How does milk spoil?

In this brief guide, we are going to answer the question “how does milk spoil” with an in-depth analysis of how milk spoils. Moreover, we are going to discuss the shelf life of milk and ways to spot spoiled milk.

So without much ado, let’s dive in and figure out more about it.

How does milk spoil?

Milk is spoiled by an overpopulation of bacteria, which reduces the quality, flavor, and texture of milk. Milk should be stored at temperatures between 45 and 32 degrees Fahrenheit and milk degrades faster if the temperature fluctuates too much as bacteria such as lactococci and lactobacilli can grow in environments with frequent and rapid temperature changes. 

Now, lactose in milk degrades and bacteria use it to make lactic acid. Lactic acid acts on the most abundant protein casein and changes it into lactic casein (acid casein precipitated from milk by lactic acid) which can curdle milk within 24 hours. When the milk begins to curdle near the bottom of the jug, it is deemed spoilt and should be dumped immediately. Mold builds on the sweet butterfat as bacteria blooms, spoiling the milk even more.

It is worth mentioning that pasteurization is used in the vast majority of commercially produced milk and most of the dangerous kinds of bacteria known to cause foodborne illness, such as E. coli, Listeria, and Salmonella, are killed during this process. But pasteurization does not remove all forms of bacteria. 

Furthermore, once a carton of milk is opened, it is exposed to additional microorganisms from the environment. These little bacterial colonies can spread over time and potentially spoil your milk.

Can milk spoil?

Yes, like all other food items milk also spoils after a certain time, and afterward, it not only develops off-flavors and off-smell but its texture also changes, and in rare cases, you can also spot a mold or organic growth within your milk.

It is worth mentioning that you should not confuse the “best by” or “best before” date with the “expiration date” as the “best by” or “best before” date that is written on the carton of milk refers to the quality rather than safety so the milk doesn’t necessarily go spoil immediately after the best before date. 

This date refers to the time during which you can enjoy the peak quality and flavor of milk but you can still use milk that is past this date as long as it was stored properly, there is no leakage in the carton and no signs of mold are present.

What are different ways to spot spoiled milk?

You should consider the texture, smell, appearance, and taste of the milk to pass a verdict of whether or not it has spoiled.

  1. If the milk is curdled or has lumps present in it, then it is an indication of spoiled milk and you should get rid of it.
  1. If you smell something foul or sour while taking a sniff test then it is an indication that the milk has gone spoil and the best thing to do is to discard it.
  1. If you spot a mold or any other organic growth in your milk carton (it is a really rare phenomenon), then the best thing you can do is to get rid of this milk as it is an indication of spoiled milk.
  1. If you notice a sour or slightly bitter taste after taking a little sip of milk then it is an indication that the milk has spoiled. 

What are the health implications of drinking a large quantity of spoiled milk?

If you have consumed spoiled milk in a large amount then it can cause severe gastrointestinal issues including diarrhea and vomiting. In this scenario not only you should seek medical help but you should also drink plenty of water to compensate for the water loss that your body is facing owing to diarrhea and vomiting. 

If you do not drink plenty of water then it will result in electrolyte imbalance in your body that can cause dizziness, weakness and can even lead to shock in severe but rare cases.

How long does milk last?

UHT milk

Unopened UHT milk lasts for about 2-4 weeks past the printed date if stored in a cool, dry, and dark corner of the pantry away from direct sunlight and heat. Moreover, it lasts for about 1-2 months in the fridge. 

On the other hand, once opened UHT  milk lasts for about 7-10 days if stored in the fridge in an air-tight bottle.

If you accidentally left the milk out overnight, smell it to check whether it is still good to use or has curdled. 

Reduced-fat or lactose-free milk

Unopened reduced-fat milk, lactose-free milk, non-fat milk lasts for about 7-10 days past the date printed on them if stored in the fridge at or below 40 degrees Fahrenheit.

It is worth mentioning that these figures are just the estimated shelf life of the milk.


In this brief guide, we answered the question “how does milk spoil” with an in-depth analysis of how milk spoils. Moreover, we discussed the shelf life of milk and ways to spot spoiled milk.


Mahnoor Asghar is a Clinical Nutritionist with a bachelor's degree in Nutrition and Dietetics. She is compassionate and dedicated to playing her part in the well-being of the masses. She wants to play a fruitful role in creating nutrition and health-related awareness among the general public. Additionally, she has a keen eye for detail and loves to create content related to food, nutrition, health, and wellness.