All yogurt recipes I’ve seen say you should preheat the milk before adding starter (= yogurt with live culture). Reasons vary. Some say it denatures the milk protein; others say it kills bacteria that might compete with the starter bacteria.
It was easy to measure the effect of preheating. I make yogurt using about a gallon of milk at a time, divided into four trays. I preheated two trays for 20 minutes and did not preheat the other two, leaving them at room temperature. After that I treated all four trays the same.
The photo above shows the results after incubation for 36 hours. The clumpy yogurt was preheated, the smooth yogurt was not. There was not a vast difference in taste. For most purposes clumpy is better so I will preheat in the future.
I was impressed that the experiment was fast, easy, safe, cheap, and conclusive, showing a large and lasting effect of a 20-minute treatment that had no visible effect. After the heated milk cooled, it looked the same as the unheated milk.