Raw milk and unhomogenised milk are different beasts. Raw milk has not been pasteurised (heated briefly to 72°C or for a longer time to a slightly lower temperature, or for a very very short time to a very high temperature). Unhomogenised milk has not had the fat droplets broken up into such tiny bits that they don’t float. You COULD have homogenised raw milk although in practise you don’t get it, and you can very easily buy pasteurised unhomogenised milk. That’s what I grew up drinking all the time. The fat (cream) floats to the top, but there is no more or less actual fat in the milk than there is in homogenised. I much prefer homogenised milk for yoghurt, because you don’t have to stir in the fat that floats to the top.
I heat new milk, but just to the temperature I incubate it at. I don’t pasteurise it. No cooling and waiting step is involved. Basically I just mix everything up, put it in the water bath and it heats up to ~40°C (I don’t worry about being too accurate and I don’t heat it to 51°C, ~40° is fine) in a very few minutes.
I do realise you were not giving instructions for raw milk yoghurt, but think about it for a minute. The milk you buy in the shops is already pasteurised, so you don’t need to pasteurise it yourself unless it has been sitting opened in the fridge and is getting close to its “best before” date. If you use raw milk for whatever health benefits it is reputed to have, the last thing you want to do is pasteurise it. Because then it’s not raw milk any more.
Adding milk powder to the milk increases the protein concentration far more easily than simmering the milk for an extended period to evaporate water from it. It’s also much easier to control the amount, and hence the thickness of the yoghurt.
What you have to remember when you’re making cheese or yoghurt is that it’s not rocket science. Unhygienic peasants have been doing it for centuries; they didn’t have thermometers, they didn’t know about germs and being clean. We overthink things these days.
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