Derivative Fourier transform infrared spectrometric determination of ethanol in beers

Abstract

A direct derivative Fourier transform infrared (FTIR) spectrometric procedure was developed for the determination of ethanol in beer samples. The method can be applied to the analysis of all types of beers, from regular to low-alcohol beers, and only requires a previous de-gassing of the samples as a preparation step. For the analysis of regular samples, with an alcohol content higher than 1% v/v, the method involves the use of the second-derivative FTIR measurements between the baseline, established from 1055 to 1037 cm^–1, and the valley at 1046 cm^–1 using a micro flow cell with ZnSe windows and a spacer of 0.029 mm after the accumulation of 10 scans. For the analysis of low-alcohol beers with less than 1% v/v of ethanol, the use of the first-derivative FTIR measurements, between the peak at 1052 cm^–1 and the valley at 1040 cm^–1, is more convenient than the use of the second-derivative measurements. However, in this instance the measurements must be corrected, taking into account the presence of maltose, which can be quantified by measuring the height between the peak at 1160 cm^–1 and the valley at 1144 cm^–1 in the first-derivative spectra. In the analysis of all types of beers, aqueous solutions of ethanol can be used as standards. The limit of detection of the proposed procedure is 0.025% v/v and typical relative standard deviations of 0.8 and 1% were obtained for five independent analyses of real samples containing 5 and 1% v/v of ethanol, respectively. The aforementioned procedures were applied to the analysis of real samples and the results obtained were coincident with those obtained by a reference method.