Today's "sour milk" is not the same as soured milk was when we were getting whole milk from our home dairy. We were able to use it to make the best sour-cream butter years ago, then what was left was perfectly delicious buttermilk and was used to make the best hot biscuits you could ask for.
Whatever the milk companies do to milk these days turns the milk into something totally unusable for anything if it goes the least bit sour. The idea behind keeping the containers in opaque plastic or even cloth bags is to avoid the least amount of light possible to reach the milk, and IT WORKS.
There are a few companies who have gotten the message and are using white plastic instead of clear now and that is probably working just fine. If you can't find those brands, then just keep your milk as "dark" as possible, and it'll remain sweet and good for a much longer period.
Always pour a little of the milk into a small cup to taste it and don't rely on the smell of the neck of the bottle, which sometimes takes on an early "sour" smell.
Source: Our son was an assistant store manager for the old Pantry Pride Grocery Stores, and a man who had previously worked for the Florida Milk Commission told him this tip. That was over 20 years ago, and I can't tell you how many other people we've shared that information with, and it always works. Being stored in the dark and good refrigeration is the key.
By Julia from Boca Raton, FL