A spot test for total esters determination in sugarcane spirits exploiting smartphone-based digital images

Cachaça is an alcoholic beverage produced from sugarcane, whose flavor and taste are related to the esters content, usually expressed as equivalent to ethyl acetate. The official method for the determination of specific esters in cachaça is based on gas chromatography with a flame ionization detector, whereas a volumetric procedure is recommended for determining the total content. Because of the high analytical demand, faster and more practical analytical procedures are required for quality control of the product. The aim of this work was to develop a spot test exploiting smartphone-based digital images for in situ determination of total esters in cachaça. The procedure was based on the reaction of the analytes with hydroxylamine, generating the corresponding hydroxamate ions, which form violet complexes with Fe(iii) in an acidic medium. Digital images were acquired under controlled illumination and converted to RGB values using the PhotoMetrix® 1.8 application. The values of channel B were taken as the analytical response because of the complementarity with the color of the reaction product. A linear response was obtained within 100-500 mg L^-1 ethyl acetate, with the coefficient of variation (n = 10) and limit of detection (99.7% confidence level) estimated at 1.1% and 30 mg L^-1 ethyl acetate, respectively. The procedure consumes only 1.4 mg NH₂OH·HCl and 115 μg Fe(iii) and generates only 900 μL of waste per determination. The results of the proposed procedure agreed with those obtained by the reference volumetric method at the 95% confidence level.