Langmuir 2001, 17, 791-797
791
Layers of Macromolecules at the Champagne/Air Interface and the Stability of Champagne Bubbles Nicolas Pe´ron,† Alain Cagna,† Michel Valade,‡ Christophe Bliard,§ Ve´ronique Aguie´-Be´ghin,| and Roger Douillard*,| I. T. Concept, Parc de Chancolan, 69770 Longessaigne, France, Comite´ Interprofessionnel du Vin de Champagne, 5 rue Henri-Martin, BP 135, 51204 Epernay Cedex, France, CNRS, Groupe de Glycotechnie, Laboratoire de Pharmacognosie, ESA 6013, Moulin de la Housse, BP 1039, 51687 Reims Cedex 2, France, and INRA, Equipe de Biochimie des Macromole´ cules Ve´ ge´ tales, CRA, 2 Espl. R. Garros, BP 224, 51686 Reims Cedex 2, France Received August 14, 2000. In Final Form: October 27, 2000 Adsorption layers formed at the air/degassed champagne interface were characterized by ellipsometry and by surface tension measurements of the samples diluted four times with water. Ultrafiltration of the samples showed that the adsorption layer is mainly formed by macromolecules with molecular masses in the range 104 to 105. Nonsparkling base wine was ultrafiltrated with a molecular mass cutoff of 104, and the resulting ultraconcentrate and ultrafiltrate were combined to yield experimental base wines with adjusted macromolecular content. These samples were submitted to bottle fermentation. The resulting experimental champagnes were tested for the extent of their bubble collar and for their surface properties. A good correlation was found between the two sets of data.
Introduction Foam appearance is an organoleptic property of utmost importance for champagne, since the first contact between wine and wine taster is visual. A few seconds after the champagne has been poured, the abundant foam disappears. Then, a collar of fine bubbles of equal size is expected from the effervescence originating from the nucleation sites which are primarily at the glass surface. This ring of bubbles is the result of a balance between formation by nucleation, growth, surface arrival, migration to the glass periphery on one hand and disappearance by resorption, bursting, or coalescence of bubbles on the other.1 Studies on sparkling wine foaming properties have been carried out on various scales. A global procedure was developed by Maujean et al. analyzing foam stability in a column,2 and later improved by Robillard et al.3 Using this procedure, a close relation was found between the foam parameters and the macromolecule concentration.4 Looking at the film surrounding a bubble, Sene´e et al.5 observed that immobile aggregates form during the drainage and that films of 16-25 nm are rather stable. Finally, at the molecular level, it has been established as a general rule that an adsorption layer formed at the gas/ liquid interfaces prevents the bursting of bubbles and thus improves their stability.6 The occurrence of an adsorption * Corresponding author. Fax 33 (0)3 26 77 35 99; Tel 33 (0)3 26 77 35 94; e-mail
[email protected] † I. T. Concept. ‡ Comite ´ Interprofessionnel du Vin de Champagne. § CNRS. | INRA. (1) Machet, F.; Robillard, B.; Duteurtre, B. Sci. Aliments 1993, 13, 73-87. (2) Maujean, A.; Poinsaut, P.; Dantan, H.; Brissonnet, F.; Cossiez, E. Bull. O.I.V. 1990, 711-712, 405-427. (3) Robillard, B.; Delpuech, E.; Viaux, L.; Malvy, J.; Vignes-Adler, M.; Duteurtre, B. Am. J. Enol. Vitic. 1993, 44, 387-392. (4) Malvy, J.; Robillard, B.; Duteurtre, B. Sci. Aliments 1994, 14, 87-98. (5) Sene´e, J.; Robillard, B.; Vignes-Adler, M. Food hydrocolloids 1999, 13, 15-26. (6) Lucassen, J. In Dynamic properies of free liquid films and foams; E. H., L.-R., Ed.; Marcel Dekker: New York, 1981; pp 217-265.
