Colourings brown polyphenols

Polyphenol oxidase Mushroom, grape, pear, kiwi, sago, tea, potato, strawberry, olive, beet, cocoa Enzymatic browning (pig-ments),ripening of fruits, flavour formation, colour e.g. strawberries) formation... 

The various conversions in the pigment composition during postharvest storage, processing and freezing are extremely important to food colour. Enzymatic browning caused by oxidative conversion of polyphenols into coloured complexes and non-enzymatic browning is also important to food colour, however, these processes are not included here. 

Phenolic compounds known as polyphenols (such as phenohc acids and flavonoids) are easily oxidised by atmospheric oxygen in reactions catalysed by oxidoreductases (o-diphenokO, oxidoreduc-tases). These reactions are known as enzymatic browning reactions (see Section 9.12). Autoxidation of plant phenols catalysed by heavy metal ions (mainly cupric and ferric ions) and oxidation by lipid hydroperoxides lead to similar products. The products of oxidation of the so-called o-diphenok (1,2-dihydroxybenzenes) are o-quinones (1,2-benzoquinones), highly reactive compounds that react with proteins (amino acids) and other food components with the formation of dark-coloured polymeric products resistant to proteolysis. 

Merely reducing the temperature reduces the reaction rate, but the colour changes are quite rapid even at 0 °C. This means that sensitive products that have not been pre-treated should be frozen as quickly as possible, for example mushrooms and sliced peaches. Rapid intensive browning occurs during defrosting, when the activity of polyphenol oxidases increases due to disruption of cellular structures by ice crystals. 

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