Effect of Processing on Pigments

Effect of Processing on Pigments

Chlorophyll

All green leaves and green-colour vegetables like beans and peas contain chlorophyll. The complex structure and conjugation of proteins makes chlorophyll very susceptible to chemical changes in cooking and consequently the pleasant green colour is difficult to retain.

 

Effect of putting in hot water: 

When a green vegetable is put in boiling water, the green colour becomes brighter. This is due to greater translucency of plant tissue due to expulsion of intercellular air collapse of the intercellular spaces.

 

Solubility in water: 

Removal of the phytyl group from the molecule of chlorophyll is catalysed by the enzyme chlorophyllase found in some vegetables. Hydrolysis of the ester linkage yields a compound chlorophyllide which iş water soluble. A limited amount of chlorophyllide produced during storage of certain green vegetables prior to cooking possibly accounts for the light green tint of the cooking water from them.

 

Effect of prolonged cooking and acid: 

As the cooking process continues the colour of the green vegetables varies from the bright green colour of chlorophyll to the olive green. Acids are formed by the respiratory processes of the plant which are accumulated as the vegetable is warmed up, then stopped as the vegetable reaches the boiling point. Both volatile and non-volatile acids are given off during the cooking of vegetables. As cooking continues cell is disrupted, constituents including organic acids diffuse from the vacuoles throughout the cell and into the cooking water. 

Magnesium present in the structure of chlorophyll is rather easily displaced when it is heated in the presence of organic acids. A pale greenish grey compound known as pheophytin-a or an olive-green pheophytin-b results. This combination together with pheophytins give the vegetable a muddy olive green colour. Green vegetables which are lower in acid retain a higher percentage of chlorophyll when they are cooked than do more acid. Since acids are present in plant tissue along with chlorophyll, the problem is how to minimise their effect during the cooking of vegetables. This may be accomplished by cooking the vegetable in an uncovered pan to eliminate volatile acids and by using enough water to cover the vegetables to dilute those acids which are not volatile. The major portion of the volatile acids is eliminated during the first few minutes, the vegetable is in contact with the boiling water. Hence greens should he cooked without lid for the first few minutes to allow the volatile acids to escape.

Effect of canning: 

Vegetables like peas, beans, greens are sometimes canned. During canning chlorophyll gets converted to pheophytin due to the high temperatures used. Sometimes, to retain the colour and to neutralise the acid, alkali is added. When sodium bicarbonate reacts with chlorophyll the phytyl and methyl groups are displaced and a bright green water-soluble chlorophyllin is formed. The sodium salt of chlorophyllin gives the cooked green vegetables an intense artificial greenness. 

 Effect of freezing: 

The better retention of colour of frozen green vegetables is due to elimination of major part of the plant acids by blanching prior to freezing.

 

Effect of copper: 

The colour of chlorophyll is enhanced in the presence of copper and weak acid.

  Effect of calcium salt: 

Addition of a small amount of calcium acetate or other calcium salt prevents the mushiness by blocking the breakdown of the hemicelluloses.

Carotenoids

Ordinary cooking conditions have little effect on the colour or the nutritive value of carotenoid. The colour is little affected by acid, alkali and the volume of the water. The nutritive value is protected during cooking by the insolubility in water.

     

Effect of heat and oxidation: 

The high degree of unsaturation of the carotenoids makes them susceptible to oxidation with the resulting loss of colour after the food containing them has been dried. The loss in intensity of colour is not only due to oxidation of the unsaturation of carotene but also shift from trans form to cis form. Although the Trans form is quite stable, the heat of cooking trans forms to cis configuration. The hue of carotenoids with a cis configuration is less intense than that of their counterparts with the trans configuration exclusively. The somewhat lighter colour of carrots and other carotenoid containing vegetables that is noted after cooking is an indication that some cis isomers have formed.

The finer the vegetable divided or cut and higher the temperatures that are used and longer cooking process can hasten the process of oxidation. Blanching prior to dehydration is helpful in reducing the likelihood of oxidation. Blanching makes lipids free from the proteins with which they are complexed as lipoproteins.

     

Effect of cooking in fat:

Appreciable amounts and enough to affect the colour will dissolve in ghee, when carrot kheer or halwa is made.

Anthocyanin

Effect of pH:

As pH changes, the colour of anthocyanin changes. 

pH	3	7	8	10	12	14
Color	Crimson	Purple	Greyish purple	Grey	Greenish grey	Bright green

The acidity of the cell in which these compounds are formed causes the molecules to have a positive charge on the oxygen atom. This form, which is the common form at a pH of 3.0 or less, maintains or shifts the hue toward red. However, the positively charged oxygen form called an oxonium, is altered to the quinone form as the pH is increased toward a weak acid or even neutral solution.

The quinone form has a violet colour. In an alkaline medium still another change takes place as a salt of the violet compound, called a colour base, is made. The alkaline salt of the colour base has a distinctly blue colour.

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.

Addition of alkali gives the pigment a bluish green shade. This shade is probably caused by the presence of anthoxanthins with anthocyanins. On the addition of alkali, the anthoxanthins turn yellow while the anthocyanins turn blue and a mixture of the two colours appear green. Such a colour can be seen in red cabbage. 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. Unusual colours ranging from green to slate blue, develop when anthocyanin’s contact iron, aluminium, tin and copper ions. 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 anthocyanin’s 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 the peeler and pigment of the cabbage.

         

Effect of method of cooking:

Anthocyanin are water-soluble cell sap pigments which can be leached from a vegetable by the cooking water. 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 appetizing blue.

 

Effect of tap water: 

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

 

Effect of pickling: 

Pickles are usually low in pH and anthocyanin’s change to deep crimson red in colour.

Betalain

Although these pigments are held tightly within cells in the raw vegetable, they diffuse rather rapidly into the cooking water resulting in the highly pigmented water associated with boiling beets. This problem is aggravated by cutting beet into small pieces and cooking leading to dull colouration in the boiled product. Since this colour is highly soluble in water, the best method of cooking beet root is to cook along with the skin, so that much of the colour does not leach out.

 

Effect of pH:

 Betacyanins undergo colour changes parallel to anthocyanin. An acidic medium promotes a reddish colour, whereas a neutral or somewhat alkaline pH brings out brownish blue of the pigment.

   

Anthoxanthins

Effect of pH: 

The colour of the vegetable will be whiter if little acid such as lime juice or vinegar is added during cooking. If the water in which cauliflower is cooked is slightly alkaline, it will have a distinctly yellow colour to it. They turn yellow or orange in the presence of alkali. This is used as a confirmatory test.

Effect of metal: 

They cause the cooking water to turn a bit yellow, when they are cooked in aluminium pans because the flavones scavenge aluminium and form a flavone aluminum chelate. Such reactions also take place in cast iron pans.