Phase behaviour of a wheat protein isolate

Abstract

We investigated the liquid–liquid phase separation of wheat storage proteins with molecular weight (MW) ranging from 25 kg mol⁻¹ to 300 kg mol⁻¹. We characterized the onset of the phase separation upon a temperature decrease by turbidity. The protein compositions at equilibrium after temperature quenches were additionally determined by size-exclusion-high performance liquid chromatography. We found that wheat proteins can be classified into two classes according to their phase behaviour. The partition of the proteins between the two phases depends on their MW above a critical size of 45 kg mol⁻¹. For those high MW proteins, we observed that the behaviour is similar to the one of neutral, linear polymers. Their partition and critical parameters are well described by Flory–Huggins theory. Proteins below 45 kg mol⁻¹, namely α-, β-and γ-gliadins, have similar interaction potentials under the physico-chemical conditions tested. Their phase diagrams, characterized by a low value of critical volume fraction, are characteristic of long-range interactions. Wheat protein behaviour can resultantly be rationalized in a much simpler way than expected from their complex composition.