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Why Does Milk Curdle in Coffee Sometimes? And How to Prevent Curdling

Have you ever tried making coffee, adding milk, and ended up with a funny-looking caffeine fix that you wonder if it is still good to drink?

One of the reasons why your coffee looks weird is curdling. It happens when milk separates from coffee, which creates lumps.

Read on and find out some of the most common reasons why milk curdles when you add it to coffee. I will also share some tips on how you can avoid such.

Why Does Milk Curdle in Coffee? 

pouring milk in a cup of black coffee

Knowledge is power, even when it comes to your coffee. So, the next time you see milk curdling in the cup you have just made, ask yourself – why does it happen? Below are some of the reasons that you should be looking at.


One of the most common culprits is the acidity of your coffee. Even if your milk is non-acidic, coffee has acid, so curdling is still possible. Combining milk and coffee results in what is known as protein denaturation, which is what causes the curdling. The more acidic your coffee, the higher the likelihood of curdling.


curdled milk in a coffee

Aside from acid, the temperature is another culprit for the curdling of milk in coffee. If your caffeine fix is too hot and you immediately pour milk, expect that there will be lumps. This is especially the case when you are using plant-based milk, such as almond, coconut, soy, and oat.

Expired Milk 

Once the milk has gone bad, it can also be a suspect when it curdles in your coffee. The overall structure of milk changes at its expiration, which can contribute to curdling. It is not as common as acidity and temperature since it is easy for most people to identify expired milk before putting it in their coffee.

How to Prevent Milk From Curdling in Coffee

While curdling milk happens a lot of times, you can do something about it. Most of the solutions are straightforward, including the following:

Use Low-Acid Coffee 

woman holding coffee mug at home

One of the best solutions is to switch to low-acid coffee. As mentioned earlier, acidity can trigger the curdling of milk, so you might want to be wiser with your choice of coffee.

As the name implies, low acid coffee has lower acidity compared to regular coffee, which makes it great for people who are prone to having an upset stomach. This is effective in preventing issues like bloating, acid reflux, and heartburn.

To avoid curdling, you should not use coffee made using Arabica beans or those that have a light roast, since they are traditionally high in acid.

Instead, you should go for dark roasts. They have fewer compounds that can trigger your stomach cells to create more acid. Espresso beans are also great, which are brewed for a short time to lessen the acid that gets in your coffee. You cannot also go wrong with cold brew coffee.

Change Your Milk 

holding a glass of expired milk

While it is more common that the coffee is the culprit when there is curdling, you can also blame the milk that you are using. Especially if it has gone bad, there is a high chance that you will be seeing lumps.

Almond and soy are two of the most common types of milk that are prone to curdling. You might want to experiment with other non-dairy alternatives if you are on a plant-based diet. For instance, oat milk is a great option.

Many companies are also selling barista editions of their milk. They are formulated to be more stable even at a high temperature, so you don’t have to worry about curdling.

Adjust the Heat 

Heat is another major contributor to the curdling of milk. Hence, you might want to change the level of heat in your coffee to prevent milk from clumping.

Most people will recommend that the best temperature of coffee ranges from 195 to 205 degrees Fahrenheit. You can lower it up to 104 degrees Fahrenheit if curdling is an issue.

Make sure to drink your coffee immediately so that you can still enjoy it hot despite brewing it at a lower temperature.

Pour the Milk First 

Preventing the milk from curdling requires changes in the preparation of your caffeine fix. Some purists might disagree but by adding milk first before the coffee, curdling is less likely to happen.

By adding milk first, you are helping to temper such, which will also give it more time to adjust to the temperature of the coffee.

Dilute Your Coffee 

Many purists will probably not like this idea, but it is worth a try if you are in desperate need of a solution to curdling milk. When coffee is too acidic, there is a higher chance that milk will curdle. So, by adding more water, you are minimizing its acidity.

Is Curdled Milk in Coffee Bad for You? 

First, you must differentiate between intentional and unintentional curdling. Intentional curdling often happens as a procedure for making milk-based products, such as yogurt and cheese. Some recipes might also call for intentionally curdled milk.

On the other hand, unintentional curdling often happens when you leave milk for a long time and it has gone bad.

If milk curdles after putting it in coffee, this is fine. It is still safe to drink since it is most likely a reaction from the acidity of coffee or the temperature of water you are using. It might not look appetizing, but you do not necessarily have to put your cup to waste.

However, when milk curdles before it gets in your coffee, then it can be bad for you. This could be a sign that milk has gone bad. To be sure, check for any discoloration or foul smell. The safest bet is to not use your milk.


Heat and acidity are two of the most common reasons why milk curdles in coffee. Such is especially true when you are using plant-based milk, such as almond and soy. It could also be because the milk has already gone bad, which is why it curdles.

Krista Haws

Known among her friends as 'the caffeine fiend', Krista loves all things coffee. From an extremely short, strong espresso to a 3 day cold brew, Krista loves them all.

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