Is the milk spoiled when a thin layered textured appears in the bottom of pan? 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:


Mostly, I see no layer or negligible layer when I first boil the milk on 4th
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?
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. During 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).