Background:  Discoloration of tuna product can be effectively prevented when carbon monoxide (CO) is bound to the Fe(II) binding site of myoglobin in tuna muscle.  Commercial CO treatment of tuna and a few other finfish species is therefore an increasingly popular practice to improve product acceptance in the commercial market.  In the US, tuna treated with CO should display labeling indicative of that process.

Methods:  CO contents in tuna and mahi-mahi were measured by a GC/MS method developed in our laboratory.  GC/MS analysis was completed by injecting 100 mL from headspace of a mixture of CO-distillated water, after bound CO was chemically released from fish tissue using sulfuric acid (5M).

Results:  Using the method described in this study, indigenous CO contents of fresh Ahi tuna and mahi-mahi samples were found to be close to or less than 150 ng/g and 100 ng/g, respectively.  Commercially CO-treated, vacuum-packaged tuna from multiple sources consistently showed CO level near or greater than 1 mg/g, while CO level in the only CO-treated frozen mahi-mahi sample was in the 500 ng/g range.  Treated specimens appear distinguishable from untreated samples as the difference in CO content between these two categories was in the range of one order of magnitude.

Conclusions:  Our findings suggest an easy quantitative and confirmative method for CO in fish using widely available instrumentation.  This method may be useful for regulatory purpose, with proper validation, in determining whether a commercially available fishery product has been exposed to CO even if not labeled as such.