CO reacts irreversibly with the dark muscle tissue of tuna and similar fish, resulting in the fish becoming bright cherry red. CO prevents oxidation and discoloration, preserving the bright red tissue for an extended period of time, making it look fresher than it is. "The Code currently permits CO as a general purpose processing aid but processing aids are not supposed to have any ongoing technological function. "A number of regulators have asked FSANZ to clarify the Code to ensure CO is not used on fish." The closing date for submissions on this proposal is 11 February 2013