Will milk go bad if it is left out overnight? 

In this brief guide, we’ll address the search query: “Will milk go bad if it is left out overnight?” Also, we’ll explore how milk should be stored, why milk is perishable, what the signs of milk being spoiled are, and what are the risks of drinking spoiled milk. 

Will milk go bad if it is left out overnight? 

Yes. Opened milk that is left out overnight will begin to go bad, and can potentially spoil, depending on the ambient temperature. Balmier temperