He shifts the pH of the fresh milk from 6 to alkaline so that in basic form it will not spoil easily. This milk takes a long time to set as curd because the lactic acid produced reacts with the baking soda.
A fresh milk is acidic and it turn sour easily to become more acidic. in the presenc eof baking soda, milk become alkaline and does not turn sour easily because the alkali does not allow the milk to become more acidic
b. when the milk set to curd,the ph decrease.. the presence of alkali does not allow it to become acidic easily..hence it take long time to set as curd