Sour cream from the grocery store is not naturally sour; it's a mass produced version of it and like so many other foods that are mass produced, it's prone to spoilage. Since it is prone to spoilage, government (FDA) guidelines must be followed and it must be 1) made from pasteurized milk only and 2) have specific bacteria introduced rather than allowing what is there naturally to reproduce.
To put it another one, it's been cooked which kills enzymes as well as naturally occurring bacteria, then it's inoculated with the "right" bacteria to sour it.
The real question that never seems to be asked is, "Does it work?" Maybe because the answer is "No." It does not work. Sour cream, no matter how it's made will eventually go bad and it will start to turn pinkish.
Never use it when it turns pink. That's the result of the life and death of a dangerous bacteria that was not killed when the cream was pasteurized. Sometimes there will appear a greenish gray mold in sour cream; don't use it when that occurs, either. This mold is the result of introduced bacteria or mold spores and was not (or was not supposed to be) there when you bought the sour cream.
If you want to make sour cream last past its expiration date, don't dip into it with a spoon that's been used for anything else and keep your fingers away from it, even if they're clean. Keep the edge of the container clean and keep the sour cream in the coldest part of the refrigerator.
If you need sour cream for a recipe and don't have it or just want to save money by not buying any, here's an easy and cheap fix:
Add a quarter cup of powdered milk and a tablespoon of melted butter to a cup of whole milk, or butter to whole milk, or a half cup of powdered milk and a tablespoon of melted butter
It may curdle; if it does, mix the curds back into it.
You can use this in any recipe that calls for sour cream.
to a cup of 2% milk. Add a tablespoon of vinegar, mix well and let it set for a few minutes.