Microstructure of whole milk powder and of insolubles detected by powder functional testing

Abstract

Summary:The functional properties of whole milk powder (performance during reconstitution in water and coffee) are routinely measured to determine powder quality. An examination of the microstructure of milk powder and of the insoluble material collected after applying a series of functional tests provided insight into why the insoluble material formed during reconstitution.The microstructures (transmission electron microscopy and confocal microscopy) of four commercial whole milk powder samples and of the insoluble material produced during functional testing were assessed. The microstructure of the whole milk powder could be related to features observed in the microstructure of some of the insolubles collected after powder functional testing. It is likely that heating conditions throughout the powder manufacturing process resulted in the denaturation of β-lactoglobulin and its interaction with other proteins and fat globule membrane components, thus influencing the solubility of the milk powder. The application of shear during processing resulted in the greatest change to powder solubility in this study. Increasing homogenisation pressure during processing resulted in fat globule size reduction, adsorption of casein micelles to the fat globule membrane, and the formation of clusters of fat globules caused by the sharing of adsorbed casein micelles. The presence of large numbers of these clusters in the milk powder increased the formation of insoluble material during powder reconstitution in water and coffee.

Controlling milk protein interactions to enhance the reconstitution properties of whole milk powders – A minireview

“…Often, surface fat or some fat spread over (part of) the surface of the particles of WMP can be observed by a microscopic examination of the reconstituted milk or the powder [3,4,14,33]. However, whether the surface fat is a "free fat" or an "uncovered fat" is still doubtful [4,25,26].…”

“…McKenna et al [14], using transmission electron microscopy in conjunction with particle size analysis and functional tests, examined a range of commercial WMPs. The most distinguishing feature in these micrographs was the significant quantity of micellar casein material that was associated with the fat globules, and the presence of hair-like structures, which were probably aggregates of denatured whey protein and κ-casein on the adsorbed casein micelles.…”

Formation and structure of insoluble particles in reconstituted model infant formula powders

“…Sediment can be collected by centrifugation, but the differentiation of SDP and WF is not straightforward. According to McKenna, Lloyd, Munro, and Singh (1999), the structure of SDP is analogous to powder particles. The reason for their slow dispersion is therefore likely due to either agglomerate structure or surface composition.…”

“…Based on the previous research (De Ruyck, 1991;McKenna et al, 1999;Mol, 1975;Ohba, Takahashi, & Igarashi, 1989) the formation of insoluble material is hypothesised to originate from heat and shear applied during spray drying resulting in changes in fat-protein interactions. Also, emulsion quality before drying (Schmidmeier, O'Gorman, Drapala, & O'Mahony, 2017) and fat crystallisation (Regost, 2016) as well as whey protein denaturation (Joyce, Brodkorb, Kelly, & O'Mahony, 2017;McKenna et al, 1999) at different stages of drying may play a role. Lactose crystallisation during humid storage has been reported to result in disruption of fat globules, higher amount of free fat and rougher particles in model infant formula powders made with different protein-to-fat ratios (McCarthy et al, 2013).…”

Fat supramolecular structure in fat-filled dairy powders: A tool to adjust spray-drying temperatures

“…Since then, the technique has been largely used and the surface composition was related to powder physical properties such as wettability, flowability and stickiness [13,18,25,30,31,39]. Few studies considered the influence of fat droplet size and the measurement method was either laser light scattering [15,18,29,39] or, more arguably, image analysis of phase-contrast light micrographs [12,21].…”