layer on still wines has recently been revealed in our laboratory using ellipsometry.7 Moreover, by ultrafiltration of champagne base wine at a 104 molecular mass cutoff, the adsorption layers were shown to be mostly composed of macromolecules above that molecular mass. Nevertheless, despite this adsorption layer, the surface tension of wine is close to that imposed by ethanol at the same concentration. In fact, macromolecules cause a decrease of surface tension of the order of 0.5 mN/m after 20 min. To increase the measurable effect of macromolecules on the surface tension, the wine was diluted four times with water to reduce the ethanol concentration. Under these conditions, the surface tension decreases, within the first 20 min of adsorption, to 4-5 mN/m below that of the corresponding hydro-alcoholic solution, and this effect is closely related to the concentration of macromolecules in the experimental base wine. These macromolecules, coming from grapes and yeast, which may be mostly proteins (10-30 mg/L) and polysaccharides (10-120 mg/L)8 are presently under investigation.9-11 In the present study, the previous characterization of the surface properties of base wine was extended to champagne where the molecular mass of surface active molecules is further specified. Moreover, it is shown that the interface has a domain structure at the optical microscope scale and that its properties are correlated with the extent of the bubble collar on a champagne glass. Materials and Methods (1) Samples. The base wine was produced from Chardonnay vine variety grown by C I V C (Comite´ Interprofessionnel du Vin de Champagne, Epernay). (7) Pe´ron, N.; Cagna, A.; Valade, M.; Marchal, R.; Maujean, A.; Robillard, B.; Aguie´-Be´ghin, V.; Douillard, R. Adv. Colloid Interface Sci. 2000, 88, 19-36. (8) Tusseau, D.; Van Laer, S. Sci. Aliments 1993, 13, 463-482. (9) Marchal, R.; Berthier, L.; Legendre, L.; Marchal-Delahaut, L.; Jeandet, P.; Maujean, A. Preprint, 1998. (10) Marchal, R.; Bouquelet, S.; Maujean, A. J. Agric. Food Chem. 1996, 44, 1716-1723. (11) Brissonnet, F.; Maujean, A. Am. J. Enol. Vitic. 1993, 44, 297301.
10.1021/la001169t CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001
792
Langmuir, Vol. 17, No. 3, 2001
Pe´ ron et al.
Using two different techniques, uItrafiltration (ultraconcentration) of base wine was performed first by tangential and second by stirred frontal ultrafiltration. In the first case the operation was performed using a 1.8 m2 hollow fiber device (Inceltech, Toulouse, France) with a 104 nominal molecular mass cutoff. The ultrafiltration membrane was made of hydrophilic polysulfone. Before running the experiment, the device was rinsed twice with 0.8 L of wine. After each ultrafiltration of a 70 L volume of wine, the device was rinsed with ultrapure water and synthesis grade ethanol. During the ultrafiltration, the flows of UF and UC were measured at regular time intervals. The speed of the peristaltic pump was adjusted to achieve a 3 times ultraconcentration of the wine. The transmembrane pressure was maintained at a constant 8 × 104 Pa value. The relative concentration factor of macromolecules in the samples, RCF, was calculated with respect to the base wine. It was 0 in ultrafiltrate, UF, and 1 in the base wine, and in the case of ultraconcentrate, UC
RCF ) (VUF + VUC)/VUC
(1)
where VUF and VUC are the volumes of UF and of UC of the experiment. An amount of wine with a RCF value of 1.0 was prepared from UF and UC; it was the reconstituted base wine RECbw. All the samples including non-ultrafiltrated base wine were stored in 0.75 L bottles. The second (or bottle) fermentation was achieved by adding yeast and 18 g of sugar to each bottle, which was finally closed by a gastight plug. The concentrations of ethanol and carbon dioxide increased during that fermentation, yielding sparkling wine. After expulsion of lees (disgorging step), the bottles were sealed with capsules. Four champagne batches were prepared by this procedure starting from wines derived of the base wine and kept at 3 °C: standard champagne (from standard base wine), UFbw champagne (from UF base wine), UCbw champagne (from UC base wine), and RECbw champagne (from reconstituted base wine). Stirred frontal ultrafiltration of standard champagne was performed after degassing by nitrogen bubbling. Prior to use, nitrogen was equilibrated with wine vapors to prevent the evaporation of volatile compounds such as ethanol. The Pyrex/ stainless steel cell (Millipore, ref XFUF 047 01) was equipped with a regenerated cellulose membrane of 47 mm diameter, with 104 (MWCO 10 000, PLCG 047 10), 3 × 104 (MWCO 30 000, PLTK 047 10), or 105 (MWCO 100 000, PLHK 047 10) nominal molecular mass cutoff. The cell and the membranes were carefully rinsed with ultrapure water and Extran (Merck) and then several times with ultrapure water. Before use, the membranes were hydrated and washed with ultrapure water for 30 min, placed in the cell, and equilibrated with wine for 2 h before the