  We measure the fluctuations of the position of a Brownian particle confined
by an optical trap in an aging gelatin droplet after a fast quench. Its linear
response to an external perturbation is also measured. We compute the
spontaneous heat flux from the particle to the bath due to the nonequilibrium
formation of the gel. We show that the mean heat flux is quantitatively related
to the violation of the equilibrium fluctuation-dissipation theorem as a
measure of the broken detailed balance during the aging process.
