A New Fluorescence Band of Anthocyanins as a Simple Oxidation Biomarker of Food Products

Abstract

The formation of a new fluorescence band of anthocyanidins and anthocyanidins, centered at about 530 nm (excitation at 460-470 nm), is proposed as a simple indicator of food oxidation. This fluorescence band appeared and increased progressively during the incubation of blueberry juice under aerobic conditions and the cooking of blueberry homogenate and black carrot. The same effect was observed upon the addition of delphinidin to rapeseed oil subjected to simulated frying. A ratiometric parameter (ratio of the fluorescence intensity at the maximum of the new band to the fluorescence intensity of native anthocyanins/anthocyanidin) is proposed as a versatile index useful for the estimation of the oxidation of food products containing anthocyanins or supplemented with anthocyanins or anthocyanidins.

Keywords: anthocyanidins; anthocyanins; black carrot; blueberry; fluorescence; lipid peroxidation; oil; oxidation.

Figure 1

Emission spectra of blueberry juice incubated under air access for up to 7 days, excitation wavelength: 460 nm (A); fluorescence intensity of the juice at 538 nm as a function of incubation time (B); fluorescence intensity ratio (538 nm)/(670 nm) as a function of incubation time (C). In some cases, the standard deviation is lower than the data symbol size; *** p < 0.001 with respect to non-boiled samples.

Figure 2

Effect of cooking blueberry homogenate for 20 min on the absorption spectrum of blueberry anthocyanins at various pH values (A); maximal absorbance as a function of the cooking time (B); effect of cooking blueberry homogenate at pH 5.0 for various times on the fluorescence spectrum of blueberry anthocyanins, excitation at 460 nm (C); effect of cooking time on the fluorescence intensity ratio at 528 nm/676 nm (D). Standard deviation values lower than the symbols in (B,D). * p < 0.05; ** p < 0.01; *** p < 0.001 (with respect to non-cooked samples); (D) p < 0.001 with respect to non-cooked samples in all cases.

Figure 3

Effect of cooking black carrots on the fluorescence of endogenous anthocyanins. Fluorescence spectra of slices of black carrots, native and cooked for 60 min, excitation wavelength: 460 nm (A); the fluorescence intensity ratio at 532/672 nm of black carrots cooked for various times in the absence and presence of broad beans (B); ** p < 0.01.

Figure 4

Oxidation of delphinidin incubated with heated rapeseed oil. Emission spectra of delphinidin incubated for 5 and 35 min with oil heated for 30 min; excitation wavelength of 360 nm (A); fluorescence intensity ratio at 520/618 nm as a function of the duration of incubation with oil heated for 30 min (B); fluorescence intensity ratio at 520/618 nm as a function of the duration of oil heating (C); Absorbance of thiobarbituric-reactive products in the oil as a function of the duration of heating (D). ** p < 0.01, *** p < 0.001.

Figure 4

Oxidation of delphinidin incubated with heated rapeseed oil. Emission spectra of delphinidin incubated for 5 and 35 min with oil heated for 30 min; excitation wavelength of 360 nm (A); fluorescence intensity ratio at 520/618 nm as a function of the duration of incubation with oil heated for 30 min (B); fluorescence intensity ratio at 520/618 nm as a function of the duration of oil heating (C); Absorbance of thiobarbituric-reactive products in the oil as a function of the duration of heating (D). ** p < 0.01, *** p < 0.001.