Why Does Milk Curdle When Heated? Explained!

Last Updated on May 28, 2022 by

Why Does Milk Curdle When Heated? When you microwave pure milk at a lower temp or medium-high, it keeps coming out curdled, almost like the start of making cheese. So, why does milk curdle when heated?

Milk will curdle as a sign of spoilage. It could be that the boiling temperature is rising too quickly or the water type in your area is the culprit. Speaking of temperature, you want to measure what you need and allow it to come to room temp, then microwave it with little blasts at a time.

By the way, curdled milk is good for making ricotta cheese/paneer, so do not throw it away. Simply, boil the milk, and lemon for your paneer or cottage cheese. It can be used for varieties of Indian dishes—desserts and savory curries.

Why does milk curdle when heated?

Your heated milk will curdle for the following reasons:

  1. Acidity

Milk typically has about 6.6 pH when coming out fresh from the cow, a little more acidic than a neutral 7 pH. However, the electrical charge on each protein molecule is reversed when the pH drops to 5.0, causing the milk to curdle or coagulate.

As such, if you add acidic ingredients such as vinegar and lemon juice to milk, it separates for making ricotta cheese.

  1. Fermentation

You would naturally find lactobacillus bacteria (LAB bacteria) strains in raw milk. If you leave the milk raw, LAB bacteria will slowly and naturally ferment it. This could happen more quickly depending on how warmer the milk gets.

The fermentation also continues in the fridge, although slowly. You could stop the fermentation in 2 ways—pasteurizing and freezing.

Pasteurizing milk destroys all living organisms, both beneficial and harmful. However, freezing raw milk causes LAB bacteria to become inactive and preserved for a while.

The LAB bacteria continue to feed on lactose, and milk sugar, creating lactic acid as a byproduct. The higher the lactic acid in the milk, the more the taste gets sour and it also becomes more acidic. This means that natural raw milk fermentation reduces the pH of milk, making it more acidic.

  1. High temperature

When you heat milk so much that it gets to a bubbling boil, it breaks down lactose into a non-digestible sugar known as lactulose, among other compounds.

If you are lactose intolerant or making cheese, this can be okay. However, it is not okay if you intend to make pasteurized milk or hot chocolate.

pH drops, just as lactose drops during the boiling of milk. This is the reason there is a possibility of splitting when you heat milk. Heating increases the souring process.

The proteins, the solid clumps of curdled milk, may coagulate and separate from the water. It does not mean the milk is necessarily bad but that you are heating it too rambunctiously.

  1. Seasoning in the beginning

Salt will cause your milk to curdle. It does not mean you should avoid salt because you have to season the sauce.

The point is to add salt to the milk at the end, instead of cooking or reducing it with the salt already inside.

  1. Not tempering milk

When you add milk to hot liquid, this can cause it to curdle. As such, you want to temper the milk. Milk tempering means slowly bringing the temperature of the milk to heat to prevent it from curdling, breaking, or cooking too fast.

Simply, add small amounts of the hot liquid into the cold milk. After the milk gets warmer, add it to the hot liquid. Alternatively, you could heat the milk gently in your pan before adding it.

Is milk spoiled if curdles when heated?

Milk proteins may separate or coagulate from water. The solid clumps in the boiled milk are the proteins making the milk curdle. As such, curdled milk does not mean spoiled milk.

You have just heated the milk “too rambunctiously”, according to Miller’s Bio Farm, making it curdle, so not spoiled.

Curdled milk is not always bad. Nonetheless, milk curdling can be a sign of spoilage. It could also be used to produce more delicious food, such as cheese.

Besides, a simple chemical reaction caused the milk to curdle, which can be set into place for many reasons.

Sour milk can be used for baking. You may not have to drink spoiled milk but can use it for baking food. The additional milk acidity adds up as the milk ages and can introduce flavor to baked goods such as muffins and cakes.

If your milk is spoiled, you will get a distinct sour odor, due to lactic acid caused by bacteria.

Another sign of milk spoilage is that it gets a lumpy texture and a slightly yellow color. Thus, if the milk smells, changes in color, or has a lumpy texture, it is spoiled and may be unsafe for consumption. It is not advisable to taste milk to know if it is spoiled.