experiment. Sixty milliliters of champagne were ultrafiltrated, with the same RCF value of three, just as in the previous ultrafiltration of the base wine. The samples obtained were standard champagne UF, standard champagne UC, and standard champagne REC (reconstituted from UF and UC with a RCF equal to one). All glass vessels were either treated with chromosulfuric acid or cleaned in a 2-propanolic potash solution and rinsed with ultrapure water before use. (2) Surface Tension Measurements. Surface tension measurements were performed with a drop (bubble) tensiometer from I T Concept, Longessaigne, France.7,12,13 Only the drop case is considered in this section, since the case of the bubble is completely analogous. A thorough description of the experimental setup has been given by Benjamins et al.14 An axisymmetric drop was formed at the tip of the needle of a syringe whose plunger position was driven by a computer. Syringe needles were thoroughly rinsed with ultrapure ethanol and water before use. The image of the drop was taken from a CCD camera and digitized. The interfacial tension γ was calculated by analyzing
the profile of the drop according to the Laplace equation:
(1/x) d(x sin θ)/dx ) 2/b - cz
The origin of the coordinates is at the drop apex; x and z are the Cartesian coordinates at any point of the drop profile, b is the radius of curvature at the drop apex, θ is the angle between the x axis and the tangent to the drop profile, and c ) 2/a2, where a is the capillary constant (a ) (2γ/(Fg))1/2, where F is the difference of density between the two phases and g is the acceleration of gravity). The area of the drop and the surface tension were calculated several times per second. Densities were measured with a PAAR (DMA 45) apparatus. This setup was made suitable to measure the surface dilational modulus :6
) dγ/d ln A
(3)
where A is the surface area of the drop. This was achieved by fluctuating sinusoidally the area of the drop at a frequency of 0.1 Hz and a relative amplitude of 0.07, which is the lowest value with an acceptable signal-to-noise ratio and where the surface tension amplitude response is in the linear range. No significant change of the modulus was noticed by changing the frequency from 0.01 to 0.1 Hz. Moreover, a 10 s period is short enough compared to the duration of the adsorption experiment, which lasts approximately 1 h. The modulus was calculated using a sliding window with a 10 s width (one period), using the A and γ variations. A Fourier transform of the signal was used to extract the main component and avoid the small perturbations linked to the inaccuracies in the movement of the plunger. The surface tension was calculated as the mean value during the same period. Drop formation at the tip of the needle was achieved in a confining cell (≈250 mL) with two parallel windows for optical measurements. Its bottom was coated with a few milliliters of wine to allow for equilibration with saturating vapors and to avoid evaporation from the drop. The confining cell was thermostated to 20.0 ( 0.1 °C. The plunger of the syringe was allowed to enter through a toric seal, ensuring a gastight cell. In most kinetics experiments, the difference between the surface tension at the beginning of the experiment and that at time t (∆γ(t) ) γ(0) - γ(t)) was used instead of surface pressure. The reason is that the surface tension of the “ pure ” solvent is not a defined physical quantity in the case of wine and cannot be used to calculate the surface pressure in the usual way. To begin the measurements as close as possible to the solvent tension, two drops were quickly expelled before forming the measuring drop. (3) Ellipsometry. Measurements were performed using a spectroscopic phase modulated ellipsometer (UVISEL, Jobin Yvon, Longjumeau, France). It was equipped with a xenon arc lamp. In the chosen configuration, the polarizer and the analyzer were set to 45°; the photoelastic modulator, activated at the 50 kHz frequency, was set to 0°. The spectroscopic measurements were monitored between 250 and 700 nm. The incidence angle was set to 53.6° for nondiluted wines and 53.5° for wines diluted four times with water. All measurements were done at the air/ liquid interface in an air conditioned room at 20 ( 1 °C. The sample was poured in a glass vessel just before acquisitions. The two ellipsometric angles Ψ and ∆15 are linked to the two reflectivity coefficients rp and rs, in the directions parallel and perpendicular to the incidence plane, respectively, by
rp/rs ) tan Ψ exp(i∆)
(4)
The fixed wavelength chosen for the kinetic measurements corresponds to the Brewster conditions defined by
∆ ) (π/2 (12) Labourdenne, S.; Gaudry-Rolland, N.; Letellier, S.; Lin, M.; Cagna, A.; Esposito, G.; Verger, R.; Riviere, C. Chem. Phys. Lipids 1994, 71, 163-173. (13) Puff, N.; Cagna, A.; Aguie´-Be´ghin, V.; Douillard, R. J. Colloid Interface Sci. 1998, 208, 405-414. (14) Benjamins, J.; Cagna, A.; Lucassen-Reynders, E. H. Colloids Surf., A 1996, 114, 245-254.
