Turns out that even though real buttermilk is fat-free or very low-fat, cultured buttermilk can come with different levels of fat in it.
Buttermilk was originally the liquid that was left over after butter was churned from cream. Since the fat was churned into butter there shouldn't have been much left over in the liquid. The process of letting the milk sit to separate the cream resulted in some fermentation in the liquid, which gives the buttermilk its sour tang.
When they make cultured buttermilk acids are added to regular milk to give it the fermentation and the sour flavor. Those acids are useful in baking because they react with baking powders and act as a leavener.
It appears, from searching online baking forums, that different types of cultured buttermilk are more readily available in different parts of the country. Some people have trouble finding fat-free, while other people can't get full-fat versions. I think that I'd only seen fat free, so when we were talking about the full-fat version I didn't even know they made that! Turns out that the people I was talking about it with didn't know that there was fat-free buttermilk, but they live in different parts of the country, so all they'd seen was the full-fat.
I started looking around for it, and I noticed that sometimes the stores that I go to did have both types, and sometimes they only had fat-free. Today I saw the rare trifecta, all three types in one place.
So watch out when you buy your buttermilk, since the fat level can affect how your recipes work. You might want to adjust things if you've been using full-fat and you suddenly find that all you can find is the lower-fat versions. Just be aware of it....
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.com and www.acaketoremember.etsy.com