Why does cream curdle in coffee?

Coffee can be a bit of a mysterious drink. If you need any proof of this, simply Google "why is coffee..." and look at all the questions people have about it. One peculiar thing you might have noticed when pouring a cup of coffee is that sometimes the cream will instantly curdle. Other times, there's no curdle and it mixes smooth as silk. What gives?

This typically happens as a result of acid. As cream ages, lactic acids build up and it eventually curdles on its own. However, if you have a cup of coffee that's overly acidic, it can speed up the curdling process with older cream. The acid in the coffee tips the pH balance of the cream and results in this instant curdling effect (via The Eagle). The heat of the coffee only adds to the likelihood of creamer that isn't extra fresh curdling. In other words, when creamer that's been in the fridge for awhile is combined with acidic, super-hot coffee it will likely curdle.

The good thing is that drinking your coffee or tea after this happens is perfectly safe. (Nestle even goes so far as to call this coffee side-effect "beverage feathering.") Adding heat or acid to milk is actually called "intentional curdling" and is how cheese is made (via First For Women). It's the unintentional curdling of milk that's past its expiration date, or has been left out all day, that can make you sick.

Not even soy milk is immune to the curdling effect in hot and particularly acidic coffee. Although, according to Cooking Light, there is a way to possibly prevent this somewhat unsightly occurrence. Pouring plant-based milk in your mug before adding the hot coffee might help because it counterbalances the milk to coffee's temperature. Opting for a less acidic coffee can be another way of preventing your creamer from curdling. 

You always want to give your creamer a good sniff before pouring it into your cup of joe. If it smells like it's on the edge of heading south, toss it. But if it's perfectly fresh and it curdles in your coffee, there's no harm in drinking it.