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Why Does Milk Curdle ? Here’s What We Found Out

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Were you expecting fresh milk from that carton, instead of a stinky, lumpy mess ? Well, curdled milk is really not a pleasant sight or smell, especially when it’s first thing in the morning and you haven’t even had your coffee yet.

This is a natural process that happens even when we take the most care possible. Sometimes milk that’s been kept only in the fridge can curdle, because it’s been left too long.

But why does milk curdle, what’s the process behind it ? Today we’re going to find out, and explain everything we know.

milk curdle

Why does milk curdle ?

Milk curdles because the pH level of the whey drops, turning it acidic. This breaks up the natural structure of the milk, forcing the proteins to clump together and the fats to separate.

This can happen two ways. First is when the milk is left at room temperature for a couple of days, and the bacteria naturally present in milk eat the lactose, and turn it into lactic acid. The second is when an acidic ingredient is added to the milk, like lemon juice, vinegar, or rennet.

In some cases curdling is just what we’re looking for, like when making cheese. In other cases, like when making sauces or using milk as a creamer it’s not great.

Now let’s explain the whole protein and lactic acid process in more detail, just to make things clearer.

Casein and lactic acid

Milk is a suspension, meaning every molecule in it repels each other, does not sink to the bottom, and does not dissolve into the liquid. Those molecules are protein, fat, lactose (a type of natural sugar), and the whey they all float in.

As you know, raw milk has natural bacteria already in it, it’s just the way it is. Bacteria spreads and grows at warm temperatures, and even room temperature is enough for a couple of days. As those bacteria eat up the lactose, they release lactic acid.

That acid breaks down the suspension, and the casein (protein) clumps together, forming the curds. The leftover liquid is whey, it has barely any protein left and nearly zero fat.

Now, this process can be greatly sped up by warming the milk a little, just enough to help bacteria spread but not hot enough to kill them. Or you could add an acidic liquid like vinegar, lemon juice, fig juice, or even rennet to curdle milk much faster.

It’s that lactic acid that gives yogurt and sour cream their tang. In cheese the whey (containing the acid) is removed and sometimes the curds are even rinsed. That’s a washed curd cheese, and it’s always going to have a sweeter, nutty note.

Read also: Why Does Milk Help With Spice ? 

Milk curds are useful

When milk curdles it can be very useful. Even if it wasn’t what you wanted for that carton of milk, you can still make use of it.

Allow it to curdle completely, it’s best to transfer everything to a clear jar or bowl to see when it’s done curdling.  Once that’s done, you can make your very own cheese !

milk curdle uses

You just need to gather all the curds together, and press them. You need a cheesecloth for this, to strain the curds and then keep them tied and pressed together.

Make sure you remove as much whey as possible, and maybe sprinkle in a bit of salt. Keep the curds tightly wrapped, and allow them to dry out. Several hours should be enough.

Depending on how tightly you packed the cheesecloth, when you open it the curds might be crumbly like cottage cheese, or they could be tightly packed. To be sure they’re tightly packed, sandwich them between two heavy objects.

Read also: Milk Boil Temperature 

Is it safe to eat curdled milk ?

Curdled milk can cause an upset stomach, and may send you straight to the bathroom. It’s acidic, tastes and smells like dirty socks, and has one of the worst textures possible.

We’re talking about milk that’s been left out too long, without the intent of making cheese or yogurt. This milk ends up stinking the whole house and it’s no good at all.

Milk that’s intentionally curdled doesn’t end up smelling, since the proteins haven’t gone off just yet. Yup, that’s the smell of rotten milk. Proteins breaking down in the lactic acid and smelling like, well, rotting meat (in a light form).

So no, don’t eat curdled milk. Always be sure to smell the milk before you confidently pour it into your coffee. If you’re not very sure, give it the smallest taste test.

Can you fix curdled milk ?

Unfortunately there’s no fixing curdled milk. Once the proteins have bound together there’s no real separation possible. What you can try is to break with the proteins, and add a little more fat to the mixture, if we’re talking about a sauce.

Get your blender, add the sauce, add a but of butter or oil or whatever you like, and blend it for a few seconds. This should be a new emulsion, and keep together better. If it’s still not good, try a thickening agent like flour or cornstarch. Add a bit more roux or starch, blend and heat it very gently in the pan.

As for the milk itself, like milk in a carton or chocolate milk, there’s no going back. We’re sorry.

How to keep milk from curdling

The best way to keep milk form curdling is to keep it away form heat sources, keep it in an airtight container that is also sterile, and don’t keep it open for more than 3-4 days.

Bacteria multiplies and spreads at room temperature, so it’s best to keep milk (raw or pasteurized) in the fridge at all times. Never leave milk on the counter. Conversely, never freeze milk or you’ll get the proteins and fat and whey separated.

If your carton of milk is already opened, don’t keep it for more than 3-4 days. And never drink straight from the carton, no matter hot convenient it is. When you do that, you add more bacteria to the milk and speed up the curling process.

And that’s about it when it come to milk and why it curdles. Milk is really an amazing product and it’s a shame to let it go to waste, so always remember to keep it in the fridge. Even the unopened cartons !

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