Roles of particle size on physical and mechanical properties of dairy model solids

• Lactose/MPI solids systems with different size particles were prepared by different spray dryers.
• The physical properties and mechanical properties of lactose/MPI solids systems with different size particles were studied.
• Dairy solids with larger size particles sorbed lower amount of water.
• The stiffness of diary solids with smaller size particles was stiffer.

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 aw–0.44 aw). 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 (Tg) values than L-lactose/MPI solids systems after equilibration at 0.11 aw, 0.23 aw and 0.33 aw, which might be due to the higher water content of S-lactose/MPI 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 aw and 0.33 aw when temperature was below Tg values. Water plasticization showed stronger effect on relaxation process of dairy solids with smaller size particles.