Chemical stability of food colorants

The common molecular features which give rise to absorption in the visible region, i.e., a conjugated double bond system, electron withdrawing and donating constituents and in the case of tetrapyrroles reactivity and oxidation 

It is well established that unsatnrated fatty acids undergo oxidation, via a radical reaction mechanism. Carotenoids undergo similar reactions and indeed do this so readily they can act as antioxidants in food materials. This antioxidant ability of carotenoids derives from their ability to form a resonance stabilised free radical. In certain controlled conditions chemical oxidation of carotenoids can give rise to epoxide formation and isomerisation of this to a furanoxide (Wong, 

The epoxide formation has been shown to occur in canned fruit juice and can give rise to considerable loss of colour. This loss of colour can be accounted for by the reduced resonance stabilisation of the product, there is basically a loss of two conjugated double bonds, one in the 6 membered ring and alternate to this (Wong, 1989). 

The benzopyran derivatives are not as easily oxidised as the carotenoids. In the heme tetrapyrrole derivatives the central iron atom may be oxidised relatively easily to form metmyoglobin. This change from Fe to Fe is observed easily when beef is exposed to the atmosphere for 2-3 days at refrigeration temperatures and faster at higher temperatures (MacDougall and Powell, 1997). The nature of the orbital changes for this have been given in Fig. 

In chlorophyll where Mg is the central atom it is oxidation of the porphoryn ring which occurs. Under mild oxidation conditions (e g. KMn04 and acetone the vinyl group of chlorophyll a and b is oxidised to carboxyhc acid substituents. Under strong oxidising conditions such as chromic acid a mixture of pyrroles results (Wong, 1989). 

---