It is well known among champagne connoisseurs that you can tell the quality of the product by the nature of its bubbles (http://www.ehow.com/how_2192062_tell-good-champagne-its-bubbles.html). The flute should be adorned with a dainty pearl necklace of delicate effervescence. But what exactly determines the character of these bubbles? The rate at which they are formed and their longevity are both dependent upon the composition of the adsorption layer at the liquid/air interface. This adsorption layer is simply the aggregation of molecules on the surface of the champagne. It is known that the adsorption layer consists primarily of macromolecules (macromolecules include proteins, carbohydrates, fats, and nucleic acids), but its specific composition remains unclear. However, Aguié-Béghin et. al. (appropriately from the Université de Reims Champagne Ardennes) report their preliminary findings on the chemical composition of the adsorption layer in champagne (which they produced in the lab—I wonder if there would be a market for that…). The article abstract can be found here.
The researchers relied primarily on three methods of investigation for this study. First, they performed ellipsiometry (http://en.wikipedia.org/wiki/Ellipsometry) allowing them to determine the amount of time required for an adsorption layer to form. They used Brewster Angle Microscopy (BAM - http://users.otenet.gr/~garof/Bam/) to visualize the interface of champagne adsorbed onto a polystyrene surface, as a model for the liquid/air interface. Finally, the group performed X-ray photoelectron spectroscopic (XPS) studies to determine the chemical composition of the champagne surface (again as modeled by application of a champagne layer to polystyrene). XPS utilizes high-energy X-rays to disrupt electrons from the surface, and the energy needed to cause these electrons to escape is characteristic of the particular elements and the way in which they are bonded (http://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy).
Ellipsiometry was used to compare the rate of formation of the adsorption layer of champagnes at significantly different concentrations. The most highly concentrated (i.e. the sample containing the most macromolecules per unit volume of liquid) formed an adsorption layer the fastest, suggesting that these macromolecules do in fact play an important role in the formation of this surface layer.
Samples of champagne on the polystyrene layer were prepared in three different ways (soaking in champagne, soaking followed by rinsing with water, applying a layer of champagne and allowing it to evaporate), and visualized using BAM. The images demonstrate that the adsorption layer is heterogeneous, with regions showing aggregation of macromolecules in organized structural forms known as domains. This is consistent with previous findings at the liquid/air interface of champagne, and gives credence to the use of the polystyrene layer as a model for the liquid/air interface.
The XPS data reveal the ratios of different elements in the surface layer, and also provide some insight into the functional groups present in the macromolecules. Upon comparison with the composition of typical proteins, polysaccharides (i.e. sugars), and lipids (i.e. fats), the group was able to support the suggestion that the adsorption layer of champagne is composed primarily of proteins and polysaccharides, in the ratio of 35% protein to 65% polysaccharide. The data suggest that lipids are not present in the surface layer in any great quantity, which is atypical for many food products and biochemical mixtures, where lipids usually have a tendency to aggregate on the surface.
The specific identities of these proteins and polysaccharides, as well as the role that each plays at the champagne/air interface and thus in the formation and stability of bubbles, remains to be elucidated. However, these results do give some insight into the make up of the adsorption layer, and point to particular targets for further analysis. I’ll drink to that – Salut!
Aguié-Béghin, V.; Adriaensen, Y.; Péron, N.; Valade, M.; Rouxhet, P.; Douillard, R. Sturcture and Chemical Composition of Layers Adsorbed at Interfaces with Champagne. J. Agric. Food Chem. [Online early access]. DOI:10.1021/jf9016948. Published Online: October 8, 2009. http://pubs.acs.org/journal/jafcau (accessed October 23, 2009).
No comments:
Post a Comment