Microfluidization of model dairy emulsions. II. Influence of composition and process factors on the protein surface concentration

Abstract

The influence of some composition variables (butter oil, sodium caseinate and monoglyceride contents) and process variables (pressure and temperature) on the relative adsorbed protein fraction (Fads,%) and on the protein load (Γ, mg m-2) of fat globules was studied in a model dairy emulsion (oil-in-water produced by microfluidization). The amount of adsorbed proteins (Fads and Γ) was evaluated immediately after emulsification by coupling the separation of oil and aqueous phases of the emulsion produced by centrifugation to the determination of protein content. The specific surface area (Asp) of oil-water interface was obtained by photon correlation spectroscopy after the protein aggregates were dissociated by an appropriate buffer. A central composite experimental design was used to obtain two nonlinear multiple regression equations relating Fads and Γ 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) and temperature (35 to 100 °C). These two functions explained 92.7 and 90.6% of the variation in Fads and in Γ, respectively, and made it possible to evaluate the independent influence of each experimental variable. The results indicate that the sodium caseinates (NaCas), as a group, seem much less easily adsorbed than monoglyceride (glycerol monostearate, GMS) molecules when the GMS:NaCas molecular ratio (R) is higher than 5: more numerous and possibly more surface active than proteins, monoglycerides probably settle at the interface quicker than proteins. A qualitative model is also presented to illustrate this possible competition and estimate its influence on the parameters that characterize protein adsorption (Fads and Γ) and fat globule size (dvs).