These jars of milk, left to right, were heated to 45, 55, 65, 75 and 85°C respectively, prior to being cultured at 45°C for 8 hours. Then a teaspoon of each placed on an incline plane tilted to about 30° for a few seconds to see how far each would run.
In a previous post I lamented that the thickness of the yoghurt I culture at home varied widely in viscosity. I suspected that heating the milk prior to culture was the variable.
Heat does indeed appear to prepare milk proteins for yoghurt culture. Holding the milk at 85°C for 30 minutes before cooling to 45°C for culturing results in a very thick consistency.