(2)
(5)
The ellipticity coefficient of the adsorption layer measured at the Brewster conditions, FjB, for the substrate is defined by (15) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and polarized light; Elsevier Science B. V.: Amsterdam, 1987.
Stability of Champagne Bubbles FjB ) tan Ψ sin ∆
Langmuir, Vol. 17, No. 3, 2001 793 (6)
(4) Brewster Angle Microscopy. This technique, BAM, gives images of the two-dimensional domains of the adsorption layer. The setup was developed by He´non and Meunier16 and improved by Lheveder et al.17 It takes advantage of a novel objective with an optical axis perpendicular to the liquid surface and has a resolution of 1 µm. The position of the objective in the horizontal plane is motor controlled, allowing the exploration of the interface. (5) Bubble Collar Extent and Effervescence. A visual procedure was developed to assess the extent of the bubble collar on the wine surface. It uses crystal glasses which are practically cylindrical (Marianna, LR Crystal, Lednicke´ Rovne, Slovakia) and which had been thoroughly washed without surfactant by domestic equipment (Zanussi, DW 474), supplied exclusively with deionized water. The glasses were dried upside down in racks. They were placed with the bottles in an air conditioned room at 20 ( 1 °C 24 h before the assessment of the collar. One hundred milliliters of sparkling wine was poured in each of the six glasses immediately after opening the bottle. The desired level of liquid was determined by referring to a horizontal line drawn on a board placed behind the glasses. The time needed to fill up the six glasses was about 3 min. The collar extent was visually assessed according to a conventional scale (Table 1) 4 and 6 min after the beginning of the wine pouring. Effervescence, which is the driving force of the collar extent, was evaluated on pictures taken 4 and 6 min after pouring. The intensity of effervescence was roughly quantified on a photograph by counting the bubbles comprised between the surface of wine and a line 4 cm below it. This procedure does not take into account the velocity of bubbles, which is an important parameter to determine the actual number of bubbles coming to the surface.18 However, this procedure was accepted as a first approximation of the effervescence intensity.
Results and Discussion At first, a characterization is given of the surface properties and of the molecules forming the adsorption layer of champagne. Then, the effect of macromolecule concentration in the base wine on the surface properties and bubble collar formation in champagne is analyzed. (1) Characterization of the Adsorption Layer of Champagne. This part of the work was performed on a sample of standard champagne which had been submitted to ultrafiltration after degassing, yielding standard champagne UF, UC, and REC samples. Ellipsometry. The Brewster wavelength with an incidence angle of 53.6° was found to be 384 nm on standard champagne UF obtained with a molecular mass cutoff of 104. As with base wine,7 it was found that the ellipticity of UF is positive and close to +0.001 (Figure 1), indicating that the main contribution to that coefficient is the roughness of the interface.19 The standard champagne (Figure 1), its UC, and the reconstituted sample exhibit an ellipticity which, at first, oscillates between a positive and a negative value and then stabilizes with a fairly significant negative value. This kinetics and negative value should be indicative of the progressive formation of an adsorption layer which looks at first inhomogeneous in the interface plane at a scale of the order of (or larger than) 1 mm (the size of the laser beam). Even when the signal is established with a negative value, significant fluctuations are obvious, indicating the likely inhomogeneity of the layer. Nevertheless, these results show that an adsorption layer forms when macromolecules with a (16) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936-939. (17) Lheveder, C.; He´non, S.; Mercier, R.; Tissot, G.; Fournet, P.; Meunier, J. Rev. Sci. Instrum. 1998, 69, 1446-1450. (18) Liger-Belair, G.; Marchal, R.; Robillard, B.; Dambrouck, T.; Maujean, A.; Vignes-Adler, M.; Jeandet, P. Langmuir 2000, 16, 18891895. (19) Meunier, J. J. Phys. 1987, 48, 1819-1831.
