The Intrinsic Effecting Factors On Quality Of Cultured Southern Pearl And Its Optimization Of Bleaching Process

Pearl, the best gift of Human being from sea, has been attracting around the worlddue to its glittering and translucent luster properties. No matter where it is, either ininternational market or domestic market, it is always the fashionable jewelry accessories.As the quality of the pearl from Southern China Sea is declining and the yield isdiminishing, it has seriously affected the survival of traditional pearl cultivation andprocessing industry. Why, how to deal with it, has become a problem, that rises in frontof the local government and pearl researchers. In order to explore the difference inmicrostructure and chemical component between good-quality and poor-quality pearls,find out the regularity, strengthen the knowledge of growing structure and themineralization theory of the nacre layer in the pearl, the nacre layer between good-quality and poor-quality pearl have been compared with, and the structure and thechemical component of the impurities between poor-quality pearl and good-quality pearlshave been investigated, by which the methods for aquaculture water quality control andthe theoretical support of optimization of the following bleaching process can beachieved.In the thesis, the scanning electron microscopy (SEM) and atomic force microscope(AFM) were used to study the outer surface, nacre layer as well as the impurity layer ofthe pearl. Especially for the impurity layer whose microstructure was extremely relatedto the quality of the pearl. The chemical components of each layer of the pearl withgood-quality and poor-quality was analyzed by thermal gravimetric analysis, infraredspectrum, Micro Raman Spectrometry and inductively coupled plasma atomic emissionspectrum. The key factors that impart the quality of the pearl and its’ regularity wereexplored and used to guide the following optimized process. The response surfacemethod was employed to optimize the bleaching process by using different volumefraction of the bleaching liquid composed of H2O2-Hydrogen Peroxide, NHD-Polyethylene glycol dimethyl ether and TAED-Tetraacetylethylenediamine. At last, theoptimal bleaching time was investigated under the optimal bleaching condition. The results as follows:1. Effect of the microstructure on the quality of pearlThe surface characteristic and internal microstructure were significantly differentbetween the good-quality and poor-quality pearls. The former surface had the regulargrowth texture. The aragonite crystallinity is excellent and has high degree of orderlyarrangement. The reflected light was bright and the image was clear. On the contrary, thepoor-quality one was scraggly and the growth texture was disorderly and unsystematic.The degree of orderly arrangement of aragonite was very low and not compact. So manypores located on the surface of nacre layer severely imparted the glossiness and mirroreffect. For the pearl with the same diameter, the better quality of the pearl had the thickerof the nacre layer and the thinner of the prismatic layer. In addition, the gray blackimpurity layer located between the pearl nucleus and nacre layer presents a laminatedstructure, which may derived from the denaturation of the matrix protein and hinder thegeneration of the crystallizing layer of calcium carbonate through wrapping in particlesand crystalline blocks. The presence of the impurity layer make the surface of pearlimpure and the reflected light dim, which leads to fuzzy reflection image.2. Effect of the chemical composition on quality of the pearlThe results show that the quality of the pearl is related to the contents of the organicsand calcium carbonate. The higher ratio of the organics and the lower ratio of the calciumcarbonate, the better the quality of the pearl. The content of calcite in the nacre also canaffect the quality, the higher content of the calcite, the poorer quality of the pearl. Theconcentration of the trace elements is directly proportional to that in nacre player, and thehigher content of the trace elements like Mg, Fe, Zn and Mn, the poorer quality of thepearl. Therefore, the concentration of the trace element could be regarded as assess indexfor the quality of aquaculture water.3. Optimization of the bleaching processThe optimal process parameters were studied by single factor experiment andresponse surface experiment. The results show that the best bleaching effect was obtainedwhen the volume fraction of H2O2is5.0%, the NHD is0.3%and the TAED is0.5%.Under this condition, the optimal bleaching time is8days, after which the standard rateachieved at65%while the rime fog pearls, the stain growth of pearls and the pearls withdesquamated nacre all have the minimum.