Ultrafast extraction of cold brew coffee in a planetary rotating bed reactor: A kinetic study on the pushback effect

Given the rising demand for cold brew coffee, innovative approaches are required to address the present difficulties in its production. This motivates the consideration of the planetary rotating bed reactor (PRBR) as a solid–liquid extraction technology. The PRBR consists of cylindrical mesh chambers that contain the solid phase and move in two superimposed rotations. This motion pattern enhances mass transfer through alternating radial forces: the pushback effect (PBE). To evaluate the utility of the PRBR and the role of PBE, we investigated the kinetics and energy demand of cold brew coffee extraction. For this purpose, we varied rotational speeds and PBE conditions in a 1 L lab-scale reactor. Kinetics was determined by in-line monitoring of electrical conductivity correlated with the eluate's weight-based total dissolved solids and caffeine by high-performance liquid chromatography. Energy consumption was derived from strain gauge torque measurements. Our findings reveal that saturation of the extraction rate with respect to the rotational speed is evident both with and without PBE. However, PBE significantly reduces extraction time by 75% while requiring 60% less mechanical energy. The fastest conditions probed 95% of the maximum extraction yield within 12 s, effectively being >99% faster than static immersion. We believe these findings, along with further procedural advantages, qualify the PRBR as a viable technology for cold brew coffee production. Practical Applications: The relevance of an economical and sustainable preparation of cold brew coffee extends from the industrial to the gastronomic to the domestic environment. Challenges and limitations relate to process time, particle separation, product and extraction yield, and taste problems due to over-extraction. The planetary rotating bed reactor (PRBR) promises to address all these issues by combining extraction and separation in a simultaneous and ultrafast process. The present study confirms the superiority of PRBR's action mechanisms in terms of rapidity and energetic efficiency for extraction. Due to the excellent scalability of both the design and the physical effect, the PRBR is intended for large-scale production of ready-to-drink products and local production in coffee shops or private households. By modifying the particle feed, even continuous operation is conceivable. Further applications are, for example, dry-hopping of beer, fast-aging of spirits, and preparation of various liquid foods.