Figure 1. Effects of ultrafiltration on the evolution of the ellipticity coefficient of the champagne/air interface. Ultrafiltration was performed on degassed standard champagne with a 104 molecular mass cutoff. The incidence angle was 53.6°; the measuring wavelength was 384 nm. The integration time of the measurements was set to 2 s: continuous line, standard champagne; dotted line, standard champagne UF.
molecular mass larger than 104 occur in the wine while practically no layer is formed when these macromolecules have been withdrawn. Moreover, the values of the ellipticity reached by the three samples with macromolecules after 20 min of adsorption are not very different. This is not surprising for standard champagne and for the reconstituted sample, which are thought to have the same composition, but it is less clear for the UC sample where the macromolecules are three times more concentrated. It may be that a factor of three in the bulk concentration of macromolecules has not a large effect on the concentration of the adsorption layer or that ellipticity is not very sensitive to the detailed structure of these adsorption layers. As shown previously,7 the dilution of wine by water allows a measurement of surface active molecules by surface tension lowering (see Surface Tension paragraph). Thus, the ellipticity of champagne samples diluted four times by water was measured under Brewster conditions. For UF obtained from standard champagne, the ellipticity is rather constant during 20 min and has a small negative value of -0.002 (Figure 2). These data are in contrast with those obtained with undiluted champagne, suggesting that some molecules which have no surface activity in the wine become surface active after having been diluted four times with water. Standard champagne, reconstituted, or UC samples have a more negative ellipticity which decreases smoothly from -0.008 to -0.010 during the first 20 min of adsorption (Figure 2). Nevertheless, the occurrence of macromolecules in the samples induces the formation of an adsorption layer which is more important than in the case of UF standard champagne. Moreover, the evolution in time of the ellipticity has a smooth kinetics, which suggests that the two-dimensional structure of the adsorption layer is more homogeneous than when champagne is not diluted with water. As in the case of samples nondiluted by water, the ellipticities of the three samples nondevoid of macromolecules are very close one to the other. No clear conclusion may be drawn regarding the effect of macromolecule concentration on the structure of the adsorption layer. Brewster Angle Microscopy. The two-dimensional organization of the adsorption layer formed by samples prepared from standard champagne was monitored to understand the previously noticed ellipticity fluctuation. Most often, several phases were visible, indicating that
794
Langmuir, Vol. 17, No. 3, 2001
Figure 2. Effects of ultrafiltration on the evolution of the ellipticity coefficient at the interface between air and champagne diluted four times with water. Ultrafiltration was performed with a 104 molecular mass cutoff. The incidence angle was 53.5°, and the measuring wavelength was 372 nm. The integration time of the measurements was set to 2 s: O, standard champagne; ], standard champagne UF; ×, standard champagne REC; +, standard champagne UC.
Figure 3. Brewster angle microscopy of air/champagne interface. The picture was taken 30 min after pouring the champagne in the dish. The interface was illuminated with a laser beam of wavelength 515 nm. The length of the image is 0.45 mm. The bright regions are domains with adsorbed layers; dark regions are gaseous domains of the interface.
the adsorption layer is in a state of transition (Figure 3). Moreover, a draught caused a large and irreversible displacement of the patterns, showing that the adsorption layer is fluid. In the case of champagne diluted four times with water, no pattern was observed, indicating a more homogeneous adsorption layer, as expected from the smooth kinetics of the ellipsometry signal. Moreover, the layer was observed to be much less mobile than when the ethanol content is larger. These effects are linked to the nature of the solvent, since the (macro)molecules involved are the same. A detailed analysis of these patterns will be published elsewhere. Surface Tension. The surface tension of UF obtained from standard champagne is stable at 46.6 mN/m. That of standard champagne is 46.3 mN/m just after the formation of the bubble and decreases 1 mN/m in the first adsorption hour, showing that some surface active compounds adsorb. Moreover, the surface tension of a water solution with the same amount of ethanol (12.4%) is 47.3, showing that some surface active compounds occur at the interface between air and standard champagne or its UF.
Pe´ ron et al.
Figure 4. Effects of ultrafiltration on the evolution of the surface tension at the interface between air and champagne diluted four times with water. Ultrafiltration was performed with a 104 molecular mass cutoff. Results are shown as ∆γ, the difference between surface tension just after drop formation and surface tension at time t: O, standard champagne; ], UF; ×, mixture of UF and UC with a 1.0 RCF; +, UC.
