Emulsification using micro porous glass (MPG): surface behaviour of milk proteins

To investigate the emulsifying properties and adsorption behaviour of high molecular amphiphilic substances such as proteins, it is important to maintain the native status of the used samples. The method of micro porous glass (MPG) emulsification could offer an opportunity to do this because of the low shear forces. The oil-in-water emulsions were produced by dispersing the hydrophobic phase (liquid butter fat or sunflower oil) through the MPG of different average pore diameters (d(p) = 0.2 or 0.5 mu m) into the flowing continuous phase containing the milk proteins (from reconstituted skim milk and buttermilk). The emulsions were characterised by particle size distribution, creaming behaviour and protein adsorption at the hydrophobic phase. The particle size distribution of protein-stabilised MPG emulsions is determined by the pore size of MPG, the velocity of continuous phase (or wall shear stress a,) and the transmembrane pressure (Delta p(TM)). A high velocity of (v) over bar = 2 m s(-1) (sigma(w) = 13.4 Pa) and low pressure (pressure of disperse phase slightly exceeded the critical pressure Delta p(TM) = 4.5 bar of 0.2 mu m-MPG) led to the smallest droplet diameter. As a consequence of average droplet diameters of d(43) > 3.5 mu m creaming was observed without centrifugation in all MPG emulsions after 24 h, but no coalescence of the oil droplets occurred. The study of protein adsorption showed that the MPG emulsification at low shear forces resulted in lower protein load values (2.5 +/- 0.5 mg m(-2)) than pressure emulsification (11.5 +/- 1.0 mg m(-2)). In addition, the various emulsification conditions (MPG or pressure homogenization) led to differences in the relative proportions of casein fractions, whey proteins and milk fat globule membranes (MFGM) at the fat globule surfaces. (C) 1999 Elsevier Science B.V. All rights reserved.