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Effect of cooling and coating treatments on storage qualities of hard-cooked eggs
The purpose of this study was to maintain the storage quality and extend the shelf life of hard-cooked eggs at room temperature using various coating materials. Paraffin wax, zein, ethyl cellulose and polyvinyl acetate were selected as coating materials in this experiment. The cooling methods of hard-cooked eggs directly affect both the production and storage quality. The degree of yolk discoloration of water cooling hard-cooked eggs was lighter than that of air cooling ones. As storage at 25℃, microbial penetration rate of water cooling hard-cooked eggs was more rapid than air cooling hard-cooked eggs, that made the former had the shorter shelf life of water cooling hard-cooked eggs. This experiment showed that reducing the microbial contamination during processes of hard-cooked and coating had marked effect on the storage quality of hard-cooked eggs. The shelf life of hard-cooked eggs stored at 25℃ was greatly extended by air cooling treatment after being cooked. Therefore, air cooling hard-cooked egg was used in coated hard-cooked storage test. The appearance of zein treated hard-cooked egg was much yellow and was significantly different from others. Scanning electron micrographs showed that both the number and the size of cracks on the eggshell surfaces increased after cooking. After coating, cracks on the eggshell surfaces were almost covered by coating materials. The bonding of coating materials was only on the eggshell surface, and there was no penetration of the coating materials below the surface. Both total plate counts and volatile basic nitrogen value of air cooling hard-cooked eggs stored at 25℃ for 12 days did not reached spoilage standard. During storage at 25℃, the pH values of albumen of all treatments tended to decline at early stage and then rise slowly. The pH values of yolk in all treated samples increased with storage time. The moisture loss of albumen of the uncoated and zein treated samples declined most seriously. The moisture content of yolk increased with storage time. The water activity in all treatments was between 0.967 and 0.975, which did not change markedly. The weight loss of all treatments increased with storage time, but of paraffin wax treatment significantly (p<0.05) lighter than other treatments. All the coating materials were only covered on eggshell surfaces and had no effect on the shell peelability of hard-cooked eggs. Coating treatment had effect on yolk color of hard-cooked eggs during storage. Paraffin wax treatment had effect on the scores of albumen and yolk color. Paraffin wax was the best coating material for holding hard-cooked egg flavor in this experiment. Results of sensory evaluation of appearance, color, flavor and texture showed that polyvinyl acetate was a good coating material for storage of the hard-cooked eggs.
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