Changes in the stainability of muscle with azan in the course of putrefaction
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Changes in color of muscle stained with azan in the course of putrefaction were examined histologically, from the viewpoint of ultrastructural density, and compared with the amount of volatile basic nitrogen (VBN) of muscle measured chemically by Conway's method.
The results obtained are summarized as follows:
1. The color of muscle as stained with azan was red just after death when no rigor mortis appeared, and gradually became purplish-red, purple, purplish-blue, and finally blue in accordance with the degree of putrefaction of muscle. Such changes in color were not due to those of pH value but to a gradual change of the ultrastructural density of muscle tissue with the progress of decomposition of meat, which allows a largemoleculed dye to invade into the tissue.
2. During the period of rigor mortis, any changes could not be demonstrated in the histological picture and the amount of VBN of muscle. However, the color of muscle stained with azan at this period had a more yellowish tint than that of the control muscle harvested just after death. It is considered that the ultrastructural density of muscle tissue became dense in this stage.
3. In the period when the VBN level reached about 30mg/100 g and which was regarded as an early stage of putrefaction, muscle was stained purplish-red with a purplish tone. It is assumed that the ultrastructural density of muscle tissue became coarse in this stage.
4. The muscle with a VBN level of about 100mg/100 g and showing remarkable morphological changes was stained purple. The ultrastructural density of muscle tissue in this stage seems to have become coarser.
5. The putrid muscle with a VBN level of more than about 250mg/100g was stained bluish-purple or blue.
6. There was a slight difference between fish and mammalian muscle in the change of staining color by azan with the progress of putrefaction. In general, fish muscle showed a more rapid and distinct change in color than mammalian muscle when stained. From these results, it may be said that the azan staining method is available, to some extent, for histological estimation of the freshness of meat.