The fluorescence of chlorophyll in fats in relation to rancidity

Summary 1.) Difficulties in applying the “chlorophyll value” test to fat samples has led us to investigate the apparent “quenching” of chlorophyll fluorescence in mineral oil solution when cottonseed oil or lard is added to it. The disappearance of chlorophyll fluorescence in ultraviolet light caused by the addition of cottonseed oil appears to be due to the absorption of the light by the cottonseed oil and to the intense white fluorescence of the oil itself rather than to a chemical reaction of some constituent of the oil with the excited chlorophyll. 2.) There was no evidence of a stoichiometric quenching reaction between chlorophyll and acceptor substances in the fats used in this study and, in consequence, no “endpoint” was observed in any of the titrations. 3.) A lack of correlation between either the peroxide value or the stabilities measured in conventional ways and the amount of chlorophyll fluorescence of several fats makes the “chlorophyll value” test appear to have doubtful value as a generally applicable test for fat rancidity or stability. 4.) The crude absorption curves here presented suggest that the greater absorption of near ultraviolet light by oxidized fats may be related to their content of fat peroxides. This work was made possible by grants from the Rockefeller Foundation, the Hormel Foundation, and the Graduate School of the University of Minnesota. Assistance in the preparation of these materials was furnished by the personnel of Work Projects Administration, Official Project No. 165-1-71-440, Subproject No. 382.