Effects of some cosmetic pigments on the bactericidal activities of preservatives
Tetsuo Sakamoto , Mitsuo Yanagi , Shoji Fukushima , Takeo Mitsui

Synopsis

The effects of eight cosmetic pigments on the bactericidal activities of p-hydroxybenzoic acid esters (methyl through butyl) were investigated by a viable cell count method. Most of the tested pigments interfered with the bactericidal action of all four perservatives. The degree of inactivation was highest for ultramarine blue followed by talc, titanium dioxide (R), titanium dioxide (A), red oxide of iron, and yellow oxide of iron. In general, the bactericidal activity of methyl p-hydroxybenzoate was affected the least.

The mechanism of inactivation was also investigated. The lack of a phenolic hydrogen for inactivated methyl p-hydroxybenzoate was demonstrated by ¹ H-NMR analysis. It is likely that the phenolic hydrogen of methyl p-hydroxybenzoate was replaced by some metal from the coexistent pigments. This is postulated to be the inactivation mechanism. The addition of EDTA prevented the inactivating action.

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