Milk gets used for a lot of things in the kitchen, from cookie dunking to custards and silky-smooth sauces. Drinking milk right from the fridge with or without cookies is a pretty low-risk proposition as long as you give it a quick sniff test first. When you cook with it, though, milk can sometimes curdle without warning and leave you with a pan full of chunky milk. That's always a nasty surprise, but you can work with it.
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Curdling Can Happen in Different Ways
Curdled milk isn't appealing, but it's inconvenient rather than dangerous. If your milk is lumpy and curdled when it comes out of the refrigerator, for example, that's usually unwelcome. It means that naturally occurring bacteria have settled in the milk, happily digesting its sugars and converting them into mild acids. Those acids cause the milk's proteins to bond together chemically, which is why you get those lumps in your sour milk.
This isn't necessarily a bad thing. The milk itself is still food safe, and in fact, a lot of dairy products — yogurt, sour cream, kefir and some cheeses, for example — are made that way quite deliberately. Even accidentally soured milk is useful because it makes a fine substitute for buttermilk in recipes that are leavened with baking soda. The acidity of the milk reacts with the soda and makes your baked goods rise. When your milk curdles over heat, though, it's for a different reason.
Curdled Milk While Cooking
Even whole milk has relatively low quantities of fat, typically between 3 and 4 percent, and low-fat or fat-free milk has even less. The remainder consists mostly of water and proteins in the form of whey and casein. When you heat your milk gently for cocoa or custards, it won't usually get lumpy. At higher temperatures, though, and especially in the presence of acidic ingredients, the proteins in your milk will begin to stick together and form lumps.
This doesn't change the flavor of your milk the way souring does when bacteria set up housekeeping in your milk. In this case, your chunky milk tastes the same; it's just not as pretty to behold. That means you can still salvage your dish if you act quickly.
Saving A Curdled Dish
There are a few ways to save a dish if your milk "breaks" and curdles. The first is to just discard your sauce and start over, but that's wasteful and not always an option. It's frequently more practical to strain out or ladle out the sauce, fix it separately and add it back in.
The simplest way to do that is with a starch thickener. Whisk flour or cornstarch into a small saucepan of cold milk and bring it to a simmer. As it thickens, slowly whisk in your salvaged sauce. The thickener should keep it from breaking again, and small lumps won't be obvious in the finished sauce. You can also strain it again through a fine-mesh strainer to remove them if you want.
A second option is to use cream. Don't use light cream or cereal cream, or you'll have curdled cream to go with your curdled milk. Instead, heat heavy whipping cream in a saucepan until it thickens on its own. Slowly whisk your broken sauce into the concentrated cream, pausing and adding another splash of cream if it shows signs of not coming together. The heavy cream has enough fat to keep it from curdling in the heat unless it's getting old and slightly acidic in its own right.
Saving a Cheese Sauce
If you were making a cheese sauce, what you're seeing in your saucepan might be curdled cheese rather than curdled milk. The proteins and fats in cheese can separate if they're subjected to too much heat — picture the little pools of fat swimming on top of your last pizza — which also makes your sauce look gritty and unappealing. If this happens, you can mask the graininess of your sauce by forcing it through a fine strainer or "buzzing" it with a stick-type immersion blender and then adding more shredded cheese.
To avoid the problem in future, always thicken your cheese sauce before you actually add the cheese. Starch thickeners like flour and cornstarch require a relatively high heat to do their job, and it's the heat that makes your cheese curdle.
Instead, stir the sauce until the starch reaches its maximum thickness and then remove the pan from the burner before you stir in the cheese. There will still be plenty of heat left to make it melt, and you won't run the risk of turning your gooey, tasty cheese into grainy little lumps.