It has been known for some time that Penicillium citrinum Thom when grown on suitable media produces a characteristic lemon yellow colour. Thus Thom in his monograph on “Cultural Studies of species of Penicillium” (1910), says, “P. citrinum produces a lemon yellow colour, soluble in alcohol, in media containing sugars, milk, gelatin, bread and potato.” This colouring matter was not isolated, however, nor was its nature further investigated. In the course of an investigation into the colour reactions given with ferric chloride by metabolism solutions produced by various species of Penicillium, it was found that a very characteristic reaction was given by each of the strains of Penicillium citrinum THOM in our possession, but was not given by any other species in pure culture whether of Penicillium or of any other genus tested.
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Hetherington et al. (1931) studied this question.