808. doi:10.1021/acs.jafc.4c12852