Cold-set thickening mechanism of β-lactoglobulin at low pH: Concentration effects

There is an interest in developing protein based thickening agents for nutritional considerations. A procedure to convert whey protein concentrates or isolates into a pH modified cold-thickening ingredient was developed. Concentration effects on thickening mechanism of this whey protein ingredient were studied with a β-lactoglobulin model system at the pH of the modification procedure, 3.35. In this study, concentration effects on thermal aggregation of β-lactoglobulin were studied at low pH using capillary and rotational viscometry, transmission electron microscopy (TEM), and high performance liquid chromatography coupled with multi-angle laser light scattering (HPLC-MALS). From the results of capillary viscometry, a critical concentration (C_c  6.9% w/w) was identified below which no significant thickening functionality could be achieved. Microscopy revealed formation of flexible fibrillar network at pH 3.35 during heating at all concentrations. These flexible fibrils had a diameter of about 5 nm and persistence length of about 35 nm as compared to more linear and stiff fibrils formed at pH 2 and low ionic strength conditions. Under similar heating conditions at concentration above C_c, larger aggregates similar to microgels were observed compared to the concentration below C_c, where isolated fibrils with an average contour length of about 130 nm were observed. These microgels and apparently stronger interactions between aggregates at concentrations above C_c were seemingly responsible for thickening functionality of heated β-lactoglobulin solutions and subsequently modified powders. Further investigation of β-lactoglobulin aggregation at this pH may provide capability to mechanistically tailor the functional attributes of modified ingredients.