As previously performed with base wine, the samples were diluted four times with water to decrease the ethanol amount and its effect on surface tension.7 Under these conditions, the surface active molecules of standard champagne induce a lowering of the surface tension of about 5 mN/m in 1 h. The standard champagne UC has a larger variation than those for standard champagne REC and standard champagne themselves, which are very close one to the other, and standard champagne UF has practically no variation of the surface tension (Figure 4). These data are generally consistent with those obtained by ellipsometry (Figure 2) even if ellipsometry is more sensitive to adsorption layers which are not clearly revealed by tensiometry on wines not diluted by water. However, the solvent used in ellipsometry is not the same as the one used for these surface tension measurements: molecules adsorbed in one situation are not necessarily adsorbed in the other situation. Moreover, the difference between standard or reconstituted champagne and UC is more visible from surface tension measurements than from ellipsometry (Figure 2), a fact which may indicate that the three times more important volume concentration of macromolecules in the UC sample has a more pronounced effect on the properties of the adsorption layer than on its structure. It can be concluded that the kinetic surface tension measurements give a better evaluation of the amount of macromolecules in the sample even though there is no proportionality between the macromolecule concentration and the surface tension variation. Molecular Mass of the Adsorbed Molecules. Standard champagne was submitted to ultrafiltration with three different molecular mass cutoffs: 104, 3 × 104, and 105. The kinetics of surface tension of the ultrafiltrate at 104 molecular mass cutoff shows, as expected, that it is devoid of surface active (macro)molecules (Figure 5). The very close values of the surface tension at any time for the standard champagne and its ultrafiltrate at 105 molecular mass cutoff make it very likely that the same pattern of surface active molecules is present in both and that most of the surface active macromolecules have a molecular mass of less than 105. This conclusion seems to be confirmed by the fact that the ultrafiltrate obtained with a molecular mass cutoff of 3 × 104 has surface properties intermediate between those of the two other ultrafiltrates. This probably means that an important fraction of the
Stability of Champagne Bubbles
Figure 5. Effect of molecular mass cutoff on the kinetics of the surface tension of ultrafiltrated champagne. Standard champagne was membrane ultrafiltrated and diluted four times with water: O, standard champagne; b, [, and ], UF of standard champagne with molecular mass cutoff of 105, 3 × 104, and 104.
Figure 6. Relation between the dilational modulus and the surface tension during the formation of the adsorption layer. Standard champagne samples were diluted four times with water. Both parameters were measured on the same droplet. ∆γ is the difference between surface tension at the drop formation and at the measuring time. Experiments were performed during 2 h: [, UF using a 3 × 104 molecular mass cutoff; +, UC using a 104 molecular mass cutoff.
surface active macromolecules has a molecular mass in the range of 3 × 104. Thus, it can be concluded that most of the surface active molecules of champagne, besides ethanol and some small molecular mass compounds, are macromolecules and that a significant fraction among them has a molecular mass of the order of 3 × 104. Dilational Modulus. Using champagne samples diluted four times with water, the dilational modulus was measured after the formation of a drop at the tip of a needle during the time course of adsorption while oscillating the volume (and the area). At a frequency of 0.1 Hz, the phase angle between stress and strain was less than 10°. Thus, the dissipative component of the modulus is relatively small and has not been further studied. The modulus increases when the surface tension drops during the formation of the adsorption layer (Figure 6). This increase is practically linear until a surface pressure drop of 4-5 mN/m with a slope (d/dπ) of the order of 7. At larger surface pressure drops, the modulus levels off. Such evolution of the modulus as a function of surface pressure has been predicted for adsorption layers of polymers with a two-dimensional behavior20 and observed with natural (20) Aguie´-Be´ghin, V.; Leclerc, E.; Daoud, M.; Douillard, R. J. Colloid Interface Sci. 1999, 214, 143-155.
