Equilíbrio higroscópico da casca do café arábica

Abstract

Brazil is the largest producer and exporter of coffee. The process of industrialization provides large amounts of waste from coffee beans (about 50% of total mass), and the coffee husks are the main residue from dry method process. As hygroscopic materials, the coffee husks have the ability to desorbs or absorb ambient water, trying to maintain a balanced relationship between their moisture content and the relative humidity of air. Even with low moisture content, the coffee husks are subject to few research studies of hygroscopic behavior. The objective was to evaluate the hygroscopic behavior of coffee husks at 25 ° C using the gravimetric static method with saturated salt solutions and select the mathematical models that describe more precisely the adsorption isotherms of arabica coffee husks. The mathematical models used were BET, GAB, Kühn, Kühn simplified, Lewicki, Halsey and Peleg. To adjust the mathematical models, nonlinear regression analysis was performed by the Gauss-Newton method, using the computational program Statistica 7.1. During the experiment under controlled relative humidity, it was also evaluated the caffeine content for each point of gravimetric equilibrium. All samples maintained in environments saturated with NaCl and KCl solutions and relative humidity 75 and 84%, respectively, was checked visually the microbial growing, and therefore disregarded for obtaining the isotherm. From the equations of several parameters used for adjusting the relative humidity data for coffee husks and calculating coefficient values (R²) percentage error (P) and standard errors (SE), it was found that the models selected had satisfactory adjustments to the experimental data, with the best fit obtained by the BET model. The isotherm obtained for the studied temperature was characterized by the behavior type II. The caffeine levels found ranged from 72,95 to 100,88 mg.100g-1, with an average content of 86,67 mg.100g-1 peel.