Optical probe for the analysis of trace indole in shrimp.

Indole is a chemical from the decomposition of shrimp and is used extensively to indicate seafood freshness. US Food and Drug Administration (FDA) sets its concentration of <25 μg/100 g shrimp as the threshold for Class I (fresh shrimp). A novel optical probe is reported to quantitatively analyze trace indole in shrimp, including the Class I threshold concentration.

Product in indole detection by Ehrlich's reagent.

Ehrlich's reagent (p-dimethylaminobenzaldehyde [DMAB, 1] in 95% EtOH with HCl as catalyst) was employed in spot tests of indoles, providing a diagnosis of, for example, liver diseases, hemolytic processes, occlusion of the common bile duct, and carcinoid syndrome. Although the reagent has been widely used for more than a century, it is not clear how many indole molecules react with a DMAB molecule and whether the reaction takes place at the α- or β-position of the indole molecule. Research here shows that the reaction of DMAB (1) with indole (2) in a 1:2 ratio gives β-bis(indolyl)methane (3).