MODELING THE SOLUBILIZATION PROCESS DURING COFFEE BREWING

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.

Swelling properties of roasted coffee particles

BACKGROUND

In this study, the swelling behavior of roasted coffee particles in water and particularly its impact on particle diameter is examined by applying laser-diffraction analysis and microscopy. Several potential influencing factors are investigated: initial particle size, roasting degree, and temperature. Additionally, the time dependency of swelling and particle shape is evaluated at two different temperatures.

RESULTS

We verify that particle erosion occurs - as observed by an increase of the fine particle fraction after wetting - and it is revealed that this effect is more pronounced with a rise in temperature. The total relative increase in particle size is determined as approximately 15% based on a broad range of different sized coffee grounds. It is demonstrated that the degree of swelling is independent of both the initial particle diameter and the roasting degree. The particle shape is found to be unaffected by swelling. This research reveals that swelling is initially quick, with 60-80% of the final steady-state diameter being reached after 30 s and completed after 4 min of wetting, i.e. within the timescale of conventional coffee brewing methods.

CONCLUSION

This work provides a better understanding of the impact of wetting as part of the coffee brewing process, thus aiding the design, modeling, and optimization of coffee extraction. It clarifies the strong deviation of previous results on coffee-particle swelling by considering particle erosion and degassing and provides a robust method for quantification. © 2020 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Physiochemical Characteristics of Hot and Cold Brew Coffee Chemistry: The Effects of Roast Level and Brewing Temperature on Compound Extraction

The role of roasting in cold brew coffee chemistry is poorly understood. The brewing temperature influences extraction processes and may have varying effects across the roast spectrum. To understand the relationship between brew temperature and roast temperature, hot and cold brew coffees were prepared from Arabica Columbian coffee beans roasted to light, medium, and dark levels. Chemical and physical parameters were measured to investigate the relationships among degree of roast, water temperature, and key characteristics of resulting coffees. Cold brew coffees showed differential extraction marked by decreased acidity, lower concentration of browned compounds, and fewer TDS indicating that cold water brewing extracts some compounds less effectively than hot water brewing. Compounds in coffee did exhibit sensitivity to degree of roast, with darker roasts resulting in decreased concentrations for both hot and cold brew coffees. Total antioxidant capacity (TAC) was only sensitive to degree of roast in cold brew coffees, while hot brew coffees had a constant TAC for all three roast levels. This indicates that the solid bean matrix and its chemical constituents interact with cold water differently than with hot water. Surface wetting, pore dynamics, and solubility all contribute to the extraction potential during brewing and are all functions of water temperature.

Analysing extraction uniformity from porous coffee beds using mathematical modelling and computational fluid dynamics approaches

Achieving a uniform extraction of soluble material from a porous matrix is a generic problem in various separation and filtration operations, with applications in the food processing, chemical and pharmaceutical industries. This paper describes models of fluid flow and transport of soluble material within a packed granular bed in the context of coffee extraction. Coffee extraction is described by diffusion of soluble material from particles of one or more representative sizes into fluid flowing through the packed bed. One-dimensional flow models are compared to computational fluid dynamics (CFD) models. A fine and a coarse coffee grind are considered. Model results are compared to experimental data for a packed cylindrical coffee bed and the influence of a change in geometry to a truncated cone is considered. Non-uniform flow in the truncated cone causes significant variation in the local extraction level. Coffee extraction levels during brewing are analysed using extraction maps and the degree of variation is represented on the industry standard coffee brewing control chart. A high variation in extraction yield can be expected to impart bitter flavours into the brew and thus is an important variable to quantify.

Toward a New Brewing Control Chart for the 21st Century

This paper describes new results from a base model of brewing from a bed of packed coffee grains. The model solves for the diffusion of soluble species out of a distribution of particles into the flow through the bed pore space. It requires a limited set of input parameters. It gives a simple picture of the basic physics of coffee brewing and sets out a set of reduced variables for this process. The importance of bed extraction efficiency is elucidated. A coffee brewing control chart has been widely used to describe the region of ideal coffee brewing for some 50 years. A new chart is needed, however, one that connects actual brewing conditions (weight, flow rate, brew time, grind, etc.) to the yield and strength of brews. The paper shows a new approach to brewing control charts, including brew time and bed extraction efficiency as control parameters. Using the base model, an example chart will be given for a particular grind ratio of coarse to fine particles, and an "espresso regime" will be picked out. From such a chart yield, volume and strength of a brew can be read off at will.

Understanding the Effects of Roasting on Antioxidant Components of Coffee Brews by Coupling On-line ABTS Assay to High Performance Size Exclusion Chromatography

INTRODUCTION

Coffee is a widely consumed beverage containing antioxidant active compounds. During roasting the phytochemical composition of the coffee bean changes dramatically and highly polymeric substances are produced. Besides chlorogenic acids that are already present in green coffee beans, melanoidins show antioxidant capacity as well.

