How does milk change to curd?

Objective: To demonstrate the need for starter to convert milk into curd.

Materials required: curd (as a starter “jaman”)(20 mL), milk (500 mL), 2 beakers, a spoon, heat source for heating the milk, measuring cylinder, thermometer.

Procedure: Take two beakers and label them as A and B. You may label them with a sketch pen or a marker pen. Put 250 mL of milk in each beaker. Heat the milk in each beaker to 45 C. Add a teaspoonful of curd in beaker ‘B’ and cover the beaker with an inverted petridish. Cover beaker ‘A’ also with an inverted petridish. Put the beakers in separate cardboard boxes or insulated containers. You may wrap each beaker in a thick towel to maintain the respective temperatures. Leave the beakers undisturbed for 3-4 hours. Record your observations in the table given below:


Interpretation: Curd is a milk product that contains milk proteins, milk fats, some minerals and vitamins and a very large number of this microorganism. It is sour to taste. The microorganism responsible for formation of curd from milk is a bacterium called Lactobacillus. The milk tastes sweetish because of the presence of a sugar called lactose. Lactobacillus converts this lactose of the milk into lactic acid of the curd.

BEAKER	Has the starter been added?	Observation (Has the milk been converted to curd)
A	No
B	Yes

Re-enforcement: What has been provided in the form of a starter in this experiment? If you boil the starter curd and then mix it with milk, will curd formation take place? Give reasons for your answer.