Impact of Heating and Shearing on Physicochemical Properties of Reconstituted Milk Protein Concentrates

Reconstituted milk products, particularly milk protein concentrates (MPC), are becoming increasingly popular for their enhanced qualities and health benefits, playing a pivotal role in protein-rich dairy beverages. However, essential thermal treatments for ensuring safety, such as pasteurization and sterilization, along with shear forces from mechanical processing, can modify their physical, nutritional, and functional properties. Additionally, the stability of these dairy beverages is influenced by factors such as protein concentration, pH, and the addition of chelators and hydrocolloids during production. Therefore, the primary objective of this study was to investigate the impact of these factors on the structural and functional behaviour of milk proteins in MPCs, specifically under the combined effects of heat/time (90°C/5 min or 121°C/2.6 min) and shear rate (100, 1000, or 1500 s-1). MPC suspensions were subjected to heat and shear treatments using a rheometer equipped with a narrow-gap coaxial cylinder pressure cell. The secondary structural changes of the proteins were analysed using Fourier transform infrared (FTIR) spectroscopy. Protein denaturation and aggregation were assessed using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and high-performance liquid chromatography (HPLC). Mineral analysis was performed using a calcium ion selective electrode and inductively coupled plasma optical emission spectrometry (ICP-OES). Particle size and zeta potential analyses were conducted using a Zetasizer. The heat stability of reconstituted MPCs at different protein concentrations (4% and 8% w/w) was evaluated under combined heat and shear. The results demonstrated significant differences in heat stability between the two concentrations. For MPC suspensions with 4% protein, the impact of shear on heat stability was relatively minor. However, this effect became more pronounced in suspensions with an 8% protein concentration, leading to noticeable reductions in heat stability. The higher protein concentration in these suspensions facilitated increased protein-protein interactions under shear, resulting in greater protein aggregation at elevated temperatures. Consequently, this adversely affected the heat stability of these reconstituted MPC’s. Subsequent investigations were conducted utilizing an 8% MPC concentration, given the marked influence of shear on heat stability observed at this level. The second study explored the behaviour of milk proteins in suspensions adjusted to pH levels of 6.1, 6.4, 6.8, or 7.5, under combined heat and shear treatments. Under alkaline conditions, shear forces notably enhanced heat-induced micellar dissociation. Conversely, at pH 6.1, shear forces facilitated heat-induced aggregation, whereas at pH 6.4, the effect of shear predominantly led to the fragmentation of aggregates. Further studies were focused upon common additives used in the dairy industry and their impact on the shear effect. The impact of calcium sequestering salts (CSS), disodium hydrogen phosphate (DSHP) and trisodium citrate (TSC), on the heat stability of milk proteins was analysed. Combined shear and heat were observed to significantly affect the milk system when CSS is present. Moreover, the lower calcium binding affinity of DSHP reduces micellar disruption, thereby enhancing the effect of shear. Finally, the heat stability of milk protein dispersions with added cocoa powder (1.5% (w/w)), sucrose (7% (w/w)), and varying levels of κ-carrageenan (0.01, 0.03, or 0.05% (w/w)) under combined heat and shear conditions was examined. Shear forces led to considerable protein aggregation, particularly at higher κ-carrageenan concentrations. The aggregation was mainly due to micelle destabilization and the presence of loosely bound caseins within the κ-carrageenan network, rendering them more prone to aggregation as shear increased collision frequencies. Shear forces present in dairy processing are critical in defining the attributes of the final product, thereby underscoring the necessity to meticulously consider these dynamics within the realm of dairy science research. This study significantly advances food science by examining how combined heat and shear treatments affect the properties of MPCs, essential for optimizing protein-rich dairy beverages. Understanding these dynamics leads to enhanced safety, quality, and functionality in dairy products, contributing to broader innovation and sustainability in the food industry.