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The effect of lecithin addition on the wettability of whey protein isolate (WPI) powders was investigated in this study. Different concentrations of lecithin solution (0.5%, 2% and 5% w/v) were added by fluidised bed agglomeration and Wurster coating processes. The formed WPI powders, with different particle sizes, were used to compare the densities, porosity and shapes properties, as well as the wetting behaviours, including wetting time, contact angles, water absorption by capillary flow and the strength of films formed at the powder/water interfaces. It was found that, if lecithin was used as bridges to adhere primary particle in agglomeration, the formed granules had large particle size, lower bulk density, higher porosity and more irregular shapes. The influences of the coating process on these properties were shown to be less significant, as lecithin was layered on the outside of the particle surfaces. Those physical and structural modifications in WPI powders caused the improvements in the wettability by different degrees. High coverage of lecithin by coating process gave the particles a more advantageous first step of wetting, showing smaller initial contact angles and weaker protein films; while the porous structures by agglomeration process resulted in quicker water penetration by capillary flow. They were both rate-controlling factors for wetting process, as the limited contribution was found if only controlled by any one of these factors. Consequently, high coverage of lecithin, high porosity of particles and large particle sizes were all necessary for enhancing the wetting behaviours of WPI powders.

Rehydration of milk protein isolate (MPI) powder in water is poor which requires more than 24 h, however it is greatly influenced by solution pH. This study analysed the rehydration of MPI powder in acidic solution followed by neutralization (CN), and in alkalized solution followed by neutralization (NC), and how these pH adjustments impact the dissolution ability and functionality of the MPI. Besides, C’N’ and N’C’ are prepared by the above neutralization sequence with increasing rehydration time. Acid gelation was chosen as an example of critical functional properties. The results showed that applying alkalization (performed by Na-carbonate) as the first step prior to neutralization was superior to acidification (using citric acid) in terms of both dissolution and gelation. MPI powder rehydrated in distilled water gelled very poorly, and this was due to slow dissolution, while neutralized MPI (CN, C’N’, NC and N’C’) produced much stronger gels, in-part due to rapid dissolution and disrupted casein micelle. NC and N’C’ exhibited better gelation characteristics than CN and C’N’ overall, showing much higher elasticity values and earlier gelation point. These were also compared with the rehydration of sodium caseinate, which displayed more rapid dissolution and consequently faster gelation; however, the gelation characteristics became similar to NC over time. N’C’ with longer rehydration time could display further improved gelation characteristics owning higher elasticity than that of SC and SC’. In addition, the neutralized MPI was able to tolerate over-acidification during the acid gelation process.

Rehydration is an important powder property and is regarded as a critical issue by the dairy industry. Traditional powder rehydration measurements are relatively empirical with poor reproducibility. Thus, more reliable techniques tailored for dairy powders should be developed based on varied rehydration behaviours and applications. In this paper, a critical assessment to identify the measurement characteristics of milk protein powder rehydration is presented. Milk protein based powders were used as model systems. Four different wettability measurements (Immersion, Capillary rise, Condensation and Spreading) and four different dispersibility measurements (Dispersibility Index, Light scattering of particles in suspension, Light transmission and Conductivity of suspension) are compared and analysed. The results show that the method based on immersional wetting procedure is only appropriate for skimmed milk powder while the method for capillary rise wetting is more useful for the agglomerated milk protein powders with porous structures. Contact angle changes in the spreading wetting approach is found to be a straightforward technique to show the hydrophobicity or hydrophilicity of milk protein powders. If compared with traditional dispersibility measurements, light transmission of suspension is suitable to reflect optical properties of slow dispersion process. Light scattering methods can also be used to measure the dynamic size change of particles during the dispersion process. Furthermore, the conductivity of suspensions is a useful indicator to quantify the dispersibility indirectly by the release of minerals during rehydration. In summary, it is necessary to understand the specialities and applications of dairy powders before choosing the appropriate rehydration measurement methods.

Milk protein isolate (MPI) powder has poor rehydration ability. An alkaline pH environment has the potential to alter and improve its rehydration behaviour. Sodium carbonate, an approved food additive, is an alkaline reagent and consequently has some potential for improving the rehydration behaviour of MPI powder. The addition of sodium carbonate in this study was adjusted between 0 and 1 g/100 mL concentrations in water. The results showed that wettability worsened with higher sodium carbonate concentrations, due to strong films developing at the powder/water interfaces which inhibited water penetration into the bulk. However, appropriate agitation could effectively wet and disperse the powder. On the other hand, particle size and centrifugal sedimentation studies showed that increasing the sodium carbonate concentration greatly improved the dissolution ability of the MPI powder. The best dissolution occurred in 0.5 g/100 mL sodium carbonate solution, while some large undesirable aggregates occurred at 1 g/100 mL addition. The improved dissolution is most likely due to the higher pH increasing negative electrostatic repulsion and decreasing hydrophobic group attractions on casein micelle surfaces.