Sparkling wine is an indispensable element of celebrations and gatherings, extending far beyond Christmas and New Year festivities. The sight of bubbles ascending through a flute glass alone can evoke a sense of festivity in the consumer.
Sparkling wine is produced worldwide. Recently, English Sparkling Wine produced in the United Kingdom has garnered significant attention, while German Sekt has also experienced increasing sales. Despite this growing diversity, Champagne maintains its preeminent position. Champagne refers to sparkling wine produced in the Champagne region of France. Japan ranks third in import volume after the United States and the United Kingdom, where it is popularly known as “Champagne.”
The defining characteristic of these sparkling wines is effervescence generated by carbon dioxide (CO₂). This article examines the bubbles, the most distinctive feature of sparkling wine.
The Mechanism of Bubble Formation
The fundamental difference between still wine, which produces no bubbles, and sparkling wine lies in the production methodology. Sparkling wine acquires its effervescent properties through a second alcoholic fermentation, whereas still wine undergoes only a single fermentation.
A distinctive production method unique to sparkling wine is the process known as secondary fermentation. The alcoholic fermentation necessary for converting grape juice into wine is termed primary fermentation. Secondary fermentation refers to conducting a second fermentation following the completion of the primary fermentation.
During fermentation, sugars are converted into alcohol (ethanol) and carbon dioxide. In primary fermentation, the carbon dioxide gas generated during fermentation is released directly into the atmosphere. In contrast, secondary fermentation is conducted in a sealed container, thereby preventing the carbon dioxide gas from escaping into the air and forcing it to dissolve into the wine.
Within a sealed container, the elevated pressure maintains the carbon dioxide in solution. However, upon opening the container, a pressure differential is created between the interior and exterior. This pressure differential prevents the previously dissolved carbon dioxide gas from remaining in solution, resulting in its release as bubbles.
Quantification of Bubble Production
Standard sparkling wines, including Champagne and Sekt, are produced to achieve a bottle pressure of 6 bar (1 bar = 100 kPa, approximately 6 times atmospheric pressure). When expressed as dissolved carbon dioxide concentration, this pressure corresponds to approximately 10-12 g/L.
When 10 g/L of carbon dioxide is maintained at 20°C, the corresponding gas volume is approximately 6 L. Thus, when 100 ml of Champagne is poured into a glass, 0.6 L of carbon dioxide will be released from that glass. Given that bubble diameter reaches a maximum of approximately 500 μm (micrometers, where 1 μm = 0.001 mm), approximately 10 million bubbles would ascend in a single glass under these conditions.
However, approximately 80% of the carbon dioxide released from Champagne poured into a glass is reported to dissolve in an invisible state. Consequently, the actual number of bubbles observable through the glass is approximately 2 million per serving.
Factors Determining Bubble Quantity
Research has established that the quantity of bubbles released when sparkling wine is poured into a glass depends on multiple factors. Among these, the most significant is the dissolved carbon dioxide concentration, specifically the quantity of carbon dioxide dissolved in the wine.
When bottle pressure is set at 6 bar during secondary fermentation, the quantity of carbon dioxide generated is fundamentally constant. This carbon dioxide quantity shows no variation based on origin—whether Champagne, Sekt, English Sparkling Wine, or South African sparkling wine. Assuming ideal conditions with strictly maintained bottle pressure of 6 bar during production, the total quantity of carbon dioxide released when poured into a glass would theoretically show no variation regardless of the wine’s country of origin.
In practice, factors affecting actual dissolved carbon dioxide concentration are production precision during vinification and aging duration.
Following the completion of secondary fermentation, sparkling wine production requires removal of the lees (sediment consisting primarily of dead yeast) remaining in the bottle. In Champagne production, this operation is termed dégorgement (disgorgement).
Dégorgement necessitates opening the bottle to expel the lees. Some loss of carbon dioxide during this operation is unavoidable, resulting in subtle variations in carbon dioxide content between bottles. Reduced carbon dioxide content sealed within the bottle correspondingly reduces the quantity of bubbles released when poured into a glass.
Additionally, extended aging has been demonstrated to reduce dissolved carbon dioxide concentration. This phenomenon explains why sparkling wines subjected to long-term aging frequently exhibit reduced effervescence.
