Recipes treat these three as if they were interchangeable, and readers reasonably conclude they are just tangy dairy that makes things taste better.
They are doing two structural jobs, and flavor is a third thing on top. Understanding the two jobs tells you when a recipe genuinely needs one of them and when it does not, and why swapping in plain milk sometimes ruins a bake and sometimes does not matter at all.
Allergen note: all three are dairy ingredients. This page describes what they do mechanically. It offers no substitutions and makes no claim about any recipe being suitable for someone avoiding dairy.
Job one: the acid reacts with baking soda
This is the job that will break a recipe if you get it wrong, so it comes first.
Baking soda is a base and it does nothing on its own. It needs an acid to react with, and that reaction is what produces the gas that lifts the bake. If a recipe calls for baking soda, something in the ingredient list is supplying the acid, and buttermilk, yogurt and sour cream are three of the most common candidates. What that looks like when it goes wrong is on why a cake comes out dense and heavy, where a leavening failure is separated from the other causes.
So swapping an acidic dairy for plain milk in a soda-leavened recipe removes the acid the leavening needed. The soda has nothing to react with, less gas is produced, and the cake comes out flatter and heavier than it should. It can also leave an unpleasant soapy or metallic taste, because unreacted baking soda has a flavor of its own.
The reverse case matters too. If a recipe uses baking powder rather than soda, the powder already contains its own acid in dry form, so an acidic dairy is not needed for the leavening. In that recipe the acid is there for the other reasons below, and the consequences of changing it are about texture rather than about rise.
The first thing to check before any swap, therefore, is which leavener the recipe uses. That single question decides whether you are making a texture change or breaking the recipe.
Job two: acid weakens gluten, so the crumb gets softer
The second job is the one nobody mentions and it is why these ingredients turn up in recipes that are already using baking powder.
An acidic environment interferes with the gluten network. The protein strands do not link as strongly, so the structure that forms is looser and shorter, and a looser structure eats as more tender. That is the same lever as fat and sugar, arriving by a different route, and it is the reason an acidic batter tolerates a little more handling than a plain one before it goes tough.
This is why a sour cream cake reads as noticeably softer than the same cake made with milk even when the leavening is identical, and why buttermilk biscuits and pancakes have the texture they do.
Acid also affects how proteins set. Egg proteins and dairy proteins both behave differently in an acidic batter, which is part of why these ingredients produce a finer, more even crumb rather than merely a softer one.
Job three: flavor, and it is not just tang
The acid tastes of something, and it also changes how everything else tastes. A slight sourness makes sweetness read as less flat and gives a plain vanilla cake somewhere to go. It works the way salt does in a sweet recipe, which is one of the roles set out in what each ingredient in a cake is actually doing: a counterweight makes the main note legible.
Why the three are not actually interchangeable
They share the acid. They differ in the two other properties that matter, and both of those change a batter.
Fat content. Sour cream carries substantially more fat than buttermilk. Fat is a tenderizer and a flavor carrier, so swapping sour cream for buttermilk changes richness as well as acidity. Yogurts vary enormously depending on whether they are full fat, low fat or strained.
Thickness, which is really water content. Buttermilk pours. Sour cream and strained yogurt do not. Substituting one for another by volume changes the total liquid in the recipe, and total liquid is one of the main things deciding whether a batter behaves. A thick dairy used where a thin one was intended gives a stiffer batter, less spread and a drier result.
We publish no conversion ratio between them, and no thinning instruction. Fat and water content vary by brand and by country in ways a general figure hides, and a swap that works in one recipe fails in another depending on how much slack that recipe had. Substitution has its own treatment on this site rather than a paragraph here.
What we will not publish
No pH figures. The numbers in circulation for these products vary by brand, by batch and by how long the tub has been open, and a figure from a general page is not a specification for what is in your fridge.
No "make your own buttermilk" ratio. The lemon-juice-and-milk trick is widely repeated and it produces something acidic, which handles job one. It does not reproduce the thickness or the protein behavior of cultured buttermilk, so it does not fully handle job two. Saying that plainly is more useful than a ratio that implies equivalence.
The practical read
Before you change one of these in a recipe, ask three questions in order.
- Does the recipe use baking soda? If yes, the acid is load-bearing and removing it changes the rise. If the recipe uses only baking powder, the acid is doing texture and flavor work.
- How thick is what I am taking out, and how thick is what I am putting in? A difference here is a difference in total liquid.
- How much fat am I adding or removing? Sour cream to buttermilk is a significant reduction in fat and the crumb will show it.
If your bake came out heavier and flatter than it should have, and you changed one of these, question one is where to look. If it came out tough rather than heavy, question two and three are the more likely culprits, and the wider read is on why a cake turns out dense.