Alex Rast wrote:

> This recipe calls for sour milk - which is literally that - milk that's
> gone sour. Ordinary "pasteurized" milk will go sour (usually about 1 week
> past expiration) but "ultra-pasteurized" milk (the stuff with expiration
> dates freakishly far out) will not go sour, so make sure your milk says
> "pasteurized" on it. Also UHT milk (the stuff that they can keep on
> unrefrigerated grocery shelves) doesn't go sour. This is a good way to use
> up milk that you accidentally left around a little too long.

What do you mean by "sour?"

All these milks will spoil and smell and taste sour in time. The
difference between them is the temperatures to which they've been
exposed and which, if any, native or contaminative microorganisms
survive the processing.

Milk has traditionally been soured by adding acids like vinegar or
lemon juice beyond any bacterial souring.

Pastorio