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I tried to make yogurt by slowly heating yogurt diluted in milk, but I forgot to watch it.

When I checked a few minutes later, it was above 70 °C and the whey had separated from the "yogurt".

The result smelled like yogurt, but it was very lumpy and quite dense, it looked a bit like mozzarella.

AFAIK, at 70 °C, the lactobacillus should have died, right ?

So what was that ? I am not familiar with many milk products, does it look like a known recipe ?

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    FWIW, I don't think it's usual to heat the milk with the starter yogurt already mixed in. Heat the milk to sterilize it. Then let it cool to ~105 F. Then add the starter yogurt. Keep warm but not hot for 8-12 hours. – The Photon Sep 19 '18 at 18:16
  • @ThePhoton- Not just to sterilize it. Also to improve the texture. cooking.stackexchange.com/a/17777/2001 – Sobachatina Sep 19 '18 at 18:21
  • @The Photon Indeed, but I'm actually testing procedures. I don't need to heat the milk above yogurt-friendly temperatures because I use UHT milk. – Skippy le Grand Gourou Sep 19 '18 at 18:55
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Short answer:

That is well above the 130°F (55°C) at which the bacteria will die.
You over coagulated your milk proteins and made cheese.

Long answer:

After scalding and cooling, yogurt is kept warm to incubate the bacteria. The bacteria munch on lactose in the milk and produce lactic acid. The acid denatures the milk protein, causing protein molecules to unravel and then tangle up with each other trapping water and other milk molecules.

By heating the milk further, those tangled proteins then started to tighten up. They squeezed out a bunch of the water and became more solid and dense.

This is how acid set cheeses like paneer, or queso fresco are made- except they use lemon juice or vinegar for the acid instead of lactic acid.
This is also very similar to some ricotta preparations that are made from rennet-produced cheese whey. In this case the lactic acid from the cheese fermentation is enough to denature the leftover, water soluble, milk proteins when the milk is heated.

If your fermentation didn't go off, and from your description of the smell it sounds like it is good, then you could use this as a mild, fresh cheese. Yogurt cheese is used for frozen yogurt but this won't work for you if your proteins tightened up so much that they aren't creamy anymore.

Personally I'd just eat it on a cracker with a little jam.

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