| In this study, fresh egg yolk was enzymatic modified with phospholipase A1 (PLA1) to improve the emulsifying properties of egg yolk. Compared to the control egg yolk powder, the PLA1 treated egg yolk powder has higher emulsifying activity, thermal stability and protein solubility. These advantages will extend the scope of application of egg yolk powder, and promote the development of the industry.First, optimized conditions of egg yolk lecithin hydrolysis with PLA1 were studied systematically. The enzyme reaction was carried out in aqueous. In order to monitor the degree of hydrolysis by PLA1, the amount of released free fatty acid was determined. After a treatment with phospholipase A1, the optimal parameters were determined through the single factor and orthogonal experiments. The conditions were as follows:reaction temperature was 55℃, reaction time was 4.5 h, the volume of enzyme was 1500 u/g, initial pH was 6.0, substrate concentration was 20%, independent of Ca2+. At the same time, the compositions of egg yolk powder were also discussed. The results found that the main difference between the composition of egg yolk powder and enzymic modified egg yolk powder was the amount of PC and FFA. The amount of PC is decreased. Accordingly, the amount of released free fatty acids was found to increase. This was mainly due to the action of phospholipase A1 which cut off of one fatty acid from PC.The size of modified egg yolk was measured by a Laser Particle Size Analyzer. Results show that the treatment of PLA1 does not lead to a complete destruction of egg yolk’s structure. The color changes of the egg yolk powder were studied before and after enzymatic treatment with Chroma Meter (Ultra scan XE, Hunter Lab).The results demonstrate that the color was changed from orange yellow to light yellow. The value of L* was increased from 83.26 to 91.33, and the b* from 48.80 to 28.35.While the value of a* was decreased from 10.76 to 3.94. Integrated tester was used to measure the universal powder properties. We found that the enzymic modified egg yolk powder dispersed better.Then the functional properties of enzymic modified egg yolk powder were also investigated. The results indicated that HEEYP proved better on moisture adsorption capacity and fat absorption capacity. It is important that emulsifying capacity of HEEYP has been improved 80%.And by the comprehensive study on the emulsion stability, the HEEYP was found can withstand more severe environmental conditions. SEM was used to measure the microstructure. We observed that the network excited in the egg yolk powder was changed from dense to loose, and the surface structure became more uniform. The foaming property was improved. On the contrary, the foaming stability was decreased.It was remarkable that the interfacial tension of PLA1 treated egg yolk was always lower than that of egg yolk, indicating that the modified egg yolk emulsion has a better emulsifying capacity.In addition, the effects of PLA1 on the properties of egg yolk fractions were investigated using scanning electron microscopy (SEM), Laser Particle Size Analyzer, Chroma Meter. The measured properties were emulsion stability, foaming property, and solubility. The results clearly demonstrated that enzymatic treatment showed a significant impact on the functional properties of egg yolk powder. This study indicated that significant differences in the functional properties of untreated and PLA1-modified egg yolk do not primarily result from the existence of lyso-phospholipids but from structural changes in egg yolk granules and LDL particles.Finally, the modified egg yolk powder was applied in the manufacture of ice-cream. This study examined the properties of ice cream added different egg yolk powder. When the control and enzymatic modified ice cream were compared, the ice-cream added HEEYP has been obviously improved on the overrun, melt resistance, and shape retention, while slightly decrease on the textural properties. A slightly increase on the flexibility was observed. The results indicated that fat size decreased while viscosity increased as the application of enzymatic modified egg yolk powder in ice cream. The melting time was slightly shorter in the enzymatic modified ice cream than in the control. No significant difference was found in sensory properties, whereas a profound difference was observed in the viscosity of ice cream. |
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