Calcium carbonate crystallization in the alpha-chitin matrix of the shell of pink shrimp, Pandalus borealis, during frozen storage

Calcium carbonate precipitates in the shell of pink shrimp, Pandalus borealis, during frozen storage (investigated for temperatures above - 30 degrees C), and as a result white spots appear in the shell. During continued frozen storage the white spots grow in size and eventually cover the entire, originally transparent, shell. Material isolated from shrimp shells was dried and subjected to infrared and Raman spectroscopy, X-ray diffraction, electron microscopy and EDX analyses. It was found that the white spots were composed of crystalline calcium carbonate in the two crystal forms of calcite and vaterite, and of amorphous alpha-chitin. It is proposed that alpha-chitin plays an important role in the crystallization process of white spots, as an integral part of the white spots. It is shown that the relative w/w-concentrations of alpha-chitin and calcium carbonate in white spots were constant (0.34 : 0.66), and it did not depend on chemical treatments comparable to those in use by the fishing industry for production of raw and frozen shrimps. However, the ratio of the polymorphic forms of calcium carbonate varied.