Depletion flocculation and thermodynamic incompatibility in whey protein stabilised O/W emulsions

The stability of whey protein stabilised emulsions, containing methylcellul ose added after emulsification in their bulk phase, was investigated. The p hase diagram of the ternary system whey proteins/methylcellulose/water was first established and used to identify the conditions permitting polymer ph ase separation within the emulsion bulk phase. Emulsions containing a whey protein and methylcellulose concentration in the bulk phase below and above the phase separation threshold could therefore be prepared. Below the phas e separation threshold, the creaming rate of the oil droplets was faster th an the one predicted by the Stokes equation, due to methylcellulose-induced depletion flocculation. Above the phase separation threshold, the destabil isation of the emulsion involved different mechanisms, depending on the emu lsifier adsorbed at the O/W interface. In the case of Tween 40 stabilised d roplets, depletion flocculation led to a complete creaming of the fat globu les while phase separation led to the formation of two polymer-rich phases, namely a protein-rich phase at the bottom of the tube and a methylcellulos e-rich phase above. In the case of whey protein stabilised droplets, phase separation between bulk whey proteins and methylcellulose occurred, and the fat globules were entrapped in the protein-rich phase. These results permi tted to describe the destabilisation mechanisms of both Tween 40 and whey p rotein stabilised emulsions in the presence of unadsorbed polysaccharide. T hey could be used to better understand the destabilisation processes arisin g in food emulsions, especially in those emulsions containing whey proteins , small surfactant molecules and polysaccharides. (C) 2000 Elsevier Science Ltd. All rights reserved.