Fluorimetry of Edible Oils

Food oils (olive oil, seed oil) have interesting optical properties (absorption, fluorescence) due to the content of optically active compounds, such as chlorophyll, beta-carotene and others. These properties can be used to recognize and characterize the various types of oil.

In the spectrum of “Extra Vergine” olive oil, obtained by cold pressing you notice the absence of the products of peroxidation of fatty acids, which give fluorescence at about 470nm. This happens both because the oil is cold worked and because the high content of natural anti-oxidants (carotenes and polyphenols) prevents oil from oxidative degradation. It is clear the presence of the fluorescence peak of the chlorophyll.

Seeds oils show all instead, to varying degrees, a clear fluorescence at about 470nm, a sign of the content in peroxides resulting from oxidative degradation of fatty acids, both because they are supposedly hot-worked and because the lower content of molecules anti-oxidants. These features make the seeds oils less suitable for use at high temperature.

“Extra Vergine” Olive Oil

Olive oil absorption spectrum
Absorption maxima in the red band at around 660-670nm and in the blue band, less than 500nm. Olive oil absorption is due to carotenoids and chlorophyll

Olive oil fluorescence spectrum excited by UV emission
The maximum at 680nm is due to chlorophyll A

Fluorescence spectrum of olive oil subjected to heating (thermal degradation)  excited by UV emission.
The chlorophyll fluorescence has been reduced whilst it appeared the signal of the presence of peroxides due to thermal degradation of fatty acids

Peanut Oil

Peanut oil fluorescence spectrum excited by UV emission at 405nm
The fluorescence band at 664nm is due to the content in chlorophyll. The fluorescence band with peaks at 463nm and 475nm  presumably is due to the products of peroxidation (degradation) of polyunsaturated fatty acids in the oil (oleic and linoleic)

Fluorescence spectrum of peanut oil subjected to heating (thermal degradation)  excited by UV emission
We note the increase in the intensity of fluorescence at 470nm – 530nm due to thermal degradation of fatty acids and the consequent production of peroxides. The chlorophyll fluorescence has virtually disappeared because the chlorophyll itself has been degraded by heating

SunFlower Oil

Sunflower oil fluorescence spectrum excited by UV emission at 405nm
The fluorescence band with peaks at 464nm and 476nm  presumably is due to the products of peroxidation (degradation ) of polyunsaturated fatty acids in the oil (oleic and linoleic)

Corn Oil

Corn oil fluorescence spectrum excited by UV emission at 405nm
The fluorescence band with the peak at 464nm is presumably due to the products of peroxidation (degradation) of polyunsaturated fatty acids present in the oil (oleic and linoleic acid). The attribution of the fluorescence band with the peak at 520nm is uncertain and may be also linked to the process of thermal degradation of fatty acids. Alternatively (or in addition) may be due to the compounds of vitamin B2 (flavin)

Soybean Oil

Soybean oil fluorescence spectrum excited by UV emission at 405nm
The fluorescence band with the peak at 477nm is presumably due to the products of peroxidation (degradation ) of polyunsaturated fatty acids present in the oil (oleic and linoleic) . The same applies to the band with a peak at 533nm, to assess the contribution of isoflavones on the emissions of the green fluorescence.

Almond Oil

Almond oil fluorescence spectrum excited by UV emission at 405nm
The fluorescence band with the peak at 477nm is presumably due to the products of peroxidation (degradation ) of polyunsaturated fatty acids present in the oil (oleic and linoleic acid). The fluorescence band with peak at 533nm and 596nm is presumably due to the compounds of vitamin B2 (flavins)

Sesame Oil

Sesame oil fluorescence spectrum excited by UV emission at 405nm
The fluorescence band with the peak at 475nm is presumably due to the products of peroxidation. The attribution of the fluorescence band with the peak at 533nm is uncertain and may be also linked to the process of thermal degradation of fatty acids, or due to the compounds of vitamin B2 (flavin)

Castor Oil

Castor oil fluorescence spectrum excited by UV emission at 405nm
The fluorescence band with the peak at 479nm is presumably due to the products of peroxidation

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