As a new bride I struggled to make thayir like amma made. I tried to emulate what I had seen her do with drastically different results. Over the years, talking to old hands at this, I have discovered a way that makes my thayir almost as good as hers. This applies to colder climes.
Stuff you will need:
2% Milk – 1 liter
Left over thayir – 1 tbsp
How to make it?
Boil milk till it really is ready to boil over. Remove from heat and let cool till you can put your hands around the vessel and hold it there for 10 secs. At this point stir in the thayir. Close the vessel. If you are doing this overnight, let the microwave run for about a minute or two empty. When done, move the vessel containing the milk and close. Leave undisturbed for about 8 hours. Thayir is ready.
I have experimented with putting it in the oven and leaving the oven light on, the thayir sets no doubt but becomes sour quick.