The Effect of High Protein Powder Structure on Hydration, Glass Transition, Water Sorption, and Thermomechanical Properties

Abstract

Poor solubility of high protein milk powders can be an issue during the production of nutritional formulations, as well as for end-users. One possible way to improve powder solubility is through the creation of vacuoles and pores in the particle structure using high pressure gas injection during spray drying. The aim of this study was to determine whether changes in particle morphology effect physical properties, such as hydration, water sorption, structural strength, glass transition temperature, and α-relaxation temperatures. Four milk protein concentrate powders (MPC, 80%, w/w, protein) were produced, i.e., regular (R) and agglomerated (A) without nitrogen injection and regular (RN) and agglomerated (AN) with nitrogen injection. Electron microscopy confirmed that nitrogen injection increased powder particles' sphericity and created fractured structures with pores in both regular and agglomerated systems. Environmental scanning electron microscopy (ESEM) showed that nitrogen injection enhanced the moisture uptake and solubility properties of RN and AN as compared with non-nitrogen-injected powders (R and A). In particular, at the final swelling at over 100% relative humidity (RH), R, A, AN, and RN powders showed an increase in particle size of 25, 20, 40, and 97% respectively. The injection of nitrogen gas (NI) did not influence calorimetric glass transition temperature (T_g), which could be expected as there was no change to the powder composition, however, the agglomeration of powders did effect T_g. Interestingly, the creation of porous powder particles by NI did alter the α-relaxation temperatures (up to ~16 °C difference between R and AN powders at 44% RH) and the structural strength (up to ~11 °C difference between R and AN powders at 44% RH). The results of this study provide an in-depth understanding of the changes in the morphology and physical-mechanical properties of nitrogen gas-injected MPC powders.

Keywords: dynamic mechanical analysis (DMA); environmental scanning electron microscope (ESEM); gas injection; glass transition; milk protein concentrate (MPC); structural strength; α-relaxation.

Figure 1

ESEM micrographs of MPC particles’ swelling captured at 800×, 1600× and 3000× magnifications. The rows show the hydration of regular (R), agglomerated (A), agglomerated NI (AN), and regular NI (RN) MPC powder particles, at 4 °C. The columns show the particles at 20% RH (1.2 Torr), 50% RH (3.0 Torr), 90% RH (5.5 Torr), 100% RH (6.1 Torr), and >100% RH (6.9 Torr). Scale bars represent 50 µm. The arrow clearly shows the occluded air bubbles below the particle’s surface created by nitrogen injection in the fractured AN powder particle.

Figure 2

ESEM micrographs showing the dynamic process of hydration (rows) captured at 800× magnification of regular (R), agglomerated (A), agglomerated NI (AN), and regular NI (RN) milk protein concentrate powder (MPC) particles at 4 °C: (A) MPC particles observed prior to hydration at 50% RH (3.0 Torr); (B) MPC particles during hydration at >100% RH (6.4 Torr) at 6, 12, 18, 24, and 30 min. Scale bars represent 300 µm.

Figure 3

ESEM micrographs showing the post-hydration of the samples in Figure 2 for regular (R), agglomerated (A), agglomerated NI (AN), and regular NI (RN) milk protein concentrate powder (MPC) particles at 4 °C. Columns show the morphology of the resultant residue captured at 800×, 3000×, and 6000× magnifications. Scale bars represent 50 µm.

Figure 4

Glass transition temperature (T_g) measured as a function of water content and water activity (a_w) of regular (R), agglomerated (A), regular nitrogen-injected (RN), and agglomerated nitrogen-injected (AN) milk protein concentrate powders (MPCs). Lines correspond to the T_g predicted by the Gordon–Taylor equation. Symbols correspond to T_g values obtained experimentally by differential scanning calorimetry.

Figure 5

Modified Williams–Landel–Ferry curves (lines) and experimental data (symbols) for: (A) regular; (B) agglomerated; (C) regular nitrogen-injected; (D) agglomerated nitrogen-injected milk protein concentrate powders, measured as a function of relative humidity at 0 ($\square$), 23 (◊), 33 (∆), and 44% (ο).