Yoghurt is made from a variety of milks but the term "curd" usually applies to fermented and thickened buffalo milk. Both products are white with a creamy consistency. Set yoghurt is a smooth, firm, white gel with a characteristic acidic taste made by fermenting cow milk. Other similar products can be made from goat or mare milk.
For curd preparation, milk is fermented with mixed natural lactic acid bacteria for making curd. It is inoculated with culture and fermentation is carried out at the right temperature. Lactic acid bacteria rapidly produces acid which at about pH 4.6 causes the coagulation of casein, making curd. Further production of acid not only increases sourness but also increases the compactness of the curd.
Yoghurt normally is prepared using pure culture. It may have only a single species or sometimes more. The selection of species is based on their ability for rapid production of lactic acid and development of flavour. Use of pure starter culture ensures uniform results batch after batch giving consistent production of yoghurt with same acidity, texture and flavour. Yoghurt and curd are widely used in Africa, Asia and Latin America to accompany a main meal. It is also used as a dessert or as a dressing for vegetable salads. It has a shelf-life of three to eight days, depending on the storage conditions. Stirred yoghurt is fermented in bulk, stirred and then dispensed into pots or sold into customers' containers. Set yoghurt is made by pouring the inoculated milk into pots and fermenting it in the pot.