Here's what I found - some interesting info here.
http://www.gourmetsleuth.com/recipe_sourcream.htmSour Cream
From the message board of CountryLife.net Here are two methods for making your own sour cream at home.
Method 1
I N G R E D I E N T S
1 cup cream
1 tablespoon cultured buttermilk
Recipe can be increased at the ratio of 1 tablespoon buttermilk to 1 cup of cream.
I N S T R U C T I O N S
In a double boiler bring the fresh cream up to 180 degrees. Cool to room temp in a cold water bath. Add the buttermilk, cover, and let sit at room temp. for 24-48 hours. Stir and refrigerate. The batch will keep approximately 3-4 weeks, refrigerated
Method 2
I N G R E D I E N T S
1 cup cream
1 1/2 cups pasteurized whole milk
1/2 cup buttermilk
I N S T R U C T I O N S
Mix all the ingredients in a bowl over warm water. Raise the temperature of the mixture to (68 degrees to 70 degrees F) and let it stand for 12 to 24 hours or until it is sufficiently sour and thick enough to cling firmly to a spoon. Keep in the refrigerator until you want to use it. For a richer heavier sour cream combine 2 cups of pasteurized heavy cream with 5 tablespoons of cultured buttermilk and incubate as before. For better texture refrigerate for 24 hours before serving.
Tips and Hints on Using Sour Cream
Sour cream is commonly used for dips, dressings, and sauces or simply "plain" as a condiment.
Never boil sour cream because it will curdle immediately. To add sour creme to a hot liquid, remove the liquid from the heat source (or turn the heat to very low) and add the cream while stirring gently.
Avoid using sour cream in dishes with a lot of salt, as the salt may cause curdling. Also dishes made with sour cream do not freeze well
Baking With Sour Cream
Cakes using acidic ingredients such as sour cream may development a metallic flavor if baked and stored in an aluminum pan. To prevent this reaction from taking place, line the bottom of the pan with parchment paper before adding the batter to the pan.
