Why Is Oat Milk Curdling In Coffee & How to Prevent It

Oat milk is one of the best dairy milk alternatives for coffee. Oat milk taste and consistency are close to cow’s milk, and you can even froth it. But sometimes, you may notice your oat milk curdles as you add it to your hot coffee.

Why does that happen? And can you still drink the brew? Read on and find out what causes oat milk curdling in coffee — and how you can stop it.

Why is oat milk curdling in coffee?

The most common reason for curling oat milk is that it reacts with the acetic acid in coffee. High heat or long storage could also make oat milk curdle.

Just like cow milk, all plant-based milk — almond, soy, or oat milk — can curdle. The curdling process is the same for all of them. The only difference is how easily it happens.

Curdling occurs when coffee reacts with milk and decreases its pH. This reaction turns the milk more acidic.

The free-floating fats and protein molecules attract each other and separate from the milk’s water content, breaking its emulsion. The fats and proteins form clumps, resulting in curdled milk.

It’s relatively rare for dairy or non-dairy milk to curdle. When it does happen, it’s due to specific conditions. Coffee is — unfortunately — primed to have those conditions.

There are several reasons why oat milk curdles in your coffee. This is related to some science, but don’t worry — it’s nothing you wouldn’t have covered in your middle school chemistry class!

frothing oat milk

Coffee acidity

Coffee has a naturally rather high acetic acid content. Its average pH sits between 4 and 5.5, depending on the coffee variety and the degree of roasting. To put this in perspective, orange juice has a pH of around 3-4, while canned sodas have a pH of 2.5.

Coffee isn’t the most acidic drink out there, but its acidity level is higher than that of oat milk (although it is slightly acidic). When you pour oat milk into coffee, the acidic environment will cause its pH to drop.

The low pH will cause a chemical reaction that results in curdling oat milk.


Heat is another common factor that can result in unwanted curdles. The higher the temperature of the coffee is, the more likely you’ll get curdled oat milk. But why is that?

As basic science tells us, higher temperatures make molecules move faster. The increased speed, in turn, makes it easier for chemical reactions to occur. And what is curdling, if not a poor chemical reaction?

Pouring oat milk into hot coffee can result in curdles. Pre-heating or steaming the oat milk makes it even more likely to happen.

For this reason, you should always ensure you don’t get your oat milk too hot. The cut-off point stands at about 158°F (70°C) — go any hotter, and you’ll likely end up with curdled oat milk.

Old milk

The oat milk has likely expired if you see clumps in the carton or notice an unpleasant smell. You should never drink expired dairy or plant-based milk, so go ahead and chuck the curdled mess away.

But even un-expired oat milk can coagulate. Old milk is much more likely to curdle, even if the date on the carton says it’s still perfectly fine.

Oat milk begins to build up lactic acid-producing bacteria as it ages. Although it might still be good to drink, that increased acidity can push the milk past the curdling point when you add it to the acidic coffee.

You should always use fresh oat milk — or any other plant-based milk, for that matter.

fresh oat milk

Can your drink coffee with curdled oat milk?

It is generally safe to drink coffee with curdled oat milk. But you should avoid drinking oat milk that has curdled because of expiration.

Curdling is perfectly normal for both dairy and plant milk. You probably eat curdled milk regularly as cheese! You can safely drink oat milk that has curdled.

Having curdled oat milk in coffee will not affect its taste. That said, oat milk curdle doesn’t look attractive and may introduce an unpleasant texture to your coffee.

You shouldn’t drink curdled oat milk that has expired, though. Consuming spoiled oat milk could upset your stomach or cause other health issues since it likely has bacteria. If you notice unintentional curdling, get rid of the carton.

woman looking at coffee cup

10 ways to prevent oat milk from curdling in your coffee

Although there’s no harm in drinking coffee with curdled oat milk, it may not be an enjoyable experience. Fortunately, there are ways to avoid it. Here are some good tips on how to stop oat milk curdling.

1. Add oat milk into your cup first

Adding cold milk directly into the coffee can curdle it. To mitigate this effect, you can pour milk into your cup first before adding hot coffee.

Get the desired amount of oat milk while your coffee brews. This way it can warm up a bit before adding the hot coffee, which makes curdling less likely.

Once your coffee is ready, slowly pour it into your cup. The high coffee temperature will temper the oat milk. The gradual heat increase can do a lot to prevent unwanted curdles.

2. Heat the oat milk to 140°F (60°C)

Heating it to 140°F (60°C) or slightly above before brewing can stop oat milk curdling. This way, you can temper the oat milk — like slowly pouring coffee over it — and the less drastic temperature difference between milk and coffee further prevents curdling.

