The effect of homogenisation pressure on the microstructure of milk during evaporation and drying: particle-size distribution, electronic scanning microscopy, water activity and isotherm.

Homogenisation is a widely used technique in manufacturing powdered milk with a direct impact on product solubility, and the homogenisation pressure is a central attribute of this process. We aimed to understand the effect of increasing homogenisation pressures (0/0, 15/5, and 75/5 MPa, 1st/2nd stages) on particle-size distribution during homogenised whole milk powder manufacture and rehydration of the final product. The fluid milk was thermally treated, homogenised, concentrated by rotary evaporation, and then dried using a spray dryer. Particle size (Dv90) was monitored at all stages of the manufacturing process. The final product (milk powder) was analysed using particle-size distribution, electronic scanning microscopy, water activity, and isotherms. The results demonstrated that increasing the homogenisation pressure leads to milk powder with smaller particle size when rehydrated (Dv90 values: 6.08, 1.48 and 0.64 µm for 0, 20 and 80 MPa, respectively). Furthermore, the volume (%) of the particles in the 'sub-micro' region (smaller than 1.0 µm) presented an inversely proportional profile to the homogenisation pressure (homogenised fluid milk: 86.1, 29.3 and 2.4%; concentrated milk: 86.1, 26.5 and 5.7%, and reconstituted milk powder: 84.2, 31.8 and 10.9%). Surprisingly, this pattern was not observed in the SPAN value (which corresponds to the width or range of the size distribution based on the volume). Additionally, the increase in the homogenisation pressure did not affect the sorption isotherm pattern. These results demonstrate that increasing the homogenisation pressure decreases the particle size of the reconstituted powdered milk, indicating the potential for future studies on how this phenomenon affects its physicochemical and final product properties.