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Maida is a finely milled and refined and bleached (either naturally due to atmospheric oxygen or using other chemical bleaches) wheat flour, closely resembling cake flour, and used extensively in making Pakistani/Indian fast food, Indian bakery products such as pastries and bread, varieties of sweets and sometimes in making traditional Afghani/Pakistani/Indian breads such as paratha and naan. It is made from the endosperm (the starchy white part) of the grain, while the fibrous bran is removed in the mill.
Originally yellowish, maida is popular in a white color, bleached with Azodicarbonamide, chlorine gas, benzoyl peroxide, or other bleaches. The use of benzoyl peroxide in food is banned in China as alternative processing methods are available and in the European Union (including the UK).
Maida contains trace amounts of alloxan, which is an undesirable side product of the chemical changes that give it softness and white color. Large amounts of alloxan is known to destroy beta cells in the pancreas of rodents and other species, causing diabetes mellitus, 
Maida is also used in Central Asian and Southeast Asian cuisine. Maida is used as an adhesive for wall posters in India.
Maida is finely milled flour and is usually refined using a fine mesh of 600 mesh per inch. In south India where there are no wheat farms wheat is moved in trucks and rakes and then milled. It is a common misunderstanding that tapioca is converted into maida, rava, atta and bran.
Pastry flours available in United States may be used as a substitute for maida. Flour of whole wheat, which includes part of the brown outer layer known as bran, is often considered healthier than maida flour as it contains a higher level of dietary fibre (around 2-3g per 100g as opposed to 0.3g in maida flour). Consuming breads and foods made with whole-wheat flours are recommended instead of maida for maximum nutrition.