Green Dye in Food Oil

To consumers of fresh produce and allied foodstuffs, green spells fresh and premium. This is why unscrupulous food producers are occasionally tempted to boost their products with banned additives. Fang et al. (2015) show how regulatory officials can monitor suspiciously green food oils using ultra-high performance liquid chromatography (UHPLC) coupled with either high resolution mass spectrometry (HRMS) or photodiode array detection (PDAD) for sensitive screening and quantitation¹.

The green in fresh plant foods comes predominantly from chlorophyll, but this vivid signal of freshness decays in storage as magnesium is lost from the pigment. However, canned food technologists noticed that vegetables apparently re-greened in this environment – with scientific exploration, they found that this effect was due to copper complexing with pheophytins to give a stable coloration. Copper (Cu)-chlorophyll is a much more stable compound, retaining the fresh green hue beloved of consumers. From this substitution, the food industry gained additive E141, a permitted Cu-chlorophyll green dye for use in foods other than oils.

The green aura of freshness also extends to food oils, with recognition that its intensity signals premium virgin olive oil. Unfortunately, lower value and less refined oils can easily be passed off as the premium product simply by tinkering with presentation. Thus, adding E141 is one way in which fraudulent producers can pass off inferior product as genuine virgin olive, extracting a high price by duping consumers.

Fang et al. developed methodologies to detect and quantify Cu-chlorophyll adulteration in batches of food oils seized from a producer who had admitted to E141 product adulteration. Following defatting using solid phase extraction (SPE), the researchers validated a UHPLC-HRMS workflow to detect adulteration. Focusing on Cu- pyropheophytin a, the predominant copper pigment found in the E141 chlorophyll substitute, they ran prepared samples through an Ultimate 3000 UHPLC system coupled with a Q Exactive benchtop orbitrap mass spectrometer (both Thermo Scientific). The team also examined samples with HPLC-PDAD, using an Ultimate 3000 Standard LC system in conjunction with an Ultimate DAD 300RS PDA detector (both Thermo Scientific), in addition to HPLC-tandem mass spectrometry (HPLC-MS/MS).

Using crude pomace and refined olive oil pomace, Fang et al. calibrated the workflows by spiking test samples with commercial Cu-cholorophyll standards. They achieved good limits of detection (LODs) and limits of quantitation (LOQs), with reliable reproducibility and accurate identification.

Quantitation using HPLC-PDAD at 430nm gave an LOD of 0.02µg/g and an LOQ of 0.05µg/g. Furthermore, this method enabled accurate evaluation of 24 samples in 3 hours. All samples from the seized products showed E141 adulteration inferred by detection of Cu-chlorophyll pigments above levels naturally occurring in refined olive oil and crude pomace, which the research team defined as below 0.05µg/g.

Fang et al. suggest that the workflows developed are suitable for indicating adulteration of virgin olive oil in addition to being valuable for monitoring the quality of food oil and compliance with food safety directives.

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Reference

1. Fang, M. et al. (2015) “Identification and quantification of Cu-chlorophyll adulteration of edible oils“, Food Additives & Contaminants: Part B, Vol. 8 (pp.157–62) http://dx.doi.org/10.1080/19393210.2015.1025861