Yoghurt making : effects of temperature and duration As I am fairly new to making yoghurt at home I would like to understand the effects that temperatures and time have on the end product in both the initial heating/cooling step and the fermentation step.

So far I have made half a dozen 1.5 litre batches with three different cultures (currently using ABY-1 from CHR Hansen that I was able to buy from the Westcombe dairy in Somerset). I have a Luvelle machine that can ferment at 36C, 38C and 40C from 1 hour to 24 hours. 

I want to understand what changes if I experiment with the following:-


When I heat the milk to 85C and hold the temperature I have made some batches with a 10 minute hold and others with a 20 minute hold period and see/feel/taste no difference - will holding at 30 minutes produce a difference?
I am currently fermenting for 24 hours at 38C and getting a strong tang/sourness to the yoghurt but what would I expect to get if I reduced the fermentation to 12 hours at 38C...or less?
I choose 38C for the fermentation process because it was recommended by the manual that came with the Luvelle machine but what happens if I use 36C or 40C and what length of time would I use?


So generally speaking I want to know more about the initial heating process and what options there are and what the different fermentation temperatures and durations can have on the end results.

Thanks in advance!
First, about the holding time: It is a safety feature. It is meant to ensure that the number of non-culturing organisms that survive is so low that the culturing organisms can overtake them and create a colony of their own, without pathogens. If you reduce it, sometimes nothing will happen, and sometimes you will get a dangerously high growth of pathogenic bacteria. To keep your yogurt safe, you have to do the 30 minutes. 

Second, fermentation time and temperature. When you stay close to the prescribed temperature for a given culture, then you can trust that a higher temperature will give you the same sourness in shorter time, or more sourness in the same time. If you go far away from the prescribed temperature, your fermentation process again becomes unsafe (because some other bacteria are likely to take over before your culture multiplies enough to establish itself). From experience, a deviation of 2C from the optimal is close enough to still make good yogurt, so you can experiment with all three temperatures. What happens if you use 36 C is that you get less tangy yogurt, and at 40 you get more tangy. The length of time you use is pretty much a matter of trial and error, until you get the taste you prefer. 

Note that, for establishing your preferred time, you should use an established culture. That is, you should either always use a new capsule of lab-bought culture, or you should first do a few generations of reinnoculation at constant conditions, and then start experimenting from there. This is because the first few re-innoculations from a new capsule are a kind of "adjusting" fermentations, and your yogurt will be changing its tanginess and consistency even when you keep the conditions (time and temperature) equal.