Cyclically pressurized extraction of solutes from ground coffee: Kinetic experiments and modeling
Introduction
Coffee is nowadays the second most important commodity worldwide, only behind petroleum [1], [2]. Brazil is the world-leading producer and exporter of green coffee beans [3], [4], but the role it plays in the global market of soluble coffee, where it still ranks first in terms of export [3], [4], deserves attention.
The main reason for such a particular concern is that consumption of soluble coffee has increased at rates higher than those for roasted ground coffee. In fact, approximately a half of the coffee available for market around the world is currently used as raw material for production of soluble coffee [1], [5]. In spite of some domestic problems, such as a number of only 7 Brazilian soluble coffee manufacturers [6] and an insufficient production of Robusta coffee used in a high proportion when compared to Arabica to produce soluble coffee [7], the Brazilian exports of soluble coffee raised from 3,120 in 2009/10 to 3,486 thousand 60 kg bags in 2012/13 [3].
From a technological point-of-view, an approach to keep the Brazilian leadership in such a sector is to improve the quality and reduce the costs involved in the manufacture of soluble coffee. In other words, it means to replace the conventional method of solute extraction from coffee ground by a better procedure in terms of these responses. Leaching by percolation through beds of ground coffee is the primary technique currently applied in the manufacture of instant (soluble) coffee [8], [9]. Because the temperature is always an important factor on extraction [10], [11] percolation takes place in a temperature range from 100 to 200 °C [8], [9], [12]. So, to keep the water in a liquid phase, the battery of extraction columns are pressurized up to about 1550 kPa. At these circumstances the yield of solute extraction varies approximately between 30% and 46% [8], [12] and the time of extraction is not higher than 1800 s [13]. However, the benefit of the high temperatures on the yield and time of solute removal has expected adverse effects on energy consumption, as well as on the aromatic properties of the end product [14]. Moreover, the high pressure for pressurized liquid extraction (PLE) has a detrimental influence on the capital cost [15]. It is important to remember that units to recovery volatiles compounds and avoid the production of poor flavor instant coffee are commonly found in large-size plants [8], but they have no negligible impact on costs.
In this framework, the kinetics of extraction of coffee solubles at low temperature by involving an innovative technique is the main aspect to be currently investigated. Among the alternative extraction techniques available in the literature, such as pulsed electric field assisted [16], ultrasound assisted [17], and supercritical fluid extraction [18], one that allows for high yields at short time, moderate pressures and ambient temperature is the cyclically pressurized extraction (pulsed hydrostatic pressure extraction or hydrostatic pressure cycling extraction) [19], [20]. Based on these believed advantages, which should contribute to improve the quality of the extract and reduce the costs of production and investment, cyclically pressurized extraction with water was applied to remove total solutes from ground coffee. The influence of pressure and cycles of pressure on the yield and time of extraction was considered, and the optimal results in terms of these responses were compared with those obtained by the conventional method of manufacture of instant coffee. Experiments for extracting coffee lipids were also performed under cyclic pressurized conditions with water and chloroform, mainly to check whether it is able or not to extract insoluble compounds, an important claimed advantage of this innovative method of extraction [21].
Section snippets
Experiments
A fraction of a package of commercial ground coffee in the size range between 4.17 × 10−4 and 4.95 × 10−4 m was used in all the extraction experiments. It represents the material retained on a 35 mesh screen in a size analysis made with a set of only two screens of the Tyler standard series (32 and 35 mesh) [22]. The moisture content of the solid determined in triplicate by oven drying at 105 °C for 24 h was 5.4 ± 0.1% in dry basis [23]. Such a preliminary characterization of the starting solid material
Kinetics of aqueous extraction of total solutes at constant pressurized condition
Fig. 1 shows the experimental and calculated extraction curves at all the examined pressures higher 91.4 kPa. In summary, two important aspects emerge from it: (i) soluble removal takes place in a constant-rate and in a falling-rate period, as early evidenced in several kinetic studies of solid–liquid extraction (e.g., [32]); (ii) the suggested diffusion model given by Eq. (5) is able to explain the change of solute mass fraction in the solid and aqueous phase as time passes (R2 = 0.97–0.99 and ɛ =
Conclusions
The yield of coffee solute extraction under cyclic pressurization at only 17 °C and 338.2 kPa was higher than 26%, which is 6% and 20% lower than the analogous responses obtained industrially by percolation at 100 °C (≈101.3 kPa) and 200 °C (≈1500 kPa) [8], [12], respectively. The time to reach 95% of the yield at equilibrium also rises from 1800 s [13] in real-size plants of soluble coffee production to approximately 11,160 s by cyclically pressurized extraction at the aforementioned examined
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