OBJECTIVE

To employ post-column derivatisation by coupling high performance size exclusion chromatography (HPSEC) to an antioxidant assay to investigate the effect of roasting on the properties of antioxidant active compounds in coffee brews.

METHODOLOGY

We have investigated the antioxidant capacity of Coffea arabica (Arabica) and C. canephora (Robusta) beans that were roasted over the full spectrum of roast conditions (four roasting speeds to three roast degrees) by comparing the results from HPSEC coupled on-line to the ABTS assay with those from two batch assays, Folin Ciocalteu (FC) and oxygen radical absorbance capacity (ORAC) assay.

RESULTS

The antioxidant capacity showed a general decrease towards slower and darker roasted coffee for all three assays, indicative of heat degradation of active compounds. Hence, low molecular weight (LMW) compounds such as chlorogenic acids (CGAs) decreased progressively already from relatively mild roasting conditions. In contrast, high molecular weight (HMW) compounds (e.g. melanoidins) increased from light to dark roast degrees with lowering magnitude towards slower roasting profiles.

CONCLUSION

By coupling HPSEC on-line to the ABTS assay we were able to separately quantify the contribution of HMW and LMW compounds to the total antioxidant capacity, increasing our understanding of the roast process. © 2016 The Authors. Phytochemical Analysis Published by John Wiley & Sons Ltd.

Coffee extraction kinetics in a well mixed system

The extraction of coffee solubles from roasted and ground coffee is a complex operation, the understanding of which is key to the brewing of high quality coffee. This complexity stems from the fact that brewing of coffee is achieved through a wide variety of techniques each of which depends on a large number of process variables. In this paper, we consider a recent, experimentally validated model of coffee extraction, which describes extraction from a coffee bed using a double porosity model. The model incorporates dissolution and transport of coffee in the coffee bed. The model was shown to accurately describe extraction of coffee solubles from grains in two situations: extraction from a dilute suspension of coffee grains and extraction from a packed coffee bed. The full model equations can only be solved numerically. In this work we consider asymptotic solutions, based on the dominant mechanisms, in the case of coffee extraction from a dilute suspension of coffee grains. Extraction in this well mixed system, can be described by a set of ordinary differential equations. This allows analysis of the extraction kinetics from the coffee grains independent of transport processes associated with flow through packed coffee beds. Coffee extraction for an individual grain is controlled by two processes: a rapid dissolution of coffee from the grain surfaces in conjunction with a much slower diffusion of coffee through the tortuous intragranular pore network to the grain surfaces. Utilising a small parameter resulting from the ratio of these two timescales, we construct asymptotic solutions using the method of matched asymptotic expansions. The asymptotic solutions are compared with numerical solutions and data from coffee extraction experiments. The asymptotic solutions depend on a small number of dimensionless parameters, so the solutions facilitate quick investigation of the influence of various process parameters on the coffee extraction curves.

Release kinetics of volatile organic compounds from roasted and ground coffee: online measurements by PTR-MS and mathematical modeling

The present work shows the possibilities and limitations in modeling release kinetics of volatile organic compounds (VOCs) from roasted and ground coffee by applying physical and empirical models such as the diffusion and Weibull models. The release kinetics of VOCs were measured online by proton transfer reaction-mass spectrometry (PTR-MS). Compounds were identified by GC-MS, and the contribution of the individual compounds to different mass fragments was elucidated by GC/PTR-MS. Coffee samples roasted to different roasting degrees and ground to different particle sizes were studied under dry and wet stripping conditions. To investigate the accuracy of modeling the VOC release kinetics recorded using PTR-MS, online kinetics were compared with kinetics reconstituted from purge and trap samplings. Results showed that uncertainties in ion intensities due to the presence of isobaric species may prevent the development of a robust mathematical model. Of the 20 identified compounds, 5 were affected to a lower extent as their contribution to specific m/z intensity varied by <15% over the stripping time. The kinetics of these compounds were fitted using physical and statistical models, respectively, the diffusion and Weibull models, which helped to identify the underlying release mechanisms. For dry stripping, the diffusion model allowed a good representation of the release kinetics, whereas for wet stripping conditions, release patterns were very complex and almost specific for each compound analyzed. In the case of prewetted coffee, varying particle size (approximately 400-1200 microm) had no significant effect on the VOC release rate, whereas for dry coffee, the release was faster for smaller particles. The absence of particle size effect in wet coffee was attributed to the increase of opened porosity and compound diffusivity by solubilization and matrix relaxation. To conclude, the accurate modeling of VOC release kinetics from coffee allowed small variations in compound release to be discriminated. Furthermore, it evidenced the different aroma compositions that may be obtained depending on the time when VOCs are recovered.