Laboratory simulation of interstellar grain processing is a unique tool to better understand the nature and evolution of cosmic dust. In recent years this approach has been crucial to outline a new model of evolution of the aliphatic component of organic matter in the interstellar medium. Here, the results of a recent laboratory research on processing of nano-sized carbon particles by H atoms under simulated dense medium conditions are discussed. The experiments show that the formation of C-H bonds in the aliphatic CH2 and CH3 functional groups does not take place, while the activation of a band at 3.47 μm, due to the C-H stretching vibration of tertiary sp3 carbon atoms, is observed. These results indicate that the assumption about inhibition of aliphatic C-H bond formation in interstellar dense clouds is correct. Moreover, they suggest that carbon grains responsible for the interstellar aliphatic band at 3.4 μm in diffuse regions can contribute to the absorption observed at 3.47 μm in dense clouds.