The Alchemy of Crema: How Processing Methods Shape Your Espresso Foam

Question: When considering a coffee for espresso, how might the processing method influence the crema it produces?

The ephemeral crown of an espresso, its crema, is more than just a visual delight; it’s a complex emulsion of CO2, water, and emulsified lipids, stabilized by proteins [1]. While the bean’s origin and roast are significant, the very journey the coffee cherry takes before roasting—its processing method—plays a pivotal, though often understated, role in shaping this essential characteristic.

Fermentation’s Fingerprint on CO2 and Acidity

Different processing methods involve varying degrees of fermentation, a biological process that can significantly alter the coffee’s chemical makeup. Research into fermentation protocols has shown that both aerobic and anaerobic treatments can positively influence parameters such as pH and acidity, and critically, the concentration of volatile compounds [2]. These volatile compounds, including those related to aroma and flavor, are intrinsically linked to the overall extraction dynamics and the potential for crema formation. A controlled fermentation, for instance, might optimize the retention of carbon dioxide within the bean, a key ingredient for generating robust crema [1]. Conversely, treatments that inhibit CO2 can lead to a less stable foam.

Pulping and Lipid Dynamics

The way coffee pulp is handled post-harvest can also influence crema. Studies comparing different coffee pulp wines, which represent byproducts of processing, reveal distinct flavor profiles influenced by various compounds like esters and alcohols [5]. While not directly studying crema, this highlights how processing can sequester or alter lipid and protein structures within the bean. Lipids, for example, are known to play a role in foam destabilization [1]. Processing methods that preserve or enhance certain lipid profiles might therefore impact how well the crema holds its structure. The distinction between Coffea arabica and Coffea canephora (robusta) co-products also shows differences in pH, with arabica co-products generally exhibiting lower pH values [3]. This variation in acidity can indirectly affect the protein denaturation and stabilization that contribute to crema.

The Influence of Water Activity and pH

Water activity, a measure of unbound water available for microbial growth and chemical reactions, varies significantly between different coffee co-products depending on their processing. Silverskin samples, for example, show considerably lower water activity (0.18-0.28) compared to cascara samples (0.39-0.64) [3]. This difference in water availability during stages before roasting can influence the degradation and alteration of key compounds. Furthermore, the pH of coffee co-products also differs notably between species, impacting the chemical environment in which crema-forming components are preserved or modified. A higher pH, for instance, might contribute to different protein interactions critical for bubble stability.

Species and Their Intrinsic Properties

While not strictly a processing method, the inherent characteristics of coffee species, which are often amplified or altered by processing, are fundamental. Pure Arabica, while richer in lipids, may be more susceptible to lipid-induced foam destabilization, suggesting that processing methods that balance lipid content or enhance stabilizing factors are crucial [1]. Robusta, often cited for its contribution to crema, generally possesses higher CO2 content, a direct benefit to foamability [1]. The interplay between these species-specific traits and the chemical modifications introduced by processing—whether fermentation, washing, or drying—dictates the ultimate crema quality.

In essence, the processing method is not merely a precursor to roasting but an active architect of the coffee bean’s internal architecture. By influencing CO2 retention, lipid profiles, and protein integrity through variations in fermentation, drying, and pulping, these methods lay the groundwork for the density, color, and longevity of the espresso’s signature crema.

References

[1] — Ernesto Illy, Luciano Navarini — Neglected Food Bubbles: The Espresso Coffee Foam. — 2011-Sep — https://pubmed.ncbi.nlm.nih.gov/21892345/ [2] — Gustavo Galarza, Jorge G Figueroa — Volatile Compound Characterization of Coffee ( — 2022-Mar-21 — https://pubmed.ncbi.nlm.nih.gov/35335365/ [3] — Katarína Poláková, Alica Bobková, Alžbeta Demianová, Marek Bobko, Judita Lidiková, Lukáš Jurčaga, Ľubomír Belej, Andrea Mesárošová, Melina Korčok, Tomáš Tóth — Quality Attributes and Sensory Acceptance of Different Botanical Coffee Co-Products. — 2023-Jul-11 — https://pubmed.ncbi.nlm.nih.gov/37509767/ [4] — Rongsuo Hu, Fei Xu, Xiao Chen, Qinrui Kuang, Xingyuan Xiao, Wenjiang Dong — The Growing Altitude Influences the Flavor Precursors, Sensory Characteristics and Cupping Quality of the Pu’er Coffee Bean. — 2024-Nov-28 — https://pubmed.ncbi.nlm.nih.gov/39682914/ [5] — Rongsuo Hu, Fei Xu, Liyan Zhao, Wenjiang Dong, Xingyuan Xiao, Xiao Chen — Comparative Evaluation of Flavor and Sensory Quality of Coffee Pulp Wines. — 2024-Jun-27 — https://pubmed.ncbi.nlm.nih.gov/38999011/ [6] — Samo Smrke, André Eiermann, Chahan Yeretzian — The role of fines in espresso extraction dynamics. — 2024-Mar-07 — https://pubmed.ncbi.nlm.nih.gov/38453983/