Lactose/milk protein isolate (MPI) solids systems were prepared by laboratory-scale spray dryer (defined as S-lactose/MPI solids systems) and pilot-scale spray dryer (defined as L-lactose/MPI solids systems). Particles of L-lactose/MPI solids systems had more rounded shape, and smoother surface than those of S-lactose/MPI solids systems. Water sorption study showed S-lactose/MPI solids systems sorbed 5-30% larger amount of water than L-lactose/MPI solids systems after equilibrated at different water activities (0.11 a(w)-0.44 a(w)). Besides, comparing the samples with same composition, dairy solids with small size particles showed higher steady water content at the end of lactose crystallisation. Moreover, S-lactose/MPI solids systems showed lower glass transition temperature (T-g) values than L-lactose/MPI solids systems after equilibration at 0.11 a(w), 0.23 a(w), and 0.33 a(w), which might be due to the higher water content of 5-lactose/MP' solids systems. The study of mechanical properties indicated the stiffness of S-lactose/MPI solids systems was higher than that of L-lactose/MPI solids systems at 0.23 a(w) and 0.33 a(w) when temperature was below T-g values. Water plasticization showed stronger effect on relaxation process of dairy solids with smaller size particles. (C) 2015 Elsevier Ltd. All rights reserved.