Static Electricity and Coffee Grinding: Exploring the Electrical Phenomenon Behind the Cup's Flavor

Coffee grinding is a crucial part of the coffee preparation process. The precision of grinding, the distribution of particles and their uniformity directly determine the extraction effect and the flavour of the coffee in the cup. During the grinding process, the electrostatic effect is triggered by the friction between the coffee particles and the grinding blades. This Triboelectric Effect, which is the result of charge transfer between material surfaces during contact and separation, is one of the classic topics of extensive research in materials science.

During the coffee grinding process, electrostatic effects are usually manifested as mutual attraction between particles and between particles and the surface of the equipment, leading to powder aggregation, increased grinder residue and uneven brewing. Since the electrostatic effect is closely related to the chemical properties of the material surface, understanding the source of electrostatic force and its mechanism of action in the grinding process is important for improving the design and brewing effect of coffee grinders.

Frictional charging usually occurs when two different material surfaces repeatedly come into contact and separate. When two surfaces come into contact, the difference in electronegativity between the materials leads to a transfer of electrons, with one material surface gaining electrons (negative charge) and the other losing electrons (positive charge). During the coffee grinding process, the contact between the coffee particles and the grinding blade (e.g. stainless steel or ceramic) leads to a significant charge build-up. Different roasting depths and coffee bean moisture content have a significant effect on the generation of static electricity. Typically, lightly roasted coffee beans are more likely to be positively charged, while deeply roasted beans are more likely to be negatively charged.

Measurements of the electrical charge of ground coffee powder from different grinders (e.g. Mahlkonig EK43) have revealed that up to tens of nanocoulombs (nC/g) of electrostatic charge can be accumulated per gram of coffee powder. Such high charge densities are even comparable to the amount of static electricity found in thunderstorm clouds or volcanic eruption smoke columns. Although the electrostatic effect is not usually dangerous, it can have a profound effect on the distribution of grinds, particle aggregation, and water flow paths during the brewing process, especially in the preparation of espresso.

Traditionally, grounding has been the primary means of eliminating static electricity. However, since coffee powder is an electrical insulator, charge transfer between particles is not easy. As a result, grounding alone is not sufficient to completely eliminate the charge. To meet this challenge, LeBrew burrs uses a unique AiTiN coating technology. This electrically conductive coating effectively neutralises the build-up of static charges by forming a thin film of highly conductive material on the surface of the grinding discs, thereby significantly reducing the attraction of particles to each other during the grinding process and preventing particle aggregation and agglomeration. LeBrew's AiTiN coating also significantly reduces the amount of residue when grinding fine powders compared to traditional materials and optimises particle distribution to improve grinding consistency.

Future mills can be designed to reduce electrostatic effects through the use of antistatic materials or special chamber geometries. For example, the use of grinding blades with antistatic coatings or the design of charge-guided channels in the grinding chamber (similar to the static dissipation devices on aircraft wings) can effectively reduce particle retention and adhesion, thus improving the consistency of the grind. leBrew is actively exploring this area, having introduced a high-end blade series that already applies charge-neutralisation technology based on AiTiN coatings, and is planning to further optimise the blade geometry to minimise the impact of static charge on coffee powder distribution and brewing performance.

Although the electrostatic effect is often seen as unfavourable, it could also be a new tool for monitoring coffee quality. Since frictional electrification relies on small changes in the chemical properties of material surfaces, it is possible to infer the chemical characteristics of coffee beans during cultivation, processing and roasting by measuring the charge characteristics during grinding. For example, the uniformity and defects of coffee beans can be monitored by comparing the change in charge during grinding of different batches of coffee beans.

The electrostatic effect has a significant impact in the coffee grinding process, and understanding its generation mechanism and its influence on particle distribution and brewing performance is crucial for optimising coffee grinding equipment design. Future research could further explore the application of the electrostatic effect in coffee quality monitoring and develop new grinding equipment based on electrostatic neutralisation. This will not only enhance the consistency of coffee preparation and flavour performance, but may also provide new technical tools and quality control methods for the coffee industry.

[1]  Joshua Mendez Harper, Connor S. McDonald, Elias J. Rheingold, Lena C. Wehn, Robin E. Bumbaugh, Elana J. Cope, Leif E. Lindberg, Justin Pham, Yong-Hyun Kim, Joseph Dufek, and Christopher H. Hendon, “Moisture Controlled Fracto- and Triboelectrification During Coffee Grinding,” Matter (2023）

[2]  Uman et al., “The Effect of Bean Origin and Temperature on Grinding Roasted Coffee,” Scientific Reports 6 (2016)