Got (Sour) Milk?

You scream, I scream, we all scream for sour cream!

Yogurt, sour cream, crème fraîche, and Mexican crema are all fermented dairy products. They are tangy, thick, and fatty. All of which makes them wonderful items to cook with.

To be better cooks, we first have to understand our ingredients. So let’s call the milkman, grab our microscopes, and pucker up.

How are sour dairy products made?

It starts with preservation, which milk will do all on its own. Milk and cream can spontaneously grow specific types of microbes that convert sugar to acid via fermentation. This process changes the taste and texture of the milk in amazing ways.

Can you make it happen at home? Yes! Here’s how:

1. Take milk or cream
2. Add bacteria to it—which can be in the form of a previous batch, buttermilk, or a starter culture
3. Let the milk sit—typically at room temperature—to let fermentation happen
4. Then wait anywhere from 4 to 24 hours

Learning this made me curious. Why don’t these dairy products make us sick? Ain’t nobody got time for old milk. 🤢 So here’s how it works. The bacteria turn lactose into lactic acid and other antibacterial substances during fermentation. The lactic acid prevents the growth of bad bacteria that can make us sick. And the antibacterial substances are what give sour milk products their tang. Pretty cool, right? 🤓

Types of Sour Dairy Products

They share a lot of similarities, but a few big differences that change flavor, texture, and how we use them.

Sour Cream

Sour cream has around 20% milk fat. And contrary to its name, it’s not that sour. It has a mild tanginess. Its acidity levels are at around 0.8%. While here in the US we put it on our baked potatoes, it actually came from central and eastern Europe where it was used as a condiment for soups and stews. Because of how acidity impacts the proteins, it’s also a secret ingredient in making baked goods more tender and moist.

Check out this article on how to make sour cream at home.

Yogurt

Whereas the others ferment at about room temperature, for yogurt, milk is actually heated up to a higher temperature. This concentrates the proteins and changes their structure so the yogurt ends up thick. Because of the high temperature, the fermentation process actually takes less time too—less than 6 hours usually. Yogurt also has higher acid levels—around 1–4%. One of my favorite ways to use yogurt is to turn it into a sauce. I thin it out with a touch of citrus juice or tahini so it’s pourable and more flavorful. Then I add citrus zest and grated garlic or ginger. It is the perfect sauce to finish roasted cauliflower, chicken breasts, and even hummus.

Check out this recipe on how to make yogurt at home.

Yogurt and sour cream are pretty good substitutes for each other. Just swap one part sour cream for one part yogurt.

Crème Fraîche

Crème fraîche is typically much higher in fat—around 30–40%—but lower in protein. You typically start with heavy cream. This is what also makes it the least tangy. It can range from 0.2–0.8% acidity, giving it a milder, creamier flavor than the others.

Crème fraîche also has a superpower. 🦸 But you’re gonna have to wait. More on that in a bit. Don’t have a cow 🐄. 😉

Mexican Crema

This one is simple. Mexican crema is just crème fraîche with lime juice and salt added. If crème fraîche is Captain America, Mexican crema is Captain America with a shield. 🛡 The combination makes it rich, tangy, and flavorful. It’s an incredible condiment for tacos, roasted potatoes, and spicy meats.

Check out this article on how to make crème fraîche and Mexican crema.

Cooking with Sour Dairy

The high protein levels in most of these products mean you have to worry about curdling. Those chunky, sand-like bits are not the most appetizing. Why does curdling happen? The combination of excessive heat and acidity causes whey proteins in the dairy products to denature and bind with casein proteins forming clumps.

That is why yogurt and sour creams are added at the last minute, off the heat, as a final topping. They enrich the flavor of your dish adding both creaminess and tang. But we don’t want to cook them. Crème fraîche is different.

Remember, I mentioned crème fraîche has a superpower? The extra fat in crème fraîche coats its proteins making it difficult for them to bind and clump together. Thus it won’t curdle. So what does that mean? You can add crème fraîche directly to dishes as you cook them. Add it to pureed vegetables for a creamy, luxurious soup. Use it as the dairy base in potato gratin for richness and tang. Incorporate it into a pan sauce as it finishes simmering for a velvety texture.

Crème fraîche is pretty amazing right? One might even call it legen-dairy. 🥁

Fun fact: The same reasoning applies to heavy cream vs milk. The cream has enough fat to prevent curdling. However, when adding cream to a dish it can be thinned out, eventually causing it to break. To reduce the chance of curdling, you can first reduce the cream to concentrate it. Or you can use crème fraîche. The fermentation makes it more stable than cream. And interestingly, it has a fresher, less rich flavor.

The tl;dr

Use Mexican crema for extra acidity. Use sour cream for a subtle tang. And yogurt for something in-between — especially if you want a thicker consistency. When you want more richness than acidity, crème fraîche is your friend. AND crème fraîche goes from friend to superhero when you cook with it. It doesn’t curdle under higher heat. The others do (well except Mexican crema since it’s basically all crème fraîche).  

Where I learned this: Mostly On Food and Cooking by Harold McGee, but also Christine Gallery at the Kitchn and Cooks Illustrated. And thanks to George, one of the readers, for the idea for this article!