The Role of Fibers in Bubble Nucleation
Sparkling wine effervesces when poured into a glass. For carbon dioxide to form bubbles, nucleation sites are required. In the absence of such nuclei within the bottle, wine does not spontaneously effervesce upon opening. Notably, the phenomenon known as gushing, where bubbles form within the bottle, occurs when nucleation sites such as crystallized tartaric acid, tannin, or foreign matter are present in the bottle.
Previously, it was hypothesized that microscopic irregularities on the glass surface served as nucleation sites for bubble formation. This theory led to explanations that variations in glass surface conditions between different glasses resulted in differences in effervescence.
However, recent research has revealed the actual mechanism of bubble formation in glasses. High-magnification super slow-motion camera footage of sparkling wine being poured into glasses has definitively confirmed that hollow fibers (fibers with hollow interiors) adhering to the glass interior walls serve as the primary nucleation sites for bubble formation.
These fibers are believed to originate from various sources, including airborne fibers or fibers transferred from wipes (cloths or paper towels) used to dry glasses after washing. Conversely, calculations have demonstrated that glass surface irregularities, previously considered the cause of effervescence, are too minute to exert actual influence on carbon dioxide bubble formation.
It should be noted that the role of fibers adhering to glass interior walls as the primary factor in effervescence is not limited to sparkling wine but is common to all carbonated beverages, including beer and cola.
Mechanism of Bubble Reduction Over Time
Immediately after opening the bottle, abundant bubbles are released, but this quantity gradually diminishes. After consuming approximately half the bottle, bubble production may virtually cease.
The primary cause of bubble cessation is the reduction in carbon dioxide dissolved in the wine. While this explanation—that bubbles cease due to gas depletion—is readily comprehensible, a second mechanism exists: the loss of suitable nucleation sites.
The loss of nucleation sites does not mean that the consumer has ingested them along with the wine, leaving the glass pristine. Rather, substances capable of functioning as nuclei no longer meet the size requirements.
While nuclei are necessary for carbon dioxide to form bubbles, research has established that the required nucleus size is inversely proportional to the dissolved gas concentration. Specifically, when dissolved gas concentration is high, even small nuclei can facilitate bubble formation, whereas as dissolved gas concentration decreases, nuclei must increase in size for gas contacting those nuclei to form bubbles.
When dissolved carbon dioxide concentration in wine is 12 g/L, the required nucleus size is merely 0.2 μm. However, when dissolved gas concentration decreases to 2 g/L, the required nucleus size exceeds 10 μm, demanding sizes more than 50 times larger. While 0.2 μm particles are invisible and unnoticeable when adhering to glass interior walls, substances 50 times larger adhering to glass may be visible to some individuals. For reference, the diameter of Japanese women’s hair is approximately 80 μm.
Because appropriately sized substances capable of serving as bubble nucleation sites are absent within the glass, bubble formation ceases as dissolved gas concentration decreases. It should be noted that the technique of intentionally creating small scratches on the glass bottom to enhance effervescence represents an eminently logical approach, as it supplies the larger nuclei required after dissolved gas concentration has decreased.
Factors Determining Bubble Quantity and Persistence
As described above, bubble formation when poured into a glass results from substances adhering to the glass surface. However, the quantity of foam generated at the liquid surface after bubble formation and the duration for which that foam persists vary according to the inherent characteristics of the sparkling wine itself. The yeast employed during secondary fermentation exerts significant influence on these properties.
Yeast is a microorganism essential to fermentation. Yeast plays a critical role both in primary fermentation, where grape juice becomes wine, and in secondary fermentation, which subsequently produces sparkling wine.
Throughout the fermentation period, yeast increases its population within the wine. While yeast cells proliferate vigorously through repeated division, numerous yeast cells simultaneously die. Upon fermentation completion, most yeast cells complete their function, die, and settle to the bottom of the container. This sediment becomes what is termed lees.
Lees gradually decompose over time and dissolve into the wine. During this process, substantial quantities of proteins and amino acids are supplied to the wine. Research has established that these proteins and amino acids influence both the foaming and foam stability (foam persistence) of sparkling wine.
The process by which lees decompose is termed yeast autolysis. Studies have confirmed that sparkling wines produced using yeast with high autolysis rates exhibit superior foaming upon pouring into glasses and improved subsequent foam stability. When producing sparkling wine, autolysis rate constitutes a critical criterion for yeast selection, in addition to factors such as alcohol tolerance and pressure tolerance.
