FSLondon said...
Justud, pasteurised milk that you buy is clean enough for human consumption--nothing in it will make you unwell--but small amounts of bacteria do remain and also get in that might impede the growth of yogurt bacteria (this is why UHT milk lasts longer--more of the bacteria in it is killed). You can try to make yogurt without boiling the milk and just warming it to 110 and it ought to work much of the time, but maybe not always. Also, boiling the milk before making yogurt makes for a thicker yogurt.
Just make sure you let the milk cool down a bit (while in a covered container) so that you do not kill out all of the bacteria that you add
just to add, when they pasteurize milk they lower the counts of the bad bacteria so that when we consume it there is not enough to cause illness if all goes well
when you make yogurt you need to leave the milk unrefrigerated for some time to allow the bacteria that we add to multiply (they make the biproduct of acid and we get a sour taste from that) this also allows the small counts of bacteria that may be left to multiply as well, also as the milk is uncovered more bacteria can fall in which is beyond the control of the dairy company