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I use Pasteurized Amul milk (see here). Last time, I purchased it on May 3rd and has expiry as today, 05-05-19.

(Outside temperate in our area is around 35-40 degrees)

I keep it (packaged when not boiled, in pan once opened and boiled) in the fridge all the time except when it is used. I first brought it out on May 4th morning, boiled it and used half of it (total 500 ml). Stored the remaining again in fridge in pan.

I boiled the remaining milk just a few minutes ago (tonight will be expiry).

Here's what I observe:

  1. Mostly, I see no layer or negligible layer when I first boil the milk on 4th
  2. When I boil it again next day in same pan (I don't change the pan during whole process), there's still fat in it, a creamy yellow layer on top but the bottom layer (highlighted in image) is perfectly noticeable this time.

Now, I don't note any sour taste or bad smell or the curd like solids in it. It seems fine to me.

But does this layer mean it has started to go sour and will rise eventually?

enter image description here

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This layer just means some milk proteins have cooked onto the bottom of the pan, and says nothing at all about the age or condition of the milk. It's more likely when you boil a smaller quantity due to the more rapid heating. Stirring can help avoid this.

However repeated heating and cooling isn't generally a good idea. With milk you can get away with it but it would be a better idea to boil only what you need (which would also be quicker and use less energy).

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  • Okay, I also want to counter confirm this. If it had started becoming sour (spoiling), will the milk fat (cream) be also spoiled? Or milk will spoil and the cream will survive?
    – Vikas
    May 5, 2019 at 9:23
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    Cream is part of the milk, so once the milk is spilled it's all spoiled. Merely soured is slightly different, but it doesn't happen the same if the milk has been boiled so isn't relevant here. The fat is irrelevant to what happened here as well. You'd get the same effect in skimmed milk.
    – Chris H
    May 5, 2019 at 9:27
  • Boiling milk again and again kills/destroys the useful stuff right?
    – Vikas
    May 5, 2019 at 9:54
  • That depends on your definition of useful stuff but in general heating and cooling foods repeatedly can cause food poisoning as not everything nasty is killed by boiling but it multiplies while the food is warm. Milk isn't a good host for species that survive boiling (and being a liquid is easy to heat to boiling throughout) so it's not the big deal it would be in other things
    – Chris H
    May 5, 2019 at 11:23
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    Pasteurised milk doesn't need boiling. If you're making something that requires boiling milk, or you want it hot, measure it first, then boil it.
    – Chris H
    May 5, 2019 at 15:26

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