A modified xylenol orange method to evaluate formation of lipid hydroperoxides during storage and processing of small pelagic fish

Lipid oxidation causes deterioration of the quality of fatty fish during processing and storage. Its development should be evaluated by measurement of several oxidation products, among them the hydroperoxides. However, most of the methods for determining hydroperoxides already in use failed when tested on fatty fish. The objective of this study was to adapt the ferrous ion xylenol orange assay (FOX) for easy and sensitive measurement of hydroperoxides in fatty fish such as small pelagics during their storage and processing. The proposed procedure is based on the assay developed by Hermes-Lima et al. that consists of a colourimetric assay performed directly on the methanol extract from minced fish. The reactant was replaced by Wolffs reagent (FOX2 reagent) to improve the solubility of the extract and to simplify the procedure. It was shown that absorbance should be measured at 560 nm instead of 580 nm. The method is valid between 0.3 and 0.6 absorbance units. The modified method is easy, sensitive and allowed differentiation between fresh horse mackerel samples and those stored for 12 h at 17 degreesC on the basis of their hydroperoxide levels.