Characterisation by drop tensiometry and by ellipsometry of the adsorptionlayer formed at the air/champagne wine interface

A foam ring composed of small bubbles on the surface of a champagne glass i s one of its hallmarks. The equilibrium state of that ring is linked with t he rate of formation and of disappearance of bubbles. The stability of bubb les is usually ascribed to the occurrence and to the properties of an adsor ption layer formed at the gas/liquid interface. Our goal is to characterise such an adsorption layer at the gas/wine interface in order to understand its role in bubble stability. Alcohol in wine lowers the surface tension to 49 mN/m. The adsorption of other molecules may cause a further decrease of 2 mN/m. Such a situation makes the study of adsorption by surface tension measurement inaccurate. To overcome this problem, we have diluted the wine four times with water before its surface tension measurement by pendant dro p shape analysis. In these conditions, ethanol lowers the surface tension t o 64 mN/m and the adsorption of other molecules of the wine can be monitore d over 6-8 mN/m. The usual behaviour of such a diluted wine is a lowering o f the surface tension during at least 20 min after drop formation. Since th e role of macromolecules on the foaming properties of wine had been previou sly observed, we have chosen to evaluate the effect of this fraction of the wine molecules on its surface properties. Thus, wines were ultrafiltrated on a membrane with a 10 000 molecular mass cut-off. The ultrafiltrate (UF) does not show any decrease of its surface tension over a 20-min period whil e the ultraconcentrate (UC) has a kinetics similar to that of unfiltered wi ne. Mixtures of UF and UC have behaviours intermediate between those of the se products. A technological treatment of the wine with bentonite, believed to lower the content of macromolecules, yields a wine similar to UF. The e ffect of ultrafiltration was also analysed by spectroscopic ellipsometry. U F has a spectrum similar to that of a water/alcohol mixture with the same e thanol content and its ellipticity is stable during at least 20 min. On the contrary, nine or UC show spectra with the features of an adsorption layer and those characteristics increase during more than 20 min. Two varieties of vine were compared: 'Chardonnay' and 'Pinot noir'. The former is known t o have better foaming properties than the latter. Its surface properties me asured in this study are also more pronounced than those of Pinot noir. How ever, the representation of the dilational modulus against the surface pres sure (which, in some instances, may be a mathematical transformation of the state equation) puts all the samples (wines, UF and UC of each) on the sam e master curve, a fact in favour of a common nature for all the adsorption layers. It can be concluded that surface properties of champagne wines are mostly determined by ethanol and by macromolecules with a molecular mass la rger than 10 000. Moreover, the adsorption layers seem to have the same nat ure, irrespective of the vine variety and of the concentration ratio of the wine. (C) 2000 Elsevier Science B.V. All rights reserved.