Gas chromatographic quantitative analysis of the gaseous emissions and carbon balance in the alcoholic fermentation

Abstract

The balance of the carbon transfers occuring during the fermentation of sugar is still poorly defined. To our knowledge, no description exists which accounts at each moment of the process for the components in the liquid phase and the evolution of the gaseous carbon components. However, this question is of prime importance for the vinification process. The gap between the concentration of fermentable sugars in a must and the theoritical potential alcoholic degree is always significant. The same is true for chaptalisation, in which vinters generally use more sugar than needed to obtain a given alcoholic degree. In both cases, the question is : what happens to the carbon matter lost? Is it transformed into liquid and/or gaseous metabolites or simply evaporated as ethanol or as other volatile components?

ln order to try to answer to this question, methods able to analyse simultaneously and quantitatively the liquid phase, the gaseous emissions and the biomass are needed. Recently, we have shown that under suitable defined conditions ¹³C NMR spectroscopy allows the quantitative measurement of at least eight components present at a concentration up to 5 x 10^-3M (glycerol, glucose, butandiols, tartric, malic, lactic, citric and succinic acids) in a short time (one hour) with a precision of about 3 p. cent. The measurements of the ethanol concentrations and of the amount of carbon transferred in the biomass are easily achieved using respectively ebullioscopic and standard combustion techniques. We are now able to extend these results and to show that, by using gas chromatography in a continuous dynamic mode and under a sweep of air at the head space of the fermentor, it is possible to measure quantitatively the mixture of volatile substances (composed mainly of air, carbon dioxide, ethanol and water) emanating from an alcoholic fermentation. The results obtained, when correlated with quantitative ¹³C NMR spectroscopy on the medium components permits the total balance sheet of the carbon transfers occuring during the fermentation process betwcen the liquid and the gaseous phases to be established. Our results indicate that the losses of ethanol during the fermentation process conducted under an air flow at the head space of the fermentor, may reach about 10 p. cent of the theoretical maximal amount of ethanol produced. The experiments presented here could explain the ethanol losses observed during some vinification processes conducted in open tanks.