Effects of pH on the structural and functional properties of soy lecithin-concentrated milk protein

Milk protein concentrate (MPC) has been widely recognized as an ideal functional ingredient in the food and nutritional industries, due to its superior nutritional profile and excellent digestibility. However, its application has been significantly constrained by the low solubility during storage. Fortunately, soy lecithin (SL) can be expected to interact with MPC upon co-dispersion in deionized water, leading to the formation of a soy lecithin–milk protein concentrate (SL-MPC) composite. The solubility of MPC can also be enhanced, with the optimal effect at an SL-to-MPC mass ratio of 0.8:1. This study aims to clarify the influence of different pH conditions on the structural and functional properties of the SL-MPC composite in artificial food matrices. Environmental pH was systematically adjusted for the different patterns. Then the optimal parameters were identified for the mechanistic pathways. The experimental protocol was set as follows. The pre-formed SL-MPC composite was first dispersed in phosphate buffer solutions. The pH values were then adjusted from 5 to 8. The mixtures subsequently underwent a 3-hour stirring reconstruction phase followed by low-temperature hydration at 4 °C. SL-MPC composites were obtainewas treatment under pH conditions (pH 5–8). Analytical techniques were employed to characterize the induced structural modifications, including particle size and ζ-potential analysis, fluorescence spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). Concurrently, a systematic evaluation of functional properties was conducted, including solubility, emulsifying activity and stability, as well as foaming capacity and foam stability. The results demonstrated that the pH treatment had a significant impact on the structural and functional behavior of the SL-MPC composites. Particle size analysis revealed that there were great variations in the distribution of particle size over the pH range. The distribution of particle size was shifted from a unimodal to a bimodal profile at pH 8, indicating a fundamental alteration in the dispersion state and aggregation dynamics in the system. SEM micrographs also showed that there were great variations in the morphologies. Furthermore, ζ-potential measurements indicated that the composite shared the highest absolute surface charge (16.5 mV) at pH 8, indicating maximum electrostatic repulsive forces, which contributed substantially to colloidal stability. Spectroscopic analysis also provided insights into conformational changes at the molecular level. FTIR spectroscopy detected that the absorption bands were shifted to amide I and II, thereby indicating pH-induced modifications in protein secondary structure and/or intermolecular interactions within the composite. UV-Vis spectroscopy showed a bathochromic (red) shift of the characteristic protein absorption peak in the far-UV region, from 195 nm at pH 5 to 204 nm at pH 8. There were alterations in the protein's conformational state and solvation environment. Fluorescence spectroscopy data indicated that there were significant variations in fluorescence intensity over the pH range, indicating the tertiary structure of protein components. Functional property assessments showed that there was a correlation between pH and performance. The most favorable functional profile was achieved in the SL-MPC composites at pH 8. Specifically, the highest levels of solubility and emulsifying stability were obtained with the commendable foaming capacity and foam stability. The high performance was achieved at pH 8. In summary, the structure of the SL-MPC composite was effectively modulated at the medium pH of 8. Treatment also enhanced the solubility, emulsifying properties, and foaming characteristics. Synergistic effects of an elevated ζ-potential were attributed to electrostatic stabilization and adaptive conformation in the protein structure. The findings can provide a robust theoretical foundation to optimize the functionality of milk protein ingredients using pH control. Furthermore, the valuable practical guidance can also be used to expand such composites into a diverse array of food applications, including liquid formulations, emulsified products, and aerated or foam systems.