Direct determination of ethanol in all types of alcoholic beverages by near-infrared derivative spectrometry
A rapid and accurate method has been developed for the direct determination of ethanol in all types of alcoholic beverages. The method, which does not require any sample treatment (except for simple de-gassing for beer samples or dilution with distilled water for spirits), is based on the use of the first derivative of the near-infrared absorbance spectrum. Measurements, carried out between the 1680 nm peak and the 1703 nm valley, provide a typical calibration graph [dA/dλ= 0.00002+ 0.0127 c(where c is the concentration of ethanol in % v/v)] for a concentration range up to 25% v/v with a regression coefficient of 0.999992 and a limit of detection (for k= 3) of 0.1% v/v. The interference of sugars in the determination of ethanol can be seen by the presence of bands in the derivative spectrum in the range between 1300 and 1800 nm and can be corrected. Results comparable to those found by a reference pycnometric procedure or by gas chromatography were obtained when the proposed method was applied to the determination of ethanol in beer, white and red wines, whisky, gin and rum samples and also in sweet wines and fruit liqueurs.