Is grape invertase a major component of the adsorption layer formed at the air/champagne wine interface?

The effects of ethanol concentration on the adsorption of beta -casein, grape invertase, and wine molecules have been studied at the air-solution interface by neutron reflectivity, by ellipsometry, and by static and dynamic surface tension measurements (bubble tensiometer). At 12.5% ethanol content, the surface pressure increase caused by either protein is of the order of 3 mN/m, a value which suggests a low amount of adsorbed protein as compared to the buffer devoid of ethanol where the surface pressure reaches 23-25 mN/m. The amount of protein adsorbed at 12.5 or 20% ethanol was measured by neutron reflectivity and by ellipsometry and found to be at least equal to the amount adsorbed when no ethanol occurs in the buffer. It can be concluded from these data that the adsorption of protein is not impeded by ethanol. A simple model considering that the adsorbed protein forms a network and that ethanol adsorbs in the vacant space of that mesh allows the understanding of the low surface pressure of the protein on a hydroalcoholic solution. The dilational modulus calculated from dynamic surface tension measurements is a linear function of the surface pressure over a limited range of surface pressure. The slope of this relation can be related to the fractal dimension of the macromolecules covering the interface. The slope is smaller with beta -casein than with invertase and decreases when ethanol concentration increases in the former case, whereas it is the contrary in the latter. In the case of wine, the slope is intermediate between those measured with invertase and with p-casein and does not change much with ethanol concentration. These data indicate that adsorption layers of grape invertase are not good models for the adsorption layer formed at the interface between air and champagne still wine.