It happened in 1951 that our exported canned bonito (skipjack) in brine had been rejected in Canada by reason of containing an acid, believed to be ascorbic acid. So, several experiments were carried out to confirm whether the substance was true vitamin C or not.
By 2, 6-dichlorphenol indophenol (INP) method vitamin C content of canned bonito was determined as about 10 mg%, but 2, 4-dinitrophenylhydrazine (DNP) method was 0.5 mg%. For the liquid of canned bonito concentrated to 92 mg% of reducing substance as vitamin 0 (by INP method) in vacuum, paper chromatography was tested, but the spot of vitamin 0 was not found. During canning process, reducing power of bonito meat by INP method increased, but the value by DNP method decreased. Therefore it seems that this reducing substance was not vitamin C, but value by DNP method was perhaps true vitamin C.