How does milk spoil?

In this brief guide, we are going to answer the question “how does milk spoil?”. Moreover, we are going to discuss the shelf life of milk, ways to spot spoiled milk and the health consequences of drinking spoiled milk.

How does milk spoil?

Milk is spoiled by an overpopulation of bacteria, which reduces the quality, flavor, and texture of milk (1). 

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 (2). 

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 (1). 

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 (3).

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 (4). 

But pasteurization does not remove all forms of bacteria (4). 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.

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 (5).

Drinking a large quantity of spoiled milk can therefore have different health implications, including foodborne illness from pathogenic bacteria (6), gastrointestinal issues (7), allergic reactions (8), and potential toxin exposure (7). 

What should you do if you suspect you have eaten spoiled milk?

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 (9). 

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 (10).

How long does milk last?

The shelf life of milk depends on factors such as temperature, packaging, processing method, fat content, and microbial contamination (11).

Thus, unopened UHT milk can last about 2-4 weeks past the printed date in a cool, dry, dark pantry, and 1-2 months in the refrigerator. Once opened, UHT milk typically lasts for 7-10 days when stored in the fridge in an airtight container.

While, reduced-fat, lactose-free, and non-fat milk, when unopened, can last for about 7-10 days in the fridge at or below 40 degrees Fahrenheit. 

It is very important to note that these are estimated shelf lifes, and the milk’s quality and freshness should be assessed by smelling it or checking for signs of curdling as previously mentioned.

How to properly handle milk to avoid spoilage?

To properly handle your milk and avoid its spoilage, yous can follow the next recommendations:

  • Temperature control: Keep milk refrigerated at or below 40 degrees Fahrenheit (4 °C) to slow down bacterial growth and extend shelf life.
  • Proper storage: Store milk in its original container, tightly sealed, to maintain its freshness. Avoid exposing it to light or heat, which can degrade its quality. Keep it away from strong-smelling foods as milk can absorb odors.
  • Hygienic handling: Practice good hygiene by washing your hands before handling milk and using clean utensils and containers. Avoid cross-contamination by not pouring leftover milk back into the original container.
  • Check expiration dates: Be aware of the expiration dates on the milk packaging and consume it before the indicated date for optimal quality. 

Different types of milk, such as UHT milk, reduced-fat milk, lactose-free milk, and non-fat milk, may have varying shelf lives, so refer to the product label for specific guidelines.

  • For short-term storage, refrigeration is essential, while for long-term storage, freezing is an option. However, freezing may affect the texture of milk, so it is best to consume it within a few months for the best quality.

Remember to always discard milk that appears spoiled to ensure your safety.


In this brief guide, we answered the question “how does milk spoil?”. Moreover, we discussed the shelf life of milk,ways to spot spoiled milk and the health consequences of drinking spoiled milk.


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