Bubbles determine the amount of alcohol in Mezcal

Abstract

Mezcal is a traditional Mexican spirit, obtained from the distillation of fermented agave juices. Its preparation has been conducted for centuries in an artisanal manner. the method used to determine the correct alcohol content is of particular interest: a stream of the liquor is poured into a small vessel to induce surface bubbles. These bubbles, known as pearls by the Mezcal artisans, remain stable for tenths of seconds only if the alcohol content is close to 50%. For higher or lower alcohol content, the bubbles burst rapidly. The long bubble lifetime is the result of surfactant-induced surface tension changes. However, the precise mechanism and its relation to alcohol content remain unexplained. In this investigation, the extended lifetime of pearls was studied both experimentally and numerically. It was found that changes in surface tension, density, viscosity (resulting from mixing ethanol and water), and the presence of surfactants are all relevant to extend the bubble lifetime. The dimensionless bubble lifetime was found to reach its maximum value when the Bond number was close to unity, corresponding to 2 mm Mezcal bubbles. These findings show that the traditional empirical method does work. Beyond this, the understanding of the process provides physical insight to many other natural and industrial problems for which the stability of surface bubbles is of importance, such as bio-foams, froth floatation, and volcanic flows. The consumption of alcoholic drinks can be traced back to the appearance of early humans. While its inception may have resulted from the need to find reliable sources of potable liquids 1 , these drinks have become part of the cultural identity of many societies 2. Most cultures around the world are known for a 'local' drink. In the case of Mexico, the most popular distilled spirit is Tequila which, belongs to a wider class of distilled agave-based products that are similar in production 3 but vary regionally. It was believed that the production of agave-distilled spirits began with the arrival of Europeans by the end of the 16th century 4. However, recent findings indicate that alcohol distillation was known in Mesoamerica long before, for at least 25 centuries 5. Our study focuses on Mezcal, which has progressively gained worldwide recognition and has distinctive cultural value in rural villages in Oaxaca 6. Although its production and denomination are normed 7 , its preparation remains mostly artisanal 8. According to popular accounts (informally documented by interviews with artisans) and a few formal reports 9 , the traditional method employed to determine the alcohol content in Mezcal consists of inspecting the lifetime of bubbles that are formed by splashing a jet of the liquor into a small container (Fig. 1(a) and S1 Video in the Supplementary Information section). As a result of the continuous splashing, the liquid surface breaks and so-called pearls form. If the amount of alcohol in the liquid is correct (about 55% volume fraction of ethanol), pearls persist for up to tenths o...

Lifetime of Surface Bubbles in Surfactant Solutions

“…The predicted trends of the lifetime being opposite for bubbles having a partially mobile interface, i.e., La 4, a maximum in the lifetime is expected at the transition, as observed experimentally by Nguyen et al (2013). In the experiments by Rage et al (2018) [5], however, no clear trend could be identified for Bo 1, meaning that the lifetime remains rather constant in this regime. We suspect that this experimental regime for Bo 1 is at the intermediate case between the extensional and the shear flow behaviors, the trend being inverted as outlined by Eq.…”

“…Indeed, Nguyen et al (2013) concluded that this maximum materializes the transition between the main mechanisms driving the drainage of the film, from capillary to gravitational forces [11]. A recent experimental study of bubble lifetime inside Mezcal, a traditional agave based distilled beverage, also reports a peak in bubble lifetime at Bo ∼ 0.25 [5]. In the next paragraph, a scaling analysis is provided to justify the existence of a maximum in bubble lifetime at the transition between capillary drainage and gravitational drainage.…”

“…1), while all the other cases (Eqs. 4,5,6) give an algebraic decay. Moreover, in practical situations, pure viscous-capillary drainage is rarely encountered.…”

The Life of a Surface Bubble

“…Note that D ∝ 1/η leads to < t b >∝ η 3 which makes a very strong impact of the viscosity on the bubble lifetime. In the case of Mezcal and water-ethanol mixture, Rage et al [62] show a dependence of the lifetime with alcohol concentration, with a maximum around 50% that they correlate to a maximum of viscosity. Lorenceau and Rouyer [59] have tested this scaling for water-ethanol mixture and found a relatively good agreement for the impact of ethanol concentration varying from 0 to 12% in mass on the bubble lifetime (Figure 7c).…”

“…Thus the first set of bubble appears to be "fragile" and to burst during film drainage for thickness much larger than 100 nm, whereas the second set of bubble presents a better resilience with respect to perturbations. For instance, films as thick as 20 µm have been measured at the rupture in Mezcal solutions [62].…”