Segregative interactions between gelatin and polymerised whey protein

Abstract

Polymerisation of whey proteins increases their potential use as food ingredients. In this investigation, mixtures of 6 wt% whey protein isolate (WPI) with 1–4 wt% type B (alkaline extract) gelatin, before and after polymerisation of WPI (heating to 85 °C and quenching to 5 °C), were characterised by measurements of solution viscosity at 45 °C (above the gelation temperature of gelatin) and by changes in optical rotation and turbidity (measured as optical density at 600 nm) over the temperature range 45–5 °C. Viscosities for mixtures of gelatin with un-polymerised WPI were only slightly higher than for gelatin alone, but after polymerisation they rose steeply above those of the individual components as the gelatin concentration was increased, giving values of 1.84 mPa s for 6 wt% polymerised WPI, 4.07 mPa s for 4 wt% gelatin, and 36.5 mPa s for the mixture of these. The large enhancements in viscosity are attributed to self-association of polymerised WPI in response to segregative interactions with gelatin. On cooling to 5 °C to gel the gelatin component, the unheated mixtures containing un-polymerised WPI showed only slight increases in turbidity, similar to those observed for WPI alone (polymerised or un-polymerised). Cooled mixtures of gelatin and polymerised WPI, however, gave very large increases in turbidity, which were fully reversible on re-heating to 45 °C. Optical rotation measurements showed no indication of direct association between gelatin and polymerised WPI at any temperature in the range studied (45–5 °C). It is suggested that association of gelatin molecules in the early stages of gelation triggers phase separation, giving a continuous matrix of gelatin gel with the polymerised whey protein dispersed through it in small droplets (which increase turbidity by scattering light).