What Happens When Milk Is Left At Room Temperature? (Detailed Answer)

Milk spoils when left at room temperature for a long time. Here is a detailed answer to the changes that happen in milk when not refrigerated.


Chemical or bacterial reaction

When milk is left at room temperature, a bacterial reaction occurs in the milk and turns lactose into lactic acid.

In general, lactose is fermented by bacteria at room temperature and transforms it into lactic acid. 

When there is a high amount of acid in the milk, there will be high amounts of hydrogen ions, which will turn the taste sour with low pH value. 

The structure of proteins such as casein will change when there is low PH, and this will cause the milk to curdle and form clumps. 

Whey is a yellowish liquid that will be left behind after the bacterial reaction occurred. There might still be small traces of lactobacilli left in milk even after it is pasteurized.  

Change in taste

When milk is left at room temperature for a certain period, it will begin to sour, and this is the way it will taste if you try to consume it.

Fresh milk will have a creamy and sweet taste after it has been processed, but when bacteria act on it and cause it to spoil, the lactic acid produced brings a sour taste.

Change in smell

Milk also has a unique smell that is nice when it is fresh. 

If it is left at room temperature, certain microbes can be in the milk and multiply in numbers. 

Milk proteins will then be broken down by bacteria, resulting in the formation of smelly byproducts. 

Also, the butterfat can be broken down, and this will have a bad smell. You can use the smell to know if milk is spoilt or not because milk will have a very unpleasant smell when it is spoiled.

Getting spoiled

At warmer temperature-time such as in the summer, if milk is kept out of the fridge for a certain period (mostly around 1 hour and above), then there is room for bacteria to grow on it. 

Once you smell the milk, you can know if it is bad or not, it has a sour odor.  

Milk that has spoilt or has exceeded its expiring date may not be suitable for consumption wholly but can be used for other purposes, so if you don’t want to waste it, learn a few things you can do with spoilt milk. 

They are as follows.

  • You can use it to make cottage cheese
  • It can be used to make pancakes and waffles
  • It can be used to make yogurt

Change in looks or thickness of the milk

Milk becomes very thick when it spoils. 

The lactic acid creates a layer and forms a curd, this changes the texture of the milk, making it thicker, and it has small dots or lumps all over it. 

If you notice this on your milk, just know it has spoilt.  

What happens when milk is left at room temperature during summer?

If milk is left at room temperature during summer, an oxidation reaction occurs on the milk, and this will result in the growth of microorganisms, which will cause the milk to sour. 

When this occurs, you will be able to see cracks on the surface of the milk.  

Bacteria found in milk can survive even after pasteurization, but they will not perform when the temperature is below 7 degrees Celsius. 

However, when the temperature is around 20-35 degrees Celsius, this occurs mostly in summer, these microorganisms begin to reproduce and cause the milk to sour.