Abstract
Sorption isotherms represent an efficient and valuable tool for predicting the equilibrium moisture content of foods under different humidities and temperatures; thus, they are useful for determining shelf-life and safe storage conditions. The aims of this work were to determine the sorption isotherms of parchment specialty coffee at water activity values of 0.1−0.8 and temperatures of 25, 30, and 40 °C using the dynamic dew point method. The experimental sorption data were modeled using 12 different equations to represent the dependence of equilibrium moisture content on water activity and temperature. Thermodynamic properties were also obtained from the experimental data. The results showed a type II sigmoid shape according to Brunauer-Emmett-Teller (BET) classification, and the double log polynomial (DLP) equation successfully modeled the effects of temperature on the sorption isotherms, obtaining a good fit (R2adj = 0.99 and RMSE = 0.1 % dry basis). The dynamic dewpoint isotherm (DDI) method was advantageous for modelling due to its high availability of experimental data. Thermodynamic analyses showed that the net isosteric heat of sorption, Gibbs free energy, and sorption entropy decreased as equilibrium moisture content increased, and the compensation theory provided evidence that the sorption process was controlled by enthalpy (Tβ > Thm). Key words: Water activity; equilibrium moisture content; hygroscopicity; non-spontaneous; molecular ordering; compensation theory.
Highlights
Coffee is one of the largest globally traded commodities and basic products exported by developing countries (Velasquez et al, 2018)
The aims of this study were to: 1) determine sorption isotherms at temperatures of 25, 30, and 40 °C and water activities of 0.1–0.8 using the dynamic dewpoint isotherm (DDI) method; 2) evaluate the ability of mathematical models to represent the dependence of the sorption isotherms on water activity and temperature; and 3) determine the net isosteric heat of sorption, Gibbs free energy, sorption entropy, and enthalpy-entropy compensation of dried coffee beans
2.1 Coffee samples Seven specialty coffee samples of Colombia and Castillo variety (Coffea arabica L.), from different farmers from the Huila region of Colombia, were wet processed, and underwent sensory analysis according to the Specialty Coffee Association of America (SCAA) methodology (SCAA, 2015) by four expert panelists at the South Colombian Coffee Research Center (CESURCAFÉ) (Di Donfrancesco; Gutierrez; Chambers, 2014)
Summary
Coffee is one of the largest globally traded commodities and basic products exported by developing countries (Velasquez et al, 2018) Due to their sensorial attributes, including aroma, body, and reputation, coffees, such as Colombian coffee, have become highly differentiated products (Özdestan et al, 2013). Specialty coffees are characterized as being products with outstanding quality attributes and are generally destined for specialized markets where their prices can reach up to six times those of traditional coffees (Tapiero; Trujillo; Gutiérrez, 2017). To this end, preservation of these desirable sensory qualities is significant and highly dependent on the storage conditions of the coffee (Borém et al, 2013). An important indicator of coffee quality is moisture content, especially in Colombia, where this parameter is difficult to control during storage due to the high humidity of the Central Mountain Range where the majority of Colombian coffee is grown (Donovan; Foster; Parra, 2019)
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