In this kitchen, microbes cook!

Objective: To find out the appropriate temperature for the setting of curd.

Materials required: curd (as a starter )(100 mL), milk (1 litre), 5 beakers, a spoon, heat source for heating the milk, measuring cylinder, and thermometer.

Procedure: Take five beakers and label them as A, B, C, D, and E. You may label them with a sketch pen or a marker pen. Put 200 mL of milk in each of the five beakers. Heat the milk in the first four beakers to the temperature mentioned in the table below. Add a teaspoonful of curd in each beaker and cover the beaker 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. Put the fifth beaker in the refrigerator after mixing a teaspoonful of curd. Leave the beakers undisturbed for 3-4 hours. Record your observations in the table given below:

BEAKER	Temperature at the time of mixing milk with curd	Observation (Has the milk been converted to curd) Yes/No
A	40 C
B	50 C
C	60 C
D	100 C
E (in the refrigerator)	5-8 C

Interpretation: Most microorganisms exhibit maximum growth in the temperature range 30-45 C. That is the reason why we put perishable substances in the refrigerator. The longer you leave the curd at room temperature, more sour it tastes because of increased growth of bacteria and further production of acid

Re-enforcement: Why do curds set slowly in winters? If you boil the starter curd and then mix it with milk, will curd formation take place? Give reasons for your answer. After this experiment, can you find out the ideal temperature for the growth of microorganisms? Why is it advised to put curd in the refrigerator after it is set?