  When frying potato snacks, it is typically observed that the dough, which is
submerged in hot oil, after some critical time increases its buoyancy and
floats to the surface. The lift-off time is a useful metric in ensuring that
the snacks are properly cooked. Here we propose a multiphase mathematical model
for the frying of potato snacks, where water inside the dough is evaporated
from both the top and bottom surfaces of the snack at two receding evaporation
fronts. The vapour created at the top of the snack bubbles away to the surface,
whereas the vapour released from the bottom surface forms a buoyant blanket
layer. By asymptotic analysis, we show that the model simplifies to solving a
one-dimensional Stefan problem in the snack coupled to a thin-film equation in
the vapour blanket through a non-linear boundary condition. Using our
mathematical model, we predict the change in the snack density as a function of
time, and investigate how lift-off time depends on the different parameters of
the problem.
