FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 15 : Vegetables- Processing and Preparation

Loss of nutrients during cooking

• Losses begin from pre-preparation onwards, like peeling the vitamins presents under the skin may be lost.
• Carrots have a valuable layer of nutrients under the skin so they should be scraped but not peeled thick.
• Throwing away outer leaves of cabbage can result in loss of carotene.
• Beet root, carrot and cauliflower leaves are very nutritious, hence discarding these leaves results in loss of nutrients.

Solvent action of water: Water soluble nutrients may be dissolved in the cooking water and the nutrients may be lost. Losses by solution can be reduced by the following methods:

• Cut the vegetable into bigger pieces so that the exposure of the vitamins to water is less. When carrots are cut in crosswise the loss of ascorbic acid is greater than that when they are cut lengthwise.
• Soaking or washing time should be reduced. So that enough time is not given for the water soluble nutrients to get dissolved in water.
• Wash the vegetables with the skin and later should be peeled and cut.
• Use as small quantity of water as possible so that there is no extra water at all.
• Cook for a short time. As period of cooking increases more and more of ascorbic acid leaches into the water.
• Cook the vegetables by steaming and pressure cooking, where no additional water is added.
• Cover the vessel with a lid to hasten the cooking process.
• Cook the vegetables with the skin so that the leaching of vitamins into the water would be less.
• Leached water can be used in cooking.

Oxidation and chemical decomposition

• Losses of nutrients can occur by chemical decomposition which may be caused by the reaction of the cooking water or by heat.
• Vitamin C is readily oxidisable and if this proceeds beyond the stage of dehydro- ascorbic acid, all vitamin activity is permanently lost.
• Oxidation may be accelerated by enzymatic action, by heat, by an alkaline medium by traces of copper and by free access to atmospheric oxygen.
• Vitamin A gets oxidized by dehydration or application of dry heat.

Prevention of losses by oxidation or chemical decomposition

• Cut the vegetables into bigger pieces so that the exposure is less.
• Cut and use it immediately. By grinding, the losses will be more due to greater atmospheric exposure. By extraction of juice the losses are greater.
• Start cooking with boiling water. The greatest destruction of ascorbic acid occurs during the first minute or two of the cooking period. This destruction is the result of the presence of both oxygen and of oxidizing enzymes in the plant tissue. Water should be boiling when the vegetable is put to cook to expel oxygen from the tissues which catalases the oxidation of ascorbic acid.
• Cover the pan so that there is no direct contact with the atmospheric oxygen.
• Use a sharp knife. When sharp knife is used to cut cabbage bruising is avoided and loss of ascorbic acid is reduced.
• The more alkaline the solution, the faster is the rate of destruction especially temperature and time of heating are increased. Ascorbic acid is protected to some extent when heated in the natural acids of certain foods such as tomatoes, vinegar, tamarind and lime juices. The use of baking soda increases the loss of thiamine and vitamin C.
• After cooking, the food has to be consumed immediately; even if it is kept in the refrigerator, the losses continue to occur.

Riboflavin and niacin are stable even at 1000C. Riboflavin is sensitive to light. Foods which are exposed to sunlight before cooking causes riboflavin loss. Minerals are not destroyed by cooking.

There is maximum retention in steaming and pressure cooking because there are less losses due to leaching. The richer the vitamin C in the raw vegetable, the more is the retention in steaming and pressure cooking.

Addition of baking soda

Sometimes, baking soda is added to vegetables to improve the colour or to hasten cooking process. At alkaline PH, thiamine are unstable and vitamin C. Alkaline medium increases losses due to heat. Folic acid is also lost in alkaline medium.