Structural analyses of unknown red dyes detected in dried strawberry

We examined two unknown red dyes (designated as red dyes "A" and "B") from a dried strawberry package with a label that indicated the presence of food red No. 40 (R40). Red dye "A" was identified as trisodium 3-hydroxy-4-[(2'-methoxy-5'-methyl-4'-sulfonatophenyl)azo]-2,7-naphthalenedisulfonate (CSA-R) by HPLC, UV-VIS spectra and MS spectra. This compound is one of the four reported subsidiary colors of R40. Detailed analyses of red dye "B" by MS and NMR demonstrated that its structure was disodium 3-hydroxy-4-[(2'-methoxy-5'-methyl-4'-sulfonatophenyl)azo]-2-naphthalenesulfonate. Red dye "B" is a structural isomer of R40, that has not been reported previously. Our results suggest that the two minor red dyes were subsidiary colors contained in R40, which had been added to the dried strawberries.