You can warm oat milk on the stove or in a microwave. If you make a latte, cappuccino, or other milk-based coffee drink, you’ll steam or froth oat milk, increasing its temperature.

frothing oat milk at home

3. Avoid overheating

If you warm your oat milk before making coffee, don’t overheat it. Hot oat milk may separate on its own simply from the high temperature. Homemade oat milk can also become slimy when heated.

To be safe, never heat oat milk past 158°F (70°C). Use a milk frother with a thermometer to control the oat milk temperature.

4. Brew with lower temperature water

Yet another way to keep your coffee mug’s heat in check is to brew your coffee using slightly cooler water. Lowering the water temperature even by a few degrees can help avoid curdles in your coffee.

This method is mostly suitable for pour-over coffee, as home espresso machines or capsule coffee makers such as Keurig or Nespresso don’t have temperature adjustment functionality. However, bear in mind that a lower water temperature will bring out more acidity in your coffee and can even make it unpleasantly sour.

Check the optimal water temperature for the brewing method you use before experimenting with your coffee. You will need a thermometer to ensure you are not going too far off the recommended range. 

measuring water temperature in gooseneck kettle

5. Use low-acidity coffee beans

The acidity level of your coffee beans is the most significant factor affecting curdling. Use low-acidity coffee beans to avoid oat milk curdle.

Specialty coffee roasters may have low-acidity bean varieties. But if you are out of luck finding them, there are other ways to ensure you get less acidic coffee.

Pay attention to the origin of your coffee. Coffee beans grown at lower altitudes with plenty of rainfall are likely to have lower acidity than those from arid, high-elevation coffee sources.

Finally, opt for darker roasted coffee. Light and medium roasts yield more acidic fruity flavors, whereas dark roasts have lost much of their acidity, making them less likely to cause curdling in coffee.

6. Buy oat milk formulated for coffee

Many commercial oat milk manufacturers know its tendency to curdle. Some brands offer barista edition oat milk specifically created for coffee. Various additives and heat treatments make these brands less likely to curdle.

Oatly, Minor Figures, Pacific Foods, and Califia are just a few examples of oat milk brands suitable for heating and frothing. They may be a bit more expensive than the usual brands in your local store, but making coffee without worrying about curdling might be worth it.

carton of oatly milk and coffee

7. Use fresh oat milk

Don’t keep your oat milk refrigerated for a long time, as its acidity increases and makes it more likely to curdle. You should use the freshest oat milk possible.

After opening an oat milk carton, consume it within one week. But even then, you may notice some curdling towards the end of the seven days.

You might think freshly homemade oat milk is the best for coffee, but that’s not entirely true. Although the oat milk will be fresh, it may curdle in heat more easily. Manufacturers often add certain minerals and additives to oat milk that reduce curdling.

Additionally, you might be unable to filter your oat milk and industrial producers. Consequently, it may have some chaff and grist from the ground oats. That’s not harmful, but it may make your coffee less enjoyable.

8. Wash your coffee mug

If you make coffee with oat milk, you must clean your mugs or cups more thoroughly than usual. Coffee remnants and oils can be highly acidic, resulting in curdling.

To ensure your mugs are as clean as possible, use dish soap and a bit of baking soda so all traces of your previous drinks are gone. You can make oat milk coffee without stressing about any bad reaction.

washing coffee mug in the sink

9. Store your oat milk properly

You must store your oat milk properly. Put it away as soon as you are done using it, and keep the oat milk refrigerated according to the manufacturer’s recommendations at all times.

Letting your oat milk sit on your kitchen table at room temperature is like putting up a big “BACTERIA WELCOME” sign. You’ll only have high-acidity or straight-up spoiled milk tailor-made for unintentional curdling.

10. Buy calcium-enriched oat milk

You can try calcium-enriched oat milk if barista edition brands are unavailable at your supermarket. Although oat milk is rich in fiber and minerals, producers often fortify it further. Calcium-enriched dairy-free milk isn’t usually much more expensive than non-fortified ones, but it can significantly reduce curdling.

Calcium is alkaline and neutralizes acids when it comes into contact with them. Therefore, calcium-enriched oat milk can lower coffee acidity, preventing it from curdling.

Low in carbs and with great flavor, oat milk is a wonderful lactose-free alternative to dairy products, whether skim, whole milk, or even cream. Now that you know multiple methods for preventing oat milk curdles, you can comfortably find out what oats can offer for your morning cup.

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