Análise direta de grãos de café torrados e moídos por fluorescência de raios X portátil

Abstract

Agribusiness is an economic sector that has been strengthened in the country with a direct impact on increased productivity and food exportation. Coffee is one of the most consumed foods on a daily basis, which can be grown in different agricultural regions presenting peculiar characteristics derived from the places where the beans are harvested. Therefore, the determination of inorganic nutrients in roasted and ground coffee beans ready for human consumption has nutritional relevance, as well as for the establishment of analytical protocols that allow tracking the origin of the product from their chemical composition. Analytical methods commonly used in routine laboratories to determine the content of inorganic nutrients in food samples are based on time- consuming sample preparation steps that involve acid digestion followed by elemental quantification. In this study, analytical strategies for the determination of inorganic macronutrients (P, K, Ca, and Mg) in commercial samples of roasted and ground coffee were evaluated by portable X-ray Fluorescence Spectrometry (XRF) using two commercial equipment. Two calibration strategies were tested: i) construction of calibration models using plant-based certified reference materials (CRMs); ii) matrix- matched calibration, using samples of whole roasted and ground coffee beans with the corresponding solid residues resulted from infusion. The analyte mass fractions in these samples were determined by the reference method (acid decomposition in digester block using HNO 3 and H 2 O 2 followed by ICP OES determination). In this method, calibration models were built using reference values ​​and analytical signals obtained by portable instruments. Pressed pellets from the samples (CRMs, roasted and ground coffee beans and infusion residues) were prepared. The samples were irradiated for 50 s using both equipment under two experimental conditions: X-ray tube voltage of 15 kV and 50 kV. Calibration models exhibiting excellent linearity were obtained (r ≥ 0.90) for all analytes investigated. The proposed method follows the principles of Green Analytical Chemistry, since there is no generation of toxic residues and can be used in routine laboratories for the determination of macronutrients (mainly P and K) in samples of roasted and ground coffee beans and in the residues generated by the infusion, an amendment that can be used as agricultural fertilizer.