Effect of pH and heat treatment conditions on physicochemical and acid gelation properties of liquid milk protein concentrate

Milk protein concentrates (MPC) are typically dried high-protein powders with functional and nutritional properties that can be tailored through modification of processing conditions, including temperature, pH, filtration, and drying. However, the effects of process -ing conditions on the structure-function properties of liquid MPC (fluid ultrafiltered milk), specifically, are understudied. In this report, the pH of liquid MPC [13% protein (70% protein DM basis), pH 6.7] was adjusted to 6.5 or 6.9, and samples at pH 6.5, 6.7, and 6.9 were subjected to heat treatment at either 85 degrees C for 5 min or 125 degrees C for 15 s. Sodium dodecyl sulfate PAGE was used to determine the distribution of caseins and denatured whey proteins in the soluble and micellar phases, and HPLC was used to quantify native whey proteins as a measure of denaturation, based on the processing conditions. Both heat treatments resulted in substantial whey protein denaturation at each pH, with beta-lactoglobulin denatured more extensively than alpha-lactalbumin. Changes in liquid MPC physicochemi-cal properties were monitored at d 1, 5, and 8 during storage at 4 degrees C. Viscosity increased after heat treatment and also over time, regardless of pH and heating condi-tions, suggesting the role of whey protein denaturation and aggregation, and their interactions with casein micelles. The MPC samples processed at pH 6.9 had a significantly higher viscosity than those heated at pH 6.5 or 6.7, for both temperature and time conditions; and samples processed at 85 degrees C for 5 min had higher viscosity than those heated at 125 degrees C for 15 s. Particle size analysis indicated the presence of larger particles after 5 and 8 d of MPC storage after heating at pH 6.9. Acid-induced gelation of the liquid MPC led to sig-nificantly higher gel firmness after processing at 85 degrees C for 5 min, compared with 125 degrees C for 15 s. Also, gels made from MPC adjusted to pH 6.5 had higher storage moduli, with both time and temperature combinations, demonstrating the role of pH-dependent association of denatured whey proteins with casein micelles in gel network formation. These findings enable a better understanding of the processing factors contributing to structural and functional properties of liquid MPC and can be helpful in tailoring milk protein ingredient functionality for a variety of food products.

Automated summary

This study investigated the structural and functional properties of liquid MPC by adjusting pH and heat treatment conditions, finding that heating led to denaturation and aggregation of whey proteins, resulting in increased viscosity. Samples processed at higher pH showed significantly higher viscosity, while those treated at 85 degrees C for 5 min had higher viscosity than those at 125 degrees C for 15 s.