We report on environmental effects on the optical properties of single-wall carbon nanotubes in a gelatin-based composite material designed to foster their photoluminescence. We show that the dielectric screening of excitons due to the surrounding medium is responsible for a sizeable shift of the luminescence lines which hardly depends on the tube geometry. In contrast, the temperature dependence (from 4 to 300 K) of the luminescence is clearly chirality dependent : the first and second excitonic lines shift in opposite directions with a magnitude that can be related quantitatively to a strain-induced modification of the electronic structure due to an expansivity mismatch between the nanotube and the matrix.PACS numbers: