  Mezcal is a traditional alcoholic Mexican spirit distilled from fermented
agave juices that has been produced for centuries. Its preparation and testing
involves an artisanal method to determine the alcohol content based on pouring
a stream of the liquid into a small vessel: if the alcohol content is correct,
stable bubbles, known as pearls, form at the surface and remain floating for
some time. It has been hypothesized that an increase in bubble lifetime results
from a decrease in surface tension due to added surfactants. However, the
precise mechanism for extended lifetime remains unexplained. By conducting
experiments and numerical simulations, we studied the extended lifetime of
pearls. It was found that both changes in fluid properties (resulting from
mixing ethanol and water) and the presence of surfactants are needed to observe
pearls with a long lifetime. Moreover, we found that the dimensionless lifetime
of a bubble first increases with the Bond number, until reaching a maximum at
$Bo\approx 1$, and then continuously decreases. Our findings on bubble
stability in Mezcal not only explain the effectiveness of the artisanal method,
but it also provides insight to other fields where floating bubbles are
relevant such as in oceanic foam, bio-foams, froth flotation and magma flows.
