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...

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