Improving thermal stability of hydrolysed whey protein-based infant formula emulsions by protein–carbohydrate conjugation

Whey protein hydrolysate (WPH) ingredients are commonly used in the manufacture of partially-hydrolysed infant formulae. The heat stability of these emulsion-based formulae is often poor, compared with those made using intact whey protein. The objective of this study was to improve the heat stability of WPH-based emulsions by conjugation of WPH with maltodextrin (MD) through wet heating. Emulsions stabilised by different protein ingredients, whey protein isolate (WPI_E), whey protein hydrolysate (WPH_E), heated WPH (WPH-H_E), and WPH conjugated with MD (WPH-C_E) were prepared and heat treated at 75 °C, 95 °C or 100 °C for 15 min. Changes in viscosity, fat globule size distribution (FGSD) and microstructure, evaluated using confocal laser scanning microscopy (CLSM), were used to monitor the effects of hydrolysis, pre-heating and conjugation on the heat stability of the emulsions. Heat stability increased in the order WPH_E < WPI_E << WPH-H_E <<< WPH-C_E; emulsions WPH_E, WPI_E and WPH-H_E destabilised on heating at 75 °C, 95 °C or 100 °C, respectively. Flocculation and coalescence of oil droplets were mediated by protein aggregation (as evidenced by CLSM) on heat treatment of WPH-H_E emulsion at 100 °C, while no changes in FGSD or microstructure were observed in WPH-C_E emulsion on heat treatment at 100 °C, demonstrating the excellent thermal stability of emulsions prepared with the conjugated WPH ingredient, due principally to increased steric stabilisation as a result of conjugation.