What to do if milk curdles while boiling?

You now know why milk curdles but how can you stop your milk from curdling when boiling it?

  1. Pasteurize

You can pasteurize milk within the first week. 5 days is the typical number of days to have the milk pasteurized.

Pasteurization keeps your milk from being too acidic and separating. When you boil the milk, it destroys every LAB bacteria and ensures that the pH of the milk remains relatively stable for reheating another time.

  1. Low heating

You want to heat the milk slowly instead of allowing it to boil. Bring the milk to heat gently at medium-low heat until it simmers, not a complete boil, to increase the pH and keep the lactose in place.

  1. Start with a small amount

You should start by heating a small amount of the milk if it is more than a week old. Make a test batch to help prevent wasting a whole gallon of raw milk you could use for baking or cooking.

Non curdled milk

For this section, we made picks of milk you should buy that will not curdle. Consider investing in the following:

  1. Silk Shelf-Stable Soy Milk, Original, Dairy-Free, Vegan, Non-GMO Project Verified, 1 Quart (Pack of 6)

We picked this silk shelf-stable soy milk because:

It is delicious

Add this soymilk to your pantry for its deliciousness and rich, creamy taste. It goes well whether you add it to coffee, pour it over cereal, or enjoy it alone.


We like that this is shelf-stable milk packed specially for convenience, non-refrigerated storage. Only put it in the fridge after opening it once.


Apart from being so deliciously wholesome, this milk does not contain any of the following:

  • Dairy
  • Lactose
  • Casein
  • Carrageenan
  • Gluten
  • Egg
  • Peanut
  • MSG


This milk is a nutritional powerhouse with 50% more calcium than your daily dairy milk. It contains soy protein but is low in saturated fat and has no cholesterol.

This non-GMO verified, vegan-friendly soy milk also uses no artificial colors or flavors, a decent healthy investment to make.

What we don’t like

We ordered this milk 8 times. However, the past 2 orders arrived damaged. Our tasters felt terrible but Amazon sent replacement packages within 3 days. Again, we received 3 ruptured cartons.

The problem here is that the box is too heavy and gets tossed around carelessly.

  1. Califia Farms – Oat Milk, Unsweetened Barista Blend, 32 Fl Oz (Pack of 6) | Shelf Stable | Non Dairy Milk | Creamer | Vegan | Plant Based | Gluten-Free | Non-GMO

Califia Farms’ milk is our second pick. We selected this milk for the following reasons:

Barista blend

We think this oat milk barista blend steams beautifully and pairs perfectly with coffee. It is also really delicious if consumed alone.


Produced from North American-grown whole rolled, gluten-free oats. Already ready to drink, this is unsweetened milk to buy.

This milk is free from or does not contain the following:

  • Gums or stabilizers
  • Nut
  • Allergen-free
  • Soy
  • GMO
  • Gluten

Perfect for shelf storage, and will not curdle when boiled.

What we don’t like

The flavor and nutritional values are what you want from milk. However, this milk does not froth well when heated. We made an exception for its price though.

Also, try this milk with a cold brew to unleash sweetness and flavor.

Is curdled milk safe to eat?

Curdled milk may be safe to eat. If your sauces and soups contain milk, simmering or boiling may result in curdling. Although curdled milk may be safe to eat, it is not particularly appetizing.

Milk may be curdled but is “far from useless“. But it is advisable to dispose of your milk if it is old, curdled, slimy, or grows mold.

The smell alone will be a complete turnoff. You may not even have to taste it since the foul smell is a deterrent in itself.

Suppose you manage to get past the unpleasant smell and taste. It is a bad idea to drink spoiled milk. Spoiled milk will cause food poisoning, leading to uncomfortable digestive symptoms, including nausea, stomach pain, diarrhea, and vomiting.

If you have accidentally ingested a small sip of spoiled milk, you have nothing to worry about. Consuming it in large or even moderate quantities is when the milk will react badly in your body. But if you can, stay away from even the slightest sip of it.