FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 15 : Vegetables- Processing and Preparation

Effect of cooking on carotene content of leafy vegetables

• Effect of cooking on carotene content of leafy vegetables (Amaranth, Koyakura, Bacchali and Palak)


• Effect of cooking on pigments


• 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.
  • Magnesium present in the structure of chlorophyll is rather easily displaced by hydrogen molecule 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.
  • Green vegetables which are lower in acid retain a higher percentage of chlorophyll when they are cooked than do more acid vegetables ex.
  • Gogu which is very sour becomes olive green as soon as it is put in hot water.
  • Cooking vegetables 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. Hence greens should be cooked without lid for the first few minutes to allow the volatile acids to escape.
  • Destruction of chlorophyll increases with cooking time. Chlorophyll-a is more readily converted to pheophytin than chlorophyll-b. Green vegetables should be cooked for a very short time, less than 7 minutes to retain the colour.
  
  
• Effect of canning
• Vegetables like peas, beans, greens are sometimes canned.
• During canning, chlorophyll gets converted to pheophytin due to the high temperatures used.


• Effect of sodium bicarbonate
• When sodium bicarbonate reacts with chlorophyll the phytyl and methyl groups 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.


• Chlorophyll can be retained best
• By starting with boiling water which in turn reduces the actual time required to tenderise. Vegetables should be cooked less than 5 minutes as pheophytin forms between 5 and 7 minutes.
• By using slight excess of water to dilute the acids.
• By initially cooking in an open pan.


• 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 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 to cis form.
• 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.