Preserved effect of nano-SiO2 and nano-TiO2 modified composite coating materials on pidan

Abstract: Pickling is one of the oldest techniques in the food preservation as it not only extends the shelf life but also enhances the flavor and acceptability of the product. Pidan is a kind of traditional pickled egg with an extremely long shelf life. Its pleasant and fragrant taste is preferred by most of the people in China, Thailand and other Southeast Asian countries. Pidan is formed by the penetration of alkali through the egg shell and membrane, which leads to chemical changes and gelation of egg proteins. Apart from the formation of gel-like texture, alkaline treatment can induce the degradation of proteins as well as Maillard reaction to turn the color of pidan into brown during the processing. However, pickling could possibly destroy the outer shell membrane of eggs, make the shell structure loose and enlarge the pores of shell, which causes water to evaporate easily through the pores and the spoilage of fresh eggs. Therefore, coating pidan with composite film is one of the most effective ways to preserve the quality of pidan. Polyvinylidene chloride and polyvinyl alcohol are 2 kinds of macromolecular polymer with gas and water barrier performance, whose emulsions are used as based material to prepare nano-SiO2 and nano-TiO2 modified polyvinyl alcohol based composite coating material and nano-SiO2 and nano-TiO2 modified poly vinylidene chloride based composite coating material. In this paper, pidans were pickled by soaking chicken eggs into NaOH/NaCl/ZnSO4·7H2O solution for a period of time and then were divided randomly into 3 groups. Uncoated pidan was chosen as control group (CK). Secondary soaking coating method was adopted to coat pidan. The coating process was that put pidan into coating liquid for 1 min, then take pidan out and blow dry, and later the above-mentioned process was carried out again. Pidan coated with nano-SiO2 and nano-TiO2 modified poly vinylidene chloride based composite coating material (Group A) and nano-SiO2 and nano-TiO2 modified polyvinyl alcohol based composite film (Group B), and uncoated pidan (CK) were stored under the same preserved condition whose temperature was 25℃ and relative humidity was 75%±2%. During the storage the physical and chemical indicators included weight loss rate, albumen pH value, free alkalinity content, moisture, texture characteristics (hardness, cohesiveness, springiness, chewiness and gumminess), chromatic aberration, brown intensity, absorbance at 294 nm, reducing sugar content, free amino content, sensory evaluation, and total number of bacterial colony, which were detected every 2 or 3 weeks to determine the pidan quality. The results showed that physical and chemical indicators of pidan in each group significantly changed (P<0.05). Two kinds of coating materials both significantly reduced moisture loss and weight loss rate (P<0.05), retarded the change of pH value, free alkalinity content and textural properties (hardness, cohesiveness, springiness, chewiness and gumminess) of pidan, inhibited the growth of microorganisms, and improved the sensory evaluation of pidan. However, a* value, brown intensity, and absorbance at 294 nm of coated pidan were higher than uncoated pidan, and L* value, b* value, reducing sugar content and free amino content were less than uncoated group, which meant the development of brown color and the production of flavor substances were enhanced because of Maillard reaction. This paper provides the reference for the application of composite film materials in the preservation of pidan.