Modeling Mass and Heat Transfer in Multiphase Coffee Aroma Extraction - PubMed (original) (raw)
Modeling Mass and Heat Transfer in Multiphase Coffee Aroma Extraction
David Beverly et al. Ind Eng Chem Res. 2020.
Abstract
Instant coffee manufacture involves the aqueous extraction of soluble coffee components followed by drying to form a soluble powder. Loss of volatile aroma compounds during concentration through evaporation can lower product quality. One method of retaining aroma is to steam-strip volatiles from the coffee and add them back to a concentrated coffee solution before the final drying stage. A better understanding of the impact of process conditions on the aroma content of the stripped solution will improve product design stages. In this context, we present a multiscale model for aroma extraction describing (i) the release from the matrix, (ii) intraparticle diffusion, (iii) transfer into water and steam, and (iv) advection through the column mechanisms. Results revealed (i) the existence of three different types of compound behavior, (ii) how aroma physiochemistry determines the limiting kinetics of extraction, and (iii) that extraction for some aromas can be inhibited by the interaction with other coffee components.
Copyright © 2020 American Chemical Society.
Conflict of interest statement
The authors declare no competing financial interest.
Figures
Figure 1
Schematic diagrams of different scales in the system with key geometries: (a) whole column, with the labeled height (Z) inlet and outlet pressures (_P_in and _P_out) and column diameter (_d_bed); (b) column section, with the labeled bed pore size (_d_b,pore) and particle diameter (_d_part); (c) particle section with mesopores distributed in a nanoporous matrix all filled with water; and (d) mesopore with a labeled diameter (_d_pore) and oil thickness (δ).
Figure 2
Scanning electron microscope image of a roasted and ground coffee particle (magnification 1000×) showing the mesopores and cell walls, taken on a tabletop TM-1000 microscope (Hitachi, Tokyo, Japan).
Figure 3
Schematic diagrams of (a) the particle–water–gas transfer stage and (b) aroma release and diffusion into the coffee particles.
Figure 4
Comparison graphs between experimental data from Mateus et al. (solid line) and fitted model outputs (dashed line); acetaldehyde yield curves using (a) monodisperse coarse particles with εcw = 3.30% and (b) bidisperse coarse and fine particles using εcw = 2.76%.
Figure 5
Comparison of experimental data from Mateus et al. (solid line) and model outputs (dashed line) for acetic acid yield curves using Henry’s volatility constant values of _H_298 = 0.025 Pa mol–1 m3 with (a) 
and (b) 
(estimated).
Figure 6
Comparison between experimental data from Mateus et al. (solid line) and fitted (dashed line) pyridine extraction curve with the estimated binding rate constant of _k_on = 1.9 × 10–5 m3 s–1 mol–1.
Figure 7
(a) Normalized concentration and (b) normalized yields (both normalized to values after 20 min of steam stripping) of some key aromas in the distillate when simulating 20 and 40 min steam stripping.
Figure 8
Normalized concentration of aromas in the exiting steam, demonstrating the limiting factors in extraction (time shown is after the column reaches saturation temperature).
Figure 9
(a) Normalized concentrations and (b) yields (normalized to values obtained at a water-to-coffee ratio of 0.7) of some key aromas in the distillate when simulating water-to-coffee ratios of 0.7 and 1.4 prior to steam stripping.
Figure 10
(a) Normalized concentration and (b) yields (normalized to the values at a height-to-diameter ratio of 3.2) of some key aromas in the distillate when simulating bed height-to-diameter ratios of 3.2 and 0.9.
Figure 11
Contour plots of furaneol concentration in steam for columns of two height-to-diameter ratios (a) 3.2 and (b) 0.9. Data is normalized to the maximum concentration.
References
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