Exploring the link between coffee matrix microstructure and flow properties using combined X-ray microtomography and smoothed particle hydrodynamics simulations.

Coffee extraction involves many complex physical and transport processes extremely difficult to model. Among the many factors that will affect the final quality of coffee, the microstructure of the coffee matrix is one of the most critical ones. In this article, we use X-ray micro-computed (microCT) technique to capture the microscopic details of coffee matrices at particle-level and perform fluid dynamics simulation based on the smoothed particle hydrodynamics method (SPH) with the 3D reconstructured data. Information like flow permeability and tortuosity of the matrices can be therefore obtained from our simulation. We found that inertial effects can be quite significant at the normal pressure gradient conditions typical for espresso brewing, and can provide an explanation for the inconsistency of permeability measurements seen in the literature. Several types of coffee powder are further examined, revealing their distinct microscopic details and resulting flow features. By comparing the microCT images of pre- and post-extraction coffee matrices, it is found that a decreasing porosity profile (from the bottom-outlet to the top-inlet) always develops after extraction. This counterintuitive phenomenon can be explained using a pressure-dependent erosion model proposed in our prior work. Our results reveal not only some important hydrodynamic mechanisms of coffee extraction, but also show that microCT scan can provide useful microscopic details for coffee extraction modelling. MicroCT scan establishes the basis for a data-driven numerical framework to explore the link between coffee powder microstructure and extraction dynamics, which is the prerequisite to study the time evolution of both volatile and non-volatile organic compounds and then the flavour profile of coffee brews.

Structure of arabinogalactan oligosaccharides derived from arabinogalactan-protein of Coffea arabica instant coffee powder.

Arabinogalactan-protein, previously isolated from instant coffee powder of Coffea arabica, has been subjected to partial mild acidic and enzymatic hydrolyses. Separation of obtained mixtures by size exclusion and HPLC chromatographies afforded series of oligosaccharides, structure of which were studied by NMR spectroscopy. Mild acidic hydrolysis afforded oligosaccharides without any αAraf substituent while after enzymatic hydrolysis αAraf was found in di-, tri-, and tetrasaccharides. In all cases αAraf was a terminal substituent linked separately to O3, O6, and to both, O3 and O6, of ßGal residues. Identification of di-, tri-, and tetrasaccharides containing α-Araf enabled to distinguish in the (1)H NMR spectra αAraf signals linked to O6 and O3 of neighboring ßGal unit. Composition of polymeric residues after enzymatic and mild acidic hydrolyses was also analyzed.