Why is my yogurt sour at the top and non-sour at the bottom I make yogurt with a yogurt maker and the yogurt is nice and thick, but it is very sour at the top only and the rest is almost sweet, like a sweet fresh cheese. Why isn't it sour all over?

This is how I prepare it:


heat 1l (goat or sheep) milk slowly (1h) to 85° Celsius,
keep it at 85° Celsius for 10 minutes,
let it cool slowly to 45° Celsius,
mix well with 50g simple yogurt,
put it in the yogurt maker at 45° Celsius for 12h.
There could be a few things that are going on here. 

My best guess from what you have said would be the temperature or mixing is a little off. 

Let the mixture cool a little more before combining with the yoghurt. Also, do this gradually, adding a small part of the cooled milk to the yoghurt each time, mixing until smooth.

Hopefully, this helps.