Surface characterisation of dairy powder particles

Dairy powders are common ingredients used by food manufactures and consumers, with high-protein dairy powders becoming more in demand, due to their nutritional and functional benefits. The functionality of these powders can be greatly influenced by their surface composition, as alterations at the surface can impede the rehydration of the powders after long-term storage. Therefore, this research investigated the surface characteristics of dairy powders and how it is influenced by composition, dryer type, degree of agglomeration and the stability of the surface components during storage of the powder, focusing mainly on the protein fraction. This research provides an understanding of the nature of the particle surface of selected dairy powders (milk protein concentrate, skim milk, whole milk) and what happens at the surface, which may provide a basis for the development of new or improved powders. Surface composition of dairy powders was determined by the main analysis tool used in this investigation, XPS (X-ray photoelectron spectroscopy). Other major experimental techniques used during this research include: scanning electron microscopy (SEM), atomic force microscopy (AFM), confocal laser scanning microscopy (CLSM), tensiometers (for surface tension work) and the Malvern Mastersizer (for particle sizing). The optimisation of XPS analysis for the study of dairy powders was evaluated. A change in the relative atomic percentage of the elements (C, O, N) was used (via a matrix formula) to determine the relative amount of protein, fat and lactose on the surface of the powders. The surface composition of dairy powders was partially influenced by the surface active components within the feed concentrate prior to spray drying, as noted in the protein/lactose powder experiments. The casein proteins were generally more surface active than the whey proteins. In another work, the application of cold stage XPS for the analysis of a milk droplets surface was developed and tested. It was thought this method would capture the surface composition of a droplet within seconds of its formation, similar to that of a droplet formed during spray drying. This work indicated that the part of the surface coverage is instantaneous, possibly due to rapid orientation and spreading of the hydrophobic moieties of the protein molecules present at the proximity of the interface. The surface morphology of dairy powders were also examined, where the morphology of the particles can be greatly influenced by the amount of fat, protein and lactose in the feed concentration consequently on the particle surface; particles high in protein had a more collapsed structure than those high in lactose, and particles high in fat were more spherical. The effect of size of the dryer on the surface composition of the powder was also studied. Commercially spray-dried powders were found to be different to those produced on a research scale (i.e. laboratory and pilot plant). The main differences can be attributed to differences in the feed concentration, size of the atomizer, spray dryer settings and complexity (size, number of chambers etc). The viscosity was also found to be one of the limiting factors on surface coverage by protein. Effect of storage condition on the changes to the surface composition was studied using MPC as a model powder. The main analytical methods that were used to analyse the powders include; AFM, XPS, SEM. The AFM scanning probe microscopy indicated that the surface became more hydrophobic, with the formation of a thin crust, however the overall surface coverage of the components remained relatively similar. The solubility of the particles also decreased (by 20 – 65%) during storage, depending on the storage temperature and relative humidity