Design of oleofoams from citric acid esters of mono-/diglycerides

Citric acid esters of mono- and diglycerides (CITREM E472c) are widely used as emulsifiers in the food industry. They result from the esterification of citric acid with mono- and diglycerides of fatty acids. CITREM was recently shown to produce stable aqueous foams but its potential in oil foam production remains unexplored. Due to its unique chemical structure, featuring free hydroxyl groups, along with its solubility in vegetable oils, CITREM is a highly promising candidate for this application. This study aimed to investigate the possibility of using CITREM as a surfactant to produce vegetable oil foams. We first determined the solubility of CITREM in sunflower oil to identify the solubility limit and the crystal formation temperature. We determined the critical aggregation concentration (CAC) of CITREM through Small Angle X-ray scattering (SAXS). The foamability was examined above the melting point at different surfactant concentrations. The impact of crystal formation on the foam stability was also examined at various storage temperatures. To determine the mechanisms leading to foam formation and stabilization, a multiscale approach was used by combining macroscopic observations of the foam, with optical microscopy and SAXS. We showed that oil foams were produced only above the CAC. We demonstrated that CITREM surfactants stabilize oil foams in the same manner as sucrose ester and sorbitan ester surfactants confirming that the key factor in forming vegetable oil foams with surfactants is the use of a lipophilic surfactant containing free hydroxyl groups, which can form hydrogen bonds with the triglycerides in the oil.