Effect of lecithin and monoglycerides on the heat stability of a model infant formula emulsion

The effect of two common emulsifiers, lecithin and monoglycerides, on the heat stability of a model infant formula emulsion was investigated. During homogenization the droplet diameter of the emulsion was more effectively reduced by monoglycerides than lecithin. The maximum in the heat coagulation time (HCT)-pH profile at 140 °C of the emulsion with no added emulsifier coincided with the unadjusted pH (∼6.8). Addition of lecithin (0–5 g l−1) progressively increased the maximum heat stability of the emulsion from 18 to 25 min and at 3–5 g l−1 slightly shifted the maximum in the HCT-pH profile to more acidic pH values. The addition of monoglycerides on the other hand decreased the heat stability; the emulsion containing 5 g l−1 had a maximum heat stability of 12 min. Particle size distribution and average particle diameter (D[4,3]) measurements following heating confirmed that lecithin conferred stability to heat-induced aggregation while monoglyceride destabilized the emulsions. The zeta potential of oil droplets/particles in the emulsions became increasingly less negative with increasing monoglycerides concentration which is indicative of protein displacement; this may explain the low heat stability of the monoglyceride containing emulsions. The addition of lecithin on the other hand had no effect on zeta potential possibly because negatively charged lecithin replaced interfacial protein or interacted with interfacial protein. Progressively, less hydrophobic sites were available for binding of a hydrophobic probe as the concentration of either surfactant increased; interaction of surfactant with exposed hydrophobic regions of aqueous phase and/or interfacial protein is a possible explanation and this type of interaction may explain the high heat stability of lecithin containing emulsions.