FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 15 : Vegetables- Processing and Preparation

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 flavones aluminium chelate. Such reactions also take place in cast iron pans.
• Pears and white potatoes sometimes develop a pinkish colour in their cut surfaces when they stand for a while after being peeled or sliced. Apparently this colour change is due to conversion of the pro-anthocyanin to the pigmented and closely-related compound cyanidin.

Effect of cooking on sulphur containing vegetables

• Cooked cauliflower yields about twice as much hydrogen sulphide as does cooked cabbage. Broccoli also contributes more hydrogen sulphide than does the cabbage. Dimethyl sulphide is a key volatile compound produced when cabbage is boiled.
• Many of the sulphur compounds contributing flavours to various members of the allium genus are quite volatile and escape during cooking unless the cooking time is short.

Effect of cooking on phenolic content

• The effect of cooking on phenolic content varies in different vegetables.
• Method of processing has no effect on phenolic content in spinach and other vegetables like onion and tomato. Boiling, pressure cooking and microwave cooking significantly decreased phenolic content in amaranth.
• In brinjal, there is a significant increase by all the methods as compared to the raw brinjals. In French beans processing increased phenolic content.