What is the science behind skyr setting without rennet

Question

I've read from multiple sources that rennet used to be one of the ingredients in making skyr but it's not used anymore outside of traditional recipes. I do understand that acid + heat causes milk to curd and indeed, when I heated up a skyr I thought didn't develop to reuse the milk it did separate because of the lactic acid. But heating the milk with bacteria inside will kill the live cultures, so surely this is not how skyr is supposed to set.

The recipe I followed (and others I read to compare) insisted that heating milk to 95C and keeping that temperature for 10 min is required for the skyr to set later after I add the bacteria. It also advised to heat the milk very slowly, 1L of milk for over an hour and let it cool slowly, without an ice bath.

My question is mostly about this: is it possible that the way in which I heat and cool the milk before mixing in live cultures can influence if the curds will separate or not? And if it does, then why does this happen?

Here is the recipe I followed: https://icelandmag.is/article/make-your-own-skyr.

Answer 1

is it possible that the way in which I heat and cool the milk before mixing in live cultures can influence if the curds will separate or not?

Yes, it is entirely possible. Proteins are very complex structures, probably the most complex one we as humans get to deal with. And they get changed by both heat and acid in unpredictable ways - people have to empirically find out what happens when protein-rich foods are heated under different conditions.

The most important part tends to be the final temperature. But the rate of heating is also known to have an influence on the final texture. This is usually seen in very sensitive preparations, such as yolk-based custards, while in other cases, the difference is too little to be noticeable in the final product. I don't know the exact mechanism, and I doubt that it has been investigated completely, but it is normal that, under different conditions, proteins deform in different ways - and apparently, they tend to create less "smooth" results if heated too quickly.

Temperature change rate isn't considered a factor in standard yogurt (bulgarian or western-european style), there you can scald at any speed and you will end up with good yogurt, as long as you take care to hit the right temperature before going on with the next step. But skyr is not entirely the same as yogurt, so it is possible that it does need the slow rate of temperature change. I cannot personally confirm or deny that, since I haven't made skyr myself. So it could be a real thing, or just a kitchen myth, both options are realistic.