Curd is a dairy product and it is obtained by coagulating milk in the process which is known as curdling. And that coagulation of milk can be caused by adding any edible acidic substance such as lemon juice, vinegar and then it is ready for coagulation.
Dahi or curd is a semi-solid product, obtained from pasteurised or boiled milk by souring, using harmless lactic acid or other bacterial cultures. It is produced from heat-treated milk after inoculation with certain species of lactic acid bacteria added to milk in the form of starter culture. Lactic acid bacteria added multiply, grow and produce lactic acid, acetic acid, and carbon dioxide by utilizing lactose present in milk. Some bacteria use citric acid of milk to produce certain volatile organic compounds, mainly diacetyl, which is responsible for the flavour of Dahi. A judicious combination of acid-producing and flavour-producing microorganisms in the starter helps in the production of Dahi with a firm body and good flavour.