Difference between rai and mustard seeds

There are two types of mustard seeds commonly used in India , black mustard seed (called Rai) and yellow mustard seed (called Sarson). The South Indian recipes generally use black mustard seeds (Rai​) while Bengali cuisine and other North Indian cuisine more frequently use yellow seeds (Sarson). Usually, the mustard seeds are either used as part of spice mixture such as garam masala or whole seeds are fried in hot oil where mustard seeds crackle and pop to give nutty flavour. Generally, the whole black mustard seeds (Rai​) are fried and used with dal, achar and other Indian vegetarian dishes and use yellow mustard with vegetable curry or fish. Mustard seeds have canonical use in India where it is used an agent to ward off evil spirits during illness, The Buddhist canons often cite the use of mustard as the spice. 

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