  Dusty starless cores play an important role in regulating the initial phases
of the formation of stars and planets. In their interiors, dust grains
coagulate and ice mantles form, thereby changing the millimeter emissivities
and hence the ability to cool. We mapped four regions with more than a dozen
cores in the nearby Galactic filaments of Taurus and Perseus using the NIKA2
camera at the IRAM 30-meter telescope. Combining the 1mm to 2mm flux ratio maps
with dust temperature maps from Herschel allowed to create maps of the dust
emissivity index $\beta_{1,2}$ at resolutions of 2430 and 5600 a.u. in Taurus
and Perseus, respectively. Here, we study the variation with total column
densities and environment. $\beta_{1,2}$ values at the core centers
($A_V=12-19$mag) vary significantly between $\sim1.1$ and $2.3$. Several cores
show a strong rise of $\beta_{1,2}$ from the outskirts at $\sim4$mag to the
peaks of optical extinctions, consistent with the predictions of grain models
and the gradual build-up of ice mantles on coagulated grains in the dense
interiors of starless cores.
