Quantitative Determination of Ethanol in Wine by Gas

A method is described for specific quantitative analysis of ethanol in wine by gas chromatography. This method, which uses an internal standard and flame ionization detector, is more accurate and more precise than methods commonly used. Statistical analysis of data from typical winery samples shows that the standard error of estimate for the method is 0.07~/~ (v/v) over the range 7 to 24% (v/v) . The estimate of the standard deviation of the chromatography for duplicate injections is 0.02% (v/v) . The gas-liquid chromatography method determines ethanol separately from the other wine components that interfere in other methods, and without distillation or chemical reaction. When large numbers of samples are analyzed, advantages include short analysis time per sample and potential for extensive automation. Determination of ethanol is perhaps the most important routine analysis in a modern winery. Frequent, fast, and accurate results are needed to control the quality of the wine from grape to bottle, as well as for state and federal government tax and regulatory purposes. The alcoholic beverage industry and various regulatory agencies have devoted much effort in recent years to developing a faster, specific, more accurate, and automated method. Gas-liquid chromatography is one of the most modern analytical techniques, dating from 1952. Even more recently, with the utilization of electronic digital data-processing equipment, gas chromatography has become increasingly preferred for accurate quantitative as well as qualitative analyses of many substances. Traditional ethanol analysis methods, involving distillation and/or mass determinations, are known to include small inaccuracies due to the presence of interfering volatile or other components in wine (12). With an appropriately chosen column packing, gas chromatography is inherently specific, separating volatile compounds on the basis of compoundspecific partit ioning properties between a gas phase and a liquid (or a solid) phase. To evaluate the accuracy, precision, and specificity of the gas-liquid chromatography method described here, it was necessary to analyze ethanol standards and typical winery samples by a reference method. The reference method used was the micro-dichromate procedure (9). Because of the high accuracy of the GC method, it was also necessary to develop a procedure for estimating the magnitude of the inaccuracy in the dichromate method caused by non-ethanol wine volatiles. The most significant non-ethanol wine components analyzed as "ethanol" by the dichromate method were determined to be methanol and the common fusel oils. Wine industry researchers were among the early investigators of quantitative gas chromatography (GC) techniques, presenting data on ethanol analysis in 1961 (3). A 1961 industry paper included data on a relatively sophisticated GC ethanol analysis method utilizing an internal standard and electronic integrator for peak quantitation (8). A variety of GC methods for the analysis of ethanol are described in a recent review article by Jain and Cravey (6). Recent papers by Martin and Tenenbaum (7), Solon et al (10), and Goodman and Dorion (5) describe gas-solid chromatographic methods using the newer porous polymer column packings. Despite the extensive research on GC ethanol analysis in the past 15 years, no published method has been refined to the high accuracy and precision of the method described here. A nonlinearity in GC detector response to ethanol has been the main obstacle to high accuracy in previous work. Additional inaccuracies can be introduced into a GC method by inappropriAmer. J. Enol. Viticult., Vol. 25, No. 4, 1974