Extração sólido-líquido dos grãos de café verde e da torta da prensagem: estudo de solventes alternativos

Abstract

This work aimed to evaluate the feasibility of the extraction of oil, soluble solids and bioactive compounds from Arabica green coffee beans and its press cake obtained from the mechanical pressing processs, using alternative solvents (pure or mixtures) and methods. The materials, green coffee beans and cake, were physically and chemically characterized. The results showed that even after oil extraction by pressing, the cake still contains substantial content of soluble solids and bioactive compounds. After being characterized, the raw materials were subjected to extraction at room temperature, in order to quantify bioactive compounds and to compare the performance of the solvent ethanol and the methanol-acetone mixture to extract these compounds. The results showed that the solvent mixture methanol-acetone presented better performance to extract antioxidant compounds, which were found in greater quantity in the cake. Further, the oil yield (or soluble solids), bioactive compounds and antioxidants were evaluated through the analysis of total phenolic compounds (TPC), ABTS, DPPH, FRAP and beta-carotene bleaching assay (BCBA) of the green coffee beans and the cake using the Soxhlet type extractor; also, alternative solvents (acetone, ethyl acetate, ethanol and isopropanol) and traditional solvents (hexane and petroleum ether) were studied in different extraction times (3 and 5 h). The results showed that ethanol was the solvent that presented the best yie ld of soluble solids, antioxidants and phenolic compounds of both materials, except for the BCBA antioxidant analysis, which presented better results when ethyl acetate was employed. It was also studied the yield of the solid-liquid extraction with pure solvents (ethanol, acetone and ethyl acetate) at different temperatures (35 to 55 C), as well as the thermodynamic parameters of the process. It was observed that high temperatures positively affected the yield of the process, and that ethanol and acetone solvents presented better performance. The thermodynamic analysis of the processes showed that the ΔH and ΔS were positive, and ΔG were negative, indicating that the processes were endothermic and spontaneous. Once the behavior of the pure solvents was studied, two solvents (acetone and ethyl acetate) were selected to be used as cosolvents in the process of optimization of the ethanolic extraction of beans and cake. The solid-liquid extraction was carried out in a stirring bath with temperature control. The Rotational Central Composite Design (DCCR) was used to plan the experiment, where the independent variables were the concentration of cosolvents (acetone and ethyl acetate, 0 to 40%) in ethanol and temperature (35 to 55 C), and the variables responses were the yield and retention index. It was observed that the temperature and the addition of acetone had a positive effect on the process yield of both materials, and for the cake, the addition of ethyl acetate also influenced this response variable. The temperature also influenced the retention index of the extraction process of both materials, however, this variable response could not be represented by an empirical model. The study showed that the process of extracting soluble solids from the press cake is more viable than for the coffee beans. As an alternative method, the solid-liquid extraction process of the green coffee beans (20 kHz, 400 W, 3 mm diameter tip) was performed, where the same parameters of the optimization performed for the stirred bath were used, except the temperature, instead, the variable time was used as an independent variable (5 to 60 min). The desirability function was used to simultaneously optimize the r esults of the analyzed response variables (yield and retention index). The optimum regions were 16.41 to 31.45% for acetone, 31.45 to 40% for ethyl acetate and 54.44 for 60 min for the time. The results showed that the extraction time was a parameter with high influence on the recovery of soluble solids and the retention index of the solid-liquid extraction process of green coffee beans. By means of the results obtained, it can be demonstrated that the solid-liquid extraction process using green solvents is feasible for the extraction of oil or soluble solids from green coffee beans and its cake obtained from the mechanical pressing of the beans. Thus, solid-liquid extraction could be used as a second extraction step to remove solids from the cake, since the pressing process was shown to have low efficiency (< 30%) of oil yield as well as for the extraction of bioactive compounds.