What should I use for old recipes that call for 'buttermilk'?

Given the variabilities in "buttermilk" from place to place and time to time, you should get sufficiently equivalent results by substituting modern cultured buttermilk. That's the job it was designed to do.

Recipes from the early 19th century and before are notoriously vague. They were generally written more as reminders of something you already knew through experience, rather than detailed instructions for creating it from scratch. Quantities and temperatures were much harder to control, and so recipes basically assumed you'd recognize a dough with enough liquid or a sufficiently-roasted quail. (One of my favorite instructions from an 18th century cookbook: "Cook until it is enough".)

Given those wide margins, you should find that simply using cultured buttermilk will make the recipe work. It's true that it will lack the tiny bits of butter present in "true" buttermilk, but there isn't enough to have a radical effect on the result. The really important parts (dairy protein, water, acidity) are present, in about the same quantities they would be in true buttermilk.

You'd certainly notice the difference if you were to just drink it straight. If that's what you want, you're just going to have to find somebody who makes it, or do it yourself. I've found it's often available at the kinds of farmers markets that insist on local producers. Personally, I find it nasty, but YMMV.

So if you're trying to revive an old recipe, just start with commercial buttermilk. You're going to have to tweak it from there anyway. If it needs richness, add butter, but probably only a teaspoon per cup.

In the end, it seems that what the usage is, determines the product being called for.

I found an interesting Slate article about buttermilk. Apparently, over the years, the word "buttermilk" has referred to three different products:

So, prior to the 20th century, buttermilk could refer to at least three different categories of beverage: regular old milk that had gone sour; the sour byproduct of churning sour milk or cream into butter; and the “sweet” byproduct of churning fresh milk or cream into butter.

The reason for the discrepancy is that butter is made in two ways - with fresh, sweet cream and with leftover, slightly soured milk due to lack of refrigeration.

The confusion surrounding this drink dates back to the 18th century or before. Until the age of refrigeration, milk soured quickly in the kitchen, and most butter ended up being made from the slightly spoiled stuff. As a result, some historical sources use the word buttermilk in the Laura Ingalls Wilder sense, to describe the byproduct of butter-making; others use it to describe butter-making's standard ingredient at the time—milk that had gone sour from sitting around too long. To make matters more confusing, the butter-byproduct kind of buttermilk could be either “sour,” if you started out with the off milk that was itself sometimes called buttermilk, or “sweet,” if you started out with fresh cream (like Laura’s mom did).

It further posits that any baking recipe after the late 1800s is calling for sour buttermilk, so it's necessary to have something with acid content so that it can react with baking soda. Today's cultured buttermilk should work fine for that.

By the late 1800s, buttermilk had taken on a more specific meaning and usage in the kitchen. Cookbooks started calling for the sour version of buttermilk in recipes for bread made with baking soda. Church & Co., the company that would later create the ubiquitous Arm & Hammer label, first started processing and packaging sodium bicarbonate as baking soda in 1846. The new product was more reliable and faster to use than yeast, but it wouldn’t work unless mixed with an acid. In the 1860s, Church & Co. began distributing instructions for making baking-soda cornbread, biscuits, muffins, pancakes, and waffles—and it recommended the use of sour milk as an activator. “The farmer’s wife has always an acid free to her hands in the shape of sour milk or buttermilk, which can be used both as an acid to neutralize the Soda or Saleratus [an old-fashioned word for baking soda], also as a means of wetting the dough,” stated a 1900 edition of the booklet. (“Sour milk” and “buttermilk” may be meant as synonyms here.)

If the recipe doesn't call for baking soda, and it seems that a sweet flavor would be more likely, you'll probably want something more like fresh buttermilk - it's most similar to low fat or skim milk. While, yes, you can certainly make this at home by leaving cream in your stand mixer for 15 minutes or so, if that's not an option, you're most likely to find it at local dairies or at a farmer's market, though be sure that they're selling actual sweet buttermilk and not their own cultured product.

Butter-byproduct buttermilk, meanwhile, remains mostly the province of small farmers and DIYers. Large butter manufacturers now dry their butter byproducts and sell them to processed-food manufacturers as means of adding body and texture. (If you’ve ever eaten ice cream or a candy bar with “buttermilk solids” on its ingredients list, you’ve consumed the byproduct of butter.) In other words, the “good, fresh buttermilk” I’d read about as a child isn’t exactly easy to get your hands on.

And, as a test, the author used her homemade buttermilk in a recipe for biscuits:

Further, my homemade buttermilk didn’t reveal any special attributes when I tried to bake with it: A batch of buttermilk biscuits made using the DIY stuff (sans baking soda, since there wasn’t any acidity in the liquid) were indistinguishable from any biscuit made with regular milk: dry, crumbly, ho-hum.

My mother and father were both raised on farms in the early 1900's. They did not use soured milk to make butter. They used fresh milk that was neither homogenized or pasteurized.

I have had fresh churned butter and the remaining buttermilk on family farms when I was a bit younger. There was no sour taste. In fact the buttermilk was quite sweet.

The cultured buttermilk sold today is absolutely nothing like it. So, I would have to say that skim milk would be the closest in flavor to the buttermilk that came from our family members' farms.

That said, I know that people in different regions may have done things differently. So, I am not saying that this was the only way things were done, just that this is the way it was done where my parents were from.

To substitute for buttermilk, mix one cup of regular milk with 1 tablespoon of white vinegar or lemon juice. Stir; let the mixture stand for approximately five minutes. Modify proportions appropriately for more or less "buttermilk substitute."

I have used this replacement and it works perfectly.

Buttermilk is usually up to 50% lactose, hence the sweet stickiness. Any manufactured/packet equivalent is not really equivalent, so don't bother. Try making butter from fresh unpasteurised grass-fed milk, it'll be genuine. I worked for 9 years in a large dairy factory. Got sick of the smell of milk!