Optical Techniques Gauge Champagne Bubble Stability
The hallmark of a good glass of champagne is a stable ring of bubbles — called a collar — at the top of the glass. The formation and disappearance of these bubbles are delicate phenomena that are sought after by champagne makers, but they are not easily reproduced. They are determined by the adsorption layers formed at the gas/liquid interface on the inside of the bubbles and at the air surface of the liquid.
Researchers in France have used optical techniques to investigate the stability of bubbles in champagne.
The adsorption layer is essential for the stabilization of bubbles and is responsible for the stability of the bubble collar of champagne. The layer is formed at the gas/wine interface by amphiphilic macromolecules that are probably polysaccharides — sugar polymers and glycoprotein. However, scientists are not entirely sure of the exact nature of the molecules.
Conventional techniques for measuring the properties of the adsorption layer monitor the lowering of surface tension. In the case of wines, surface tension is lowered by ethanol that is present in the wine and at the gas/liquid interface. Consequently, the adsorption of other molecules — such as macromolecules — produces only a tiny decrease of the surface tension. Surface tension measurement techniques do not provide much information on the formation of the adsorption layer, and they provide none on its two-dimensional organization.
Roger Douillard of Université de Reims — Champagne-Ardenne and his colleagues at the French National Institute for Agricultural Research and at Comité Interprofessionnel du Vin de Champagne therefore have investigated using Brewster angle microscopy to characterize the macromolecule adsorption layer formed at the air/champagne interface to determine its correlation with bubble collar stability.
Thickness of the adsorption layer is on the order of a few nanometers and is of particular interest to the researchers, as is the refractive index of the adsorption layer where the macromolecule concentration is very high, at approximately 500 g/l.
Ellipsometry measures the depolarization of a polarized light beam reflected on an interface — in this case, the air/wine interface. This depolarization is sensitive to the structure of the few nanometers around the interface and provides information on the refractive index and on the thickness of an adsorption layer, which is distinguished from the bulk on the basis of its refractive index.
Brewster angle microscopy provides a map of the ellipsometry experimental parameters at the scale of optical microscopy. It reveals an image of the organization of the adsorption layer over an area of roughly half a millimeter.
A single point measurement takes only a few milliseconds, but the adsorption layer is not always homogeneous at the millimeter or smaller scale, so its formation takes several minutes. Consequently, the researchers’ experiments were conducted over a period of 30 min.
“The main advantage of optical techniques over conventional measurement techniques is the correlation of the adsorption layer parameters with the bubble collar stability parameters,” Douillard said.
With the experimental champagnes exhibiting various macromolecule concentrations, the main result is that the adsorption layer is correctly described by the mean value of its ellipticity during the 30 min after pouring the wine into the measuring device.
To their surprise, the investigators found that wine macromolecules adsorb onto the glass of the bottles and that this effect is particularly significant when the number of macromolecules in the wine is low. Champagnes with low macromolecule concentration may be practically deprived of them through that phenomenon. This results in an unstable collar.
The solution to this problem is as yet unknown, but Douillard hopes that his group’s work will go some way toward helping champagne makers to achieve the perfect glass of bubbly.
Contact: Roger Douillard, Uni-versité de Reims — Champagne-Ardenne;
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