Stopped-flow near-infrared spectrometric determination of ethanol and maltose in beers

Abstract

A near-infrared spectrometric procedure has been developed for the determination of ethanol in beer samples, based on the measurement of the ethanol absorbance maximum at 1693 nm above a base-line established between 1657 and 1720 nm, employing an 1 cm pathlength cell, and using a 4.5% (w/v) aqueous solution of maltose as a reference. For the analysis of real samples the method only requires a previous degassing of the samples, by filtration, and aqueous solutions of ethanol can be employed as standards. All types of beers, from regular beers containing about 5% (v/v) ethanol to low-alcohol beers with less than 1% (v/v) can be analyzed by using the same procedure. The method has a limit of detection of 0.07% (v/v) and a relative standard deviation of 0.2% for regular beer samples, and less than 2% for low-alcohol beer analysis. The use of the stopped-flow strategy provides a fast filling and cleaning of the measurement cells and a sample frequency of 120 injections per hour. Maltose can also be determined by additional measurement at ca. 1410 nm.