FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 15 : Vegetables- Processing and Preparation

Flavonoids

Anthocyanins

Effect of PH :

• As PH changes, the colour of anthocyanin change as follows:



PH	3	7	8	10	12	14
COLOURS	Crimson	Purple	Greyish purple	Grey	Greenish gray	Bright green
Photographs - Colours



• Red cabbage exhibits unusually wild swings in colour with a change in PH because of the presence of more than four hydroxyl groups on the anthocyanin molecule.
• Red cabbage is frequently cooked with the addition of some slices of a tart apple to ensure that the PH is sufficiently acidic to avoid the development of a blue, highly unpalatable pigment colour.
• The addition of alkali alters the structure of anthocyanin molecule and produces a salt.



Effect of metal

• Special enamel linings in the cans are used when canning anthocyanin containing fruits and vegetables.
• The presence of ascorbic acid with copper or iron accelerates the oxidation and undesirable colour changes of anthocyanin compounds.
• The metal iron precipitates the pigment. Iron and aluminium produce considerable discolouration; iron reacts with anthocyanins to form intense black discolouration.
• Red cabbage shredded with a non stainless steel blade turns blue very rapidly from the reaction between the iron of peeler and pigment of the cabbage.



Effect of method of cooking

• Cooking in a steamer or in a pressure pan which limit the contact of the vegetable with water are better methods than boiling in water.
• To retain the red colour in red cabbage, the cooking water should be acidified. Otherwise, the pigment will change to a dull and unappetizing blue.



Effect of tap water

• Since tap water is slightly alkaline, the anthocyanins are changed to unattractive blue or grey colour.



Effect of pickling

• Pickles are usually low in pH and anthocyanins change to deep crimson red in colour.