I used to scald milk (in an Instant Pot this is the yogurt "high" setting) before making yogurt, but no longer do so.
Now I buy UHT pasteurized milk, which encompasses most organic milk, and simply run 18 hours of the Instant Pot "medium" setting. I start the instant pot, add two half gallons of milk, then pour in some Bulgarian yogurt as a starter and stir, then lid on and leave it alone until it beeps at me.
I normally get around three gallons of yogurt out of a one quart jar of the Bulgarian, which comes in a conveniently Mason-threaded quart jar, where I hoard the leftovers for storing my own yogurt.
Why did you switch from scalding regular milk to UHT? 18 hours is a long time to make yogurt. I do the scalding, let it cool in a fridge for an hour, and then make yogurt for 6.5 hours. This works for non-UHT milk (doesn't for UHT).
Not OP, but I've found that using UHT (or semi-UHT as many refrigerated organic milks are in Australia) works just as well as a heat->cool of non-UHT (after all, UHT is just milk that has been heated then cooled). So it's less work/fussing around/friction, and I end up making yoghurt more often. I usually do a 12 hour cycle. The heating (either at home, or via UHT) is necessary to modify the proteins so it thickens.
Interesting. For me (in the US), UHT was usually a failure - but as I said, I do it for only 6.5 hours. I could try doing it for longer, and I guess that's convenient for overnight. But otherwise, heating + 6.5 hours for regular milk is faster.
What I don't understand is: Why does heating + 6.5 hours work for non-UHT milk but not for UHT? It's not just me - I Googled at the time and many others had the same experience: UHT milk often fails. Like me, they all were doing 6-8 hours.
Googling now, I see lots of people arguing if UHT makes it harder to make yogurt or not. Personally, I'd like to see examples of people making decently thick yogurt (without straining) in under 7 hours with UHT (whether they scald or not).
This site[1] says there was a study done that showed unheated UHT was runnier than heated non-UHT. I didn't check the study to see how long they set it for.
This site[2] also points out that UHT for 10 hours was still quite runny. They had to add powdered milk to get it thicker.
So I haven't tried actual UHT; I use 'ultra-pasteurised' organic milk, which is refrigerated, but has a much longer expiry date than you would expect from milk, and you can taste the extra heat treatment. 8 hours works great and gives a thick yoghurt (haven't tried less than that; usually do overnight so around 12 hours). It does tend to get progressively less thick generation after generation. I use a supermarket organic plain yoghurt as a starter (there's a certain ratio of starter yoghurt to milk required - I use 4 Tbsp yoghurt to 1L milk)
When I try to use UHT milk, I usually fail in making yogurt. But that may be because I do 6.5 hours and he's doing 18 hours. Perhaps you need to go so long with UHT?
Now I buy UHT pasteurized milk, which encompasses most organic milk, and simply run 18 hours of the Instant Pot "medium" setting. I start the instant pot, add two half gallons of milk, then pour in some Bulgarian yogurt as a starter and stir, then lid on and leave it alone until it beeps at me.
I normally get around three gallons of yogurt out of a one quart jar of the Bulgarian, which comes in a conveniently Mason-threaded quart jar, where I hoard the leftovers for storing my own yogurt.