Milk contains lactose which is not present in many other plants/animals. This means it takes time for bacteria to produce their own lactase to break down lactose into sugar so their can multiply. The exceptions are lactoabacilli and lactococci which get a head start on other microbes. They convert lactose into lactic acid which makes it hard for other microbes. So milk turns sour but becomes less hospitable for spoiling bacteria
Milk fat is contained in globules, which are surprisingly resistant to heat (unless turned acidic first). Milk and cream can be heated up for hours without splitting, but these globules can be broken by ice crystals when freezing.
so don’t heat up previously frozen milk as the fat will split from the liquid
I hate the smell of warmed milk. Turns out that the lactoglobulin protein denatures at 78C which releases sulfur atoms which creates hydrogen sulphide gas that gives it the aroma! This is also present in UHT milk which is why it’s horrible
Milk will curdle if it’s too acidic, which happens if it’s too old as bacteria will create lactic acid