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.

Influencing emulsifying properties of egg yolk by enzymatic modification by phospholipase D from Streptomyces chromofuscus Part 1: technological properties of incubated egg yolk.

Impact of a direct incubation of egg yolk with phospholipase D (PLD) from Streptomyces chromofuscus was investigated with respect to its effect on rheological and emulsifying properties. Egg yolk has been shown to be a suitable substrate for incubation with PLD after a slight dilution with water (70/30, w/w). A considerable increase of egg yolk viscosity during incubation was observed correlating with formation of phosphatidic acid due to enzymatic activity. Emulsions prepared with such an incubated egg yolk showed a higher viscosity, in particular at high egg yolk concentration. Especially in the case of lower concentrations, smaller droplets indicating better emulsifying activities were detected in emulsions prepared with the incubated egg yolk compared to the application of untreated egg yolk. Stability after heat treatment of the emulsions could be significantly improved by the application of egg yolk incubated with PLD.