MICROFLUIDIZATION OF MODEL DAIRY EMULSIONS .2. INFLUENCE OF COMPOSITION AND PROCESS FACTORS ON THE PROTEIN SURFACE CONCENTRATION

The influence of some composition variables (butter oil, sodium casein ate and monoglyceride contents) and process variables (pressure and te mperature) on the relative adsorbed protein fraction (F-ads,%) and on the protein load (Gamma, mg m(-2)) of fat globules was studied in a mo del dairy emulsion (oil-in-water produced by microfluidization). The a mount of adsorbed proteins (F-ads and Gamma) was evaluated immediately after emulsification by coupling the separation of oil and aqueous ph ases of the emulsion produced by centrifugation to the determination o f protein content. The specific surface area (Asp) of oil-water interf ace was obtained by photon correlation spectroscopy after the protein aggregates were dissociated by an appropriate buffer. A central compos ite experimental design was used to obtain two nonlinear multiple regr ession equations relating F-ads and Gamma to sodium caseinate (0.5 to 3.9 wt%), butter oil (5.2 to 14.7 wt%) and monoglyceride (0.08 to 0.88 wt%) contents, and to the emulsification pressure (7.8 to 76.3 MPa) a nd temperature (35 to 100 degrees C). These two functions explained 92 .7 and 90.6% of the variation in F-ads and in Gamma, respectively, and made it possible to evaluate the independent influence of each experi mental variable. The results indicate that the sodium caseinates (NaCa s), as a group, seem much less easily adsorbed than monoglyceride (gly cerol monostearate, GMS) molecules when the GMS:NaCas molecular ratio (R) is higher than 5: more numerous and possibly more surface active t han proteins, monoglycerides probably settle at the interface quicker than proteins. A qualitative model is also presented to illustrate thi s possible competition and estimate its influence on the parameters th at characterize protein adsorption (F-ads and Gamma) and fat globule s ize (d(vs)).