When I made butter a couple weeks ago the byproduct was "churn buttermilk" which I found to be good, but jmaynard did not as he prefers cultured buttermilk. While cultured buttermilk is available around here, it's the low fat version, which starts with skim milk. That has been something of a complaint of Jay's for a while, that the buttermilk, especially one brand, is, he says, "wimpy."
Last year, when I was experimenting with homemade soda-pop, it started partly because of this page and I remembered the page about making buttermilk. This was not the byproduct of butter production, but the
infecting inoculating of milk with specific bacteria, thus the use of the word cultured. It seemed simple enough: Get a bit of fairly fresh cultured buttermilk, pour a cup or so into a clean jar, fill with milk, seal, shake, let sit in a warm room for a day or so, and it should be ready but for refrigeration.
A couple days ago I cleaned up a couple quart Mason jars, bought a half pint of cultured buttermilk with the expiration date the farthest out I could find (best chance of live and lively bacteria), and a half gallon of whole milk (Oddly, I didn't see any brand calling it "Whole milk" as every brand said it was "Vitamin D milk" - but isn't vitamin D added to skim and 2% as well?). I followed the directions and started a jar going.
About 24 hours later it looked like it had clabbered (turned thick) to the point it looked lumpy to me. I sampled it and it tasted like cultured buttermilk to me (a unique but not pleasing taste, IMO) and so chilled it. Jay sampled it and said it tasted right, then said something about seeing if he'd get sick in a while. An understandable concern as leaving milk out intentionally warm has a certain weirdness to it to someone used to it being cold almost all the time. Later I knew things has worked as Jay had more of the stuff. I've started another jar going.
Once chilled, the whole milk derived buttermilk is quite thick indeed and seems reluctant to leave the side of the jar. I haven't heard any complaints about it being wimpy. It still seems if not lumpy, very nearly so, to me.