Study On Preparation Of Functional Dried Egg Yolk With Phospholipase A₂

Egg yolk is widely used as a natural emulsifying agent in numerous processed food such asice-cream, mayonnaise, margarine and baking food owing to its effective emulsifying property.However, it is heat-sensitive and begins to coagulate at high temperature, which results indrastically decreasing in emulsifying property. Phospholipase A2(PLA2) can hydrolyze lecithinof yolk into lysolecithin,providing modified egg yolk better functional properties, especiallyheat stability. As a result, modified egg yolk is more widely used in processing food.First, Dole's titrimetric method for determination of the free fatty acids(FFA) wasmodified ,then correlation analysis of determining of the degree of hydrolysis by modifiedDole's procedure and HPLC was discussed. The results showed that the correlation between twomethods was good, which proved feasible to determine the degree of conversion by modifiedDole's procedure.Then technologcial conditions of egg yolk lecithin hydrolysis with PLA2 and spray dryingparameters were studied systematically. Based on the released FFA, the effect of factors on thedegree of conversion was studied, and the optimal parameters were determined as follows: theinitial pH value was 7.0, processed temperature was 50℃, E/S was 2000U/g yolk, reaction timewas 1.5 hours, independent of Ca2+, and the degree of hydrolysis was 81.62%.And the conditionsof spray drying were investigated, showing that addition of 4% maltodextrin or 5% cyclodextrincould improve the manufacture of dried egg product. The optimal inlet air temperature of spraydrying was 180~200℃.The functional properties of dried modified egg yolk(DEY-M)were also investigated.DEY-M proved better in in moisture adsorption capacity and fat absorption capacity, andespecially in protein solubility index(PSI) at high temperature. It was more meaningful thatDEY-M had an increase by 25% in emulsifying capacity (EC), and better emulsion stability (ES)than DEY-C at different pH value. DSC and rheological experiments revealed that thedenaturation temperature and gelling temperature of fermented yolk were both about or above100℃, suggesting heat tolerance of fermented egg yolk.In addition, the effects of PLA2 on the characteristics of the yolk pellet remaining (YPR)after extraction of IgY were studied. The results showed that the PSI of modified YPR had amarked increase by 220%, and that the EC had a considerable increase by 26%, and thatsuggested better emulsion stability at different pH value.Finally, The application of DEY-M in mayonnaise and bread was studied. Rheologicalexperiments of mayonnaise showed the shear storage modulus (G') and the shear loss modulus(G") of fermented egg yolk mayonnaise was much higher than those of normal egg yolkmayonnaise, showing thickening effect and great stability. The mayonnaise containing DEY-Mcould withstand heat treatment including boiled water bath, microwave and stream-heatedwithout the emulsion breaking, suggesting bearing thorough sterilization. The enzyme treatmentof egg yolk had a positive effect on the characteristic of bread, and more effective in preventingbread staling.