I find you can even make yogurt from slightly sour milk, just by adding a little baking soda. Details: to make good yogurt you need to bring it to 85-90C, whic

I find you can even make yogurt from slightly sour milk, just by adding a little baking soda. Details: to make good yogurt you need to bring it to 85-90C, which denatures the milk proteins and improves thickening. If the milk is acidic, it will separate into curds and whey when heated. Sour milk is acidic, so adding a little baking soda to raise the pH allows you to heat the milk without separation. Then, during the later fermentation process, the pH falls again, and the yogurt is tart.