Phase-separated biopolymer mixture rheology: Prediction using a viscoelastic emulsion model

Abstract

The relationship between the morphology and rheology of phase-separated biopolymer mixtures is investigated. Biopolymer mixtures, which are utilized in the food industry for their textural and structuring properties, often phase separate and demix to form water-in-water emulsions. Controlling the morphology of biopolymer mixtures during flow processing and inducing gelation of one or both phases lead to products with novel microstructures and material properties [B. Wolf et al., Food Hydrocolloids 14, 217–225 (2000)]. An emulsion model [J. F. Palierne, Rheol. Acta 29, 204–214 (1990)], commonly used for the prediction of the linear viscoelastic properties of polymer blends, is used here to relate the rheology to the morphology of water-in-water emulsions. The system under investigation is a gelatin–maltodextrin mixture which phase separates at 60 °C for particular concentrations, characterized by a binodal curve, into a gelatin-rich and maltodextrin–rich phase. Emulsions with phase volumes of 10% and 30% were examined with either phase as the dispersed phase. The morphology varies with the preshear rate such that the radius of droplets after a preshear of 10 s−1 is around 20–50 μm while after a preshear of 100 s−1 the droplets are typically less than 10 μm. Despite the low viscosity, elasticity, and interfacial tension of the gelatin–maltodextrin emulsion, the emulsion model is found to predict the rheology and morphology of the mixtures subjected to preshear rates of 1–100 s−1. The interfacial tension for the gelatin–maltodextrin system studied is approximately 50 μN/m at 60 °C.