Substitution of commercial milk powders by fluid milk concentrates as ingredients in dairy products, may improve product quality and increase sustainability. This study compared the functional properties of skim milk concentrates made by reverse osmosis (SMC) with standard reconstituted high- and low-heat skim milk powders (POW-SMC). SMC had larger Z-average size values, higher viscosities, wider particle size distributions and lower extent of whey protein denaturation than POW-SMC. During storage, SMC showed no acid-induced gelation, but improved rennet-induced gelation compared with POW-SMC. Emulsification activity improved at 20 compared with 10% solids in all concentrates, while emulsion stability was lowest for SMC. Better foaming was observed for high-heat POW-SMC, except after 30 days, where SMC showed highest overrun. In certain dairy products, SMC can thus replace commercial powders to reduce processing costs and improve functionality. SMC showed to be especially advantageous for rennet-gelled products.

The viscosity of skim milk concentrates increases during storage as result of interactions between milk proteins, minerals and other solids. The present study demonstrates how these interactions can be controlled by addition of calcium or glucono-delta-lactone. Milk concentrates, produced from reverse osmosis with 28% total solids, were heat-treated at 75 or 110 degrees C for 18 s, followed by addition of CaCl2 (0, 25 or 50 mM) and glucono-delta-lactone (0 or 80 mM), before storage at 5 degrees C for 20 days. Addition of CaCl2, glucono-delta-lactone or their combination were found to reduce the viscosity build-up during storage, but the extension depends on the intensity of heat-treatment and pH of the concentrates. The more pronounced effects were observed for concentrates heat-treated at 110 degrees C, which were also the more viscous concentrates. An increase of colloidal calcium reduce the partial specific volume of casein micelles leading to a less viscous milk matrix. (C) 2020 Elsevier Ltd. All rights reserved.

Skim milk concentrates have important applications in the dairy industry, often as intermediate ingredients. Concentration of skim milk by reverse osmosis membrane filtration induces water removal, which reduces the free volume between the colloidal components, in particular the casein micelles. Thermal treatment before or after concentration impacts the morphology of casein micelles. These changes affect the flow behavior and viscosity, but the consequences for supermicellar structure have not been elucidated. In the present study, skim milk concentrates with different total solid contents from 8.7% (control) up to 22.8% (w/w), prepared by reverse osmosis membrane filtration of non-heated and pasteurized skim milk, were heat treated at 75 degrees C for 18 s, and compared with non-heated concentrates. The structure of the concentrates was studied using Ultra Small Angle X-ray Scattering (USAXS), and the viscosity of concentrates was measured. The USAXS intensity I(q) was fitted at small and intermediate q-regions (0.0005 < q < 0.003 angstrom(-1) and 0.0035 < q < 0.03 angstrom(-1), respectively) with a power law. The value of the power law exponent was used to assess the heat-and concentration-induced aggregation of the milk solids and correlate it with the apparent viscosity. The results showed that increased viscosity of skim milk concentrates, due to water removal and heat-load, can be explained by increased aggregation of the casein micelles into elongated aggregates and increased smoothening of the casein micelle surface.

Milk is a ubiquitous foodstuff and food ingredient, and milk caseins are key to the structural properties of milk during processing and storage. Caseins self-assemble into nanometer-sized colloids, referred to as "micelles", and particles of this size are ideally suited to study by small-angle scattering (SAS). Previous SAS measurements have almost exclusively focussed on the internal structure of the micelles. While important for milk's properties, this attention to the interior of the micelles provides limited information about the structure-forming properties of milk and milk ingredients. The ultra-small-angle X-ray scattering (USAXS) measurements and analysis in this study extend to the micrometer scale, which makes it possible to characterize the interaction between the micelles. Until now, SAS studies have generally excluded a consideration of the interparticle interactions between casein micelles. This is inconsistent with these new data, and it is not possible to model the data without some interparticle attraction. If the micelles are treated as sticky spheres, excellent agreement between experimental data and model fits can be obtained over the length scales studied, from micrometers to ångströms. The stickiness of casein micelles will impact ultra-small-angle scattering and small-angle scattering measurements of casein micelles, but it particularly limits the application of simple approximations, which generally assume that particles are dilute and noninteracting. In summary, this analysis provides an approach to modelling scattering data over many orders of magnitude, which will provide better understanding of interactions between caseins and during food processing.

Concentrated dairy products are of increasing interest within the dairy industry. Skim milk concentrates can be produced by reverse osmosis membrane filtration, which can be considered a non-thermal process. Therefore, the physical properties of concentrates differ from the properties of concentrates produced by evaporation. In this study, reverse osmosis filtration of raw and pasteurised skim milk was carried out in batch up to 28% total solids content and the effect of thermal treatment (65-110 degrees C, 15 s) and storage at 5 degrees C up to ten days on rheological and physical properties of concentrates, were evaluated. Concentrates produced from pasteurised skim milk required longer concentration times and showed larger average casein micelle sizes, but limited structure buildup capability during storage compared to concentrates produced from raw milk which more readily created structural networks between the milk constituents and consequently had higher viscosity. Thermal treatment of concentrates increased their average particle size and viscosity, an effect enhanced by increasing the total solids content. Concentrates produced from non-pasteurised milk showed the strongest shear-thinning behaviour during storage. Thus, the thermal treatment of milk before or after the concentration process, controls the structure formation of skim milk concentrates during storage.