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I had some old milk in the fridge that smells like it’s on the edge. I decided to make yogurt with it. While I was heating it, lots of curds started to form. Is it safe to eat the yogurt that I’m making with it?

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I wouldn’t. If the milk curdled, it’s a sign of spoilage and the very last thing you want to do now is keep it in a warm environment for a prolonged time.

And while many yogurt recipes include a pasteurization step where you heat the milk to near boiling (not all do, btw.), this will not turn clearly unsafe milk (as indicated by the curdling) into a base for a new product. Especially if the process includes a fermentation at warm temperatures for many hours.

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    Milk curdling a sign of spoiling... source for that?. You can curdle fresh.milk by adding lemon juice to it.
    – Neil Meyer
    Commented Dec 5, 2022 at 11:32
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    @NeilMeyer exactly. For fresh milk to curdle you need an acidic component. If the user didn’t add anything like lemon juice, vinegar or similar, the acid is a byproduct of bacterial activity. It’s the same process as in yogurt making or other fermentations, but as we can not determine which bacteria caused the curdling we must err on the side of safety and assume unwanted bacteria and unsafe food. This has been discussed at length in other Q/As here. Or simplified: Controlled growth of known microorganisms = good, spontaneous growth of microorganisms = potentially very bad.
    – Stephie
    Commented Dec 5, 2022 at 11:45
  • What you are describing here is known as простокваша; what's the worst that could happen 🤷 Commented Dec 5, 2022 at 16:52
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    @OmarL Worst? Food poisoning. To make good soured milk / Dickmilch / … “spontaneously”, you need the right biome in the milk. Modern milking techniques and ironically the hygiene standards of milking equipment mean that even in raw milk today the natural bacteria profile is different than in the farms of our (great-great-…) grandparents. If you add processing steps like pasteurization (as minimum for store-bought milk), all bets are off. Think of it as the difference between freshly squeezed apple juice and pasteurized: one will ferment and give you hard cider, the other will just mold.
    – Stephie
    Commented Dec 5, 2022 at 17:50
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    @MichaelMior soured milk. Most countries with a farming/milking tradition have it in one way or other
    – Stephie
    Commented Dec 5, 2022 at 17:59
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A typical process of making tvorog (Russian cottage cheese) is letting the milk go slightly sour (producing soured milk, as mentioned in the comments), then heating up so that it curdles, finally pressing the remains of the liquid out.

Sounds like you accidentally followed these steps. It's unlikely that at this point you can make yogurt, because much of the lactose has already been used up by other bacteria (which may be unsafe anyway).

UPD: for safety reasons, if you do make your own soured milk, use a starter culture.

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    I must caution all readers here. Yes, this is the traditional method, but for a safe product when using pasteurized (or even worse, UHT) milk, there is a significant risk of undesired bacteria taking over instead of the desired naturally occurring lactic acid producing strains. In modern processes, the cultures are added back in and/or another acid or small amounts of rennet to start the curdling. So proceed at your own risk. For milk that started to go off in the fridge (so not the best environment for the desired kind of bacteria), I would very much discard it over trying to salvage it.
    – Stephie
    Commented Dec 6, 2022 at 6:55
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    @Stephie thanks, I thought my remark in parentheses addressed this, but I added an explicit warning
    – IMil
    Commented Dec 6, 2022 at 10:31

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