Langmuir, Vol. 17, No. 3, 2001 795
polymers such as proteins.13,14,21 These arguments show that the properties of the interfacial layer of champagne are similar to those of polymers, as expected from the ultrafiltration experiments. Moreover, it can be noticed that the value of 7 of the slope is close to that of polymer in the θ conditions (the theoretical value in these conditions is 8), which means that the two-dimensional coil is partly collapsed. This value was observed for standard, for UC, and for reconstituted champagne but also for the ultrafiltrate with a molecular cutoff of 3 × 104. This points to the fact that, for the polymers forming the adsorption layer, in all these cases, the conformation is the same. Finally, in the case of base wine, the same behavior had been observed but the slope is claimed to be of the order of 5.5.7 This is a calculation mistake, the true value being between 6.5 and 7. Thus, the conformations of the polymers forming the adsorption layer of base wine or of champagne are very similar. (2) Correlations between the Adsorption Layer and the Collar Extent. The amount of macromolecules in an effervescent champagne cannot be conveniently adjusted by ultrafiltration. However, it is usually accepted that the foaming behavior is determined by the properties of the base wine and that the bottle fermentation has no major effect. Thus, bottle fermentation was conducted on standard base wine and on three lots of base wines previously submitted to ultrafiltration treatments: ultrafiltrated (UFbw champagne), ultraconcentrated with an RCF of 3 (UCbw champagne), and reconstituted base wine from the two previous samples (RECbw champagne). This experimental setup is expected to yield champagnes with significantly different amounts of macromolecules, since it has been shown previously that ultrafiltration with a molecular mass cutoff of 104 changes drastically the content of macromolecules in base wine.7 First, the surface properties of the samples have been characterized, then the collar extent has been quantitated, and finally a correlation has been checked between the two sets of data. Surface Properties of the Samples. The general trends of the ellipsometric and surface tension properties of the four samples are in the expected order (Figure 7a): the UF having the smallest surface properties, the UC having the largest, and the two other samples being intermediate. However, some experimental facts need to be commented upon. (i) The UF champagne has a constant and positive ellipticity, just as the sample ultrafiltrated after the second fermentation (Figure 1). Thus, the yeast fermentation in the bottle does not seem to produce many surface active (macro)molecules. However, the surface tension change of the sample for 20 min is small but very significant (Figure 7b), indicating that all surface active molecules are not detected by ellipsometry or that the dilution of the champagne by water induces the surface activity of some molecules. (ii) The ellipsometric signals, as discussed previously, are often noisy, probably because of a phase transition occurring in the adsorption layer. For this reason, it seems more convenient and accurate to chose a surface tension measurement to characterize the surface properties of champagne by a single quantity. Thus, velocity of surface tension change, extrapolated at the time of drop formation, was used to characterize surface properties. Collar Evaluation. The collar was evaluated according to the arbitrary scale of Table 1 with six glasses for each sample. As previously noticed by Machet et al.,1 a good (21) Benjamins, J.; Lucassen-Reinders, E. H. Surface dilational rheology of proteins adsorbed at air/water and oil/water interfaces; Mo¨bius, D., Miller, R., Ed.; Elsevier: Amsterdam, 1998; pp 341-384.
796
Langmuir, Vol. 17, No. 3, 2001
Pe´ ron et al.
Figure 8. Relation between the collar extent and the rate of surface tension decrease at the time of drop formation. The collar extent was determined visually (mean of six measurements) and the velocity of the surface tension decrease was determined by adjusting an hyperbolic function to the curves of Figure 7b. Champagne obtained from (]) UF base wine (RCF ) 0); (∆) reconstituted base wine (RCF ) 1); (0) UC base wine (RCF ) 3); (O) standard base wine.
Figure 7. Effect of the relative macromolecular concentration of base wines on the kinetics of the surface properties of the corresponding champagnes. Champagne was obtained by bottle fermentation of wines prepared by ultrafiltration of a chardonnay base wine with a 104 molecular mass cutoff hollow fiber membrane: (a) ellipticity coefficients, same conditions as in Figure 1; (b) surface tension of champagnes diluted four times with water, same conditions as in Figure 4. Champagne obtained from (O) standard base wine; (]) UF base wine (RCF ) 0); (0) UC base wine (RCF ) 3); (∆) reconstituted base wine (RCF ) 1). Table 1. Visual Evaluation and Scoring of the Bubble Collar score 0 1 2 3 4 5 6
collar and/or foam appearance no bubble at the periphery of the glass dicontinuous ring of bubbles at the periphery of the glass continuous thin (one to five bubbles thick) ring the colar is broad enough: a bubble monolayer covers less than half of the glass area more than half of the glass area is covered by bubbles; a single hole keeps near the center wine is completely covered by a “monolayer of bubbles” wine is covered by a three-dimensional foam
correlation was noted between the height of the collar on the side of the glass and the extent of the collar (data not shown). Nevertheless, this scale was chosen rather than the height of the foam meniscus because it seems to stick more closely to the sought-after organoleptic properties. A large distribution of the effervescence intensity was noticed for each sample; it was probably linked with the state of the glass surface. No correlation was noted between effervescence in the glasses and RCF. Under our experimental conditions, the collar extent was not modified over a period of at least 0.5 h (data not shown). It was found convenient to keep the value of the collar extent determined 6 min after the end of pouring champagne in the glasses to characterize the extent of the collar. Correlation. When the surface properties and the collar extent are evaluated as described, a good correlation is
observed between these two quantities (Figure 8). This observation is not completely unexpected, since correlations had been observed between the macromolecular concentration of base wines and their foaming properties measured in a test tube4 and since some correlations had been observed between the protein content and the foaming behavior of base wines.11,22 Nevertheless, to our knowledge, it is the first time that this correlation has been established directly between first the macromolecular content and the surface properties of champagne and second the stability of its foam collar. 3. General Discussion. Effect of Bottle Fermentation on Macromolecules. As shown throughout the work, the macromolecular content of champagne governs its surface properties. Since the macromolecule content of base wine also determines the surface properties of champagne, it can be concluded that bottle fermentation does not modify to any large extent the macromolecular content and the surface properties of wine when it increases carbon dioxide and ethanol concentrations. This is consistent with the usual observation that foaming properties of champagne are well correlated with those of base wine.2 Effect of Ultrafiltration on Surface Properties. Ultrafiltration of standard champagne with a 104 cutoff yields, as expected, a standard champagne UC with a surface tension lowering larger than that of untreated sample (Figure 4). Similarly, the bottle fermentation of UC base wine results in larger surface tension lowering for the UCbw champagne (Figure 7b). The reconstituted champagne and the champagne obtained from reconstituted base wine exhibit surface tensions which are not very different from those of the nontreated samples (Figures 4 and 7b). These results are consistent with the view that ultrafiltration results in the separation of molecules on both sides of the membrane according to their molecular weight. However, in a previous study on base wines, ultrafiltration has yielded UC samples which had surface properties smaller than those of the nontreated sample.7 A difference between the two studies could explain these discrepancies. In the former study, the ratio was 1.3 L/m2 between the filtered sample volume and the surface area of the ultrafiltration membrane. In the present study, this (22) Brissonnet, F.; Maujean, A. Am. J. Enol. Vitic. 1991, 42, 97102.
Stability of Champagne Bubbles
ratio was 39 L/m2 for the ultrafiltration of base wine and 35 L/m2 for the ultrafiltration of champagne. These differences seem significant enough to explain a part of the differences noted between the two studies. Thus, it is very likely that, in the former study, the polarization layer was not completely formed after rinsing the membrane twice with 0.8 L wine. Consequently, it may be concluded that when ultrafiltration of base wine or of champagne is conducted under conditions where the adsorption of macromolecules on the membrane may be neglected, this procedure results, in actual fact, in a physical separation of molecules according to their molecular mass without any visible side effect. Structure and Properties of the Adsorption Layer. Macromolecules have been clearly evidenced in the adsorption layer of champagne. Their molecular mass is in the range 104 to 105. However, the surface tension of UF champagne shows very clearly that small molecular mass compounds are also involved in the formation of the adsorption layer. The chemical nature and the respective role of these compounds need to be understood. Surface Properties and Collar Stability. The results clearly show a correlation between surface properties and collar extent. However, this correlation occurs between two arbitrary parameters. The collar extent is a visual evaluation according to an arbitrary scale; it is difficult to choose any continuous parameter to quantify such
Langmuir, Vol. 17, No. 3, 2001 797
phenomenon which obviously exhibit different regimes. Nevertheless, this scale gives a correct ranking of the collar extent. The surface properties may have been characterized by numerous parameters. A kinetics approach has been chosen which gives a preeminent weight to the increase in stability during the early life of interfaces and of bubbles. In fact, this approach reflects the velocity of formation of the adsorption layer. Thus, a clear correlation is observed between the macromolecular content and the collar stability and extent of champagne. Moreover, since the experimental system used is concerned with macromolecule concentration only, it is clear that this parameter is strongly involved in bubble and collar stability. However, at the present time, it is not known if this parameter is the only one concerned. The chemical nature of the macromolecules or the occurrence of antifoam compounds in some champagne may also be involved. Acknowledgment. Thanks to B. Monties for his interest in this project and to J. Meunier for his help in Brewster angle microcopy experiments as well as for making the equipment available. Related discussions with B. Robillard, A. Maujean, Ph. Jeandet, and R. Marchal are acknowledged. Thanks also to the European Community contract no. IN 10381 D for financial support. LA001169T
