The interaction of milk sphingomyelin and proteins on stability and microstructure of dairy emulsions

The interaction between dairy proteins [micellar casein (MC) vs. whey protein isolate (WPI)] and phospholipids [PL; soy phosphatidylcholine (PC) vs. milk sphingomyelin (SM)] in an oil-in-water emulsion system was investigated. Sole PC-stabilized emulsion (1%, wt/vol) showed a significantly larger mean particle diameter (6.5 μm) than SM-stabilized emulsions (3.8 μm). The mean particle diameters of emulsions prepared by the combination of protein (1%, wt/vol) and PL (1%, wt/vol) did not significantly differ from the emulsions prepared with a single emulsifier (MC, WPI, and SM). Emulsion instability differed significantly among samples by a centrifugation-mediated accelerated stability test. Emulsion instability increased in the order of MC+SM < MC+PC, WPI+SM < WPI+PC < MC < SM < WPI < PC. Protein surface load determined by aqueous phase depletion was significantly decreased only in WPI+PC emulsion, whereas no significant difference was found between the MC+SM and WPI+SM emulsions. Topographic and phase images of emulsion surface by atomic force microscopy showed surface layers prepared by protein+PL combinations were composites with different mechanical properties, and PL formed a more compact domain than proteins. A smoother phase image was observed in MC+PL combinations than in WPI+PL counterparts. Based on the microstructure analysis using confocal laser scanning microscopy, combination and MC+SM formed a uniform and thick surface coating of fat droplets. More PC aggregates were observed in the emulsions containing PC (sole PC, MC+PC, and WPI+PC) compared with their SM counterparts. Based on these results, the appropriate selection of the PL matrix is important to modulate the emulsion stability of dairy emulsion products.