Purpling Reaction of Sinapic Acid Model Systems Containing L-DOPA and Mushroom Tyrosinase

To explain the formation of purple lesions in washed mushrooms during posthawest storage, the reactions of phenolic acids with quinones formed in the oxidation of catechol and L-DOPA by mushroom tyrosinase were investigated using a diode array UV-visible spectrophotometer. Among the phenolic acids tested, only gallic acid induced a purple color in the oxidation of catechol by tyrosinase, while gallic, ferulic, and sinapic acids induced a purple color in the oxidation of L-DOPA by tyrosinase. Sinapic acid was highly reactive with a melanogenic quinone derived from L-DOPA in producing a stable purple pigment with λ max at 320 and 530 nm