Validação de método para determinação das impurezas cascas e paus em café torrado e moído/Validation of a method to determine the impurities husks and straw in roast ground coffee

The determination of impurities in roast ground coffee is part of the analyses that contribute to ensuring the control of product quality. The aim of this study was to validate the methodological procedure of microscopy to detect and quantify the impurities husks and straw in roast ground coffee. The method is based on the principle of organic solvent extraction of the coffee fat, elimination of the fine powder by screening and sorting of the impurities husks and straw by stereoscopic microscopy. Seventy samples of roast ground coffee were collected in commercial establishments in Belo Horizonte - MG - Brazil and analysed in duplicate by three analysts, totalling 420 determinations, with the results expressed in percentages of the impurities husks and straw. The coefficients of variation under conditions of repeatability ranged from 7.0 to 27.9%, being considered satisfactory for microscopy. The analyst factor had a strong influence on the precision and accuracy of the method. The method was shown to be accurate at concentrations above 0.2% impurities. The accuracy of the method was satisfactory and the recovery in an acceptable range. The uncertainties varied with the impurity concentration range and tended to be inversely proportional. The method was considered suitable for the analysis of these impurities in roast ground coffee, and has the potential to be indicated for use in inspection procedures.