Influencing emulsifying properties of egg yolk by enzymatic modification with phospholipase D. Part 2: Structural changes of egg yolk due to incubation

The effects of a direct incubation of egg yolk (EY) by phospholipase D (PLD) with respect to EY structure and interface properties of oil-in-water (o/w)-emulsions prepared with this EY were investigated. PLD incubation of EY mainly converts phosphatidylcholine into phosphatidic acid (PA). EY structures were characterised using circular dichroism and scanning electron microscopy. Composition (protein and phospholipid contents), charge, and appearance of interfaces in EY stabilised o/w-emulsions were described, too. The improved emulsifying activity of EY after PLD incubation could be attributed both to a higher interface activity of the PA itself and to modified interactions of protein and phospholipids in EY. PA possesses a negative charge resulting in droplet interfaces with higher charge. This effect increased repulsion forces between droplets compared to application of EY without PLD incubation. A higher portion of alpha-helix Structures could be detected in the PLD-incubated EY enabling a higher flexibility of proteins on droplet surfaces. Additionally, a better preservation of EY protein structures during heat treatment due to the presence of PA enabled a higher thermal stability of such emulsions. (C) 2009 Elsevier B.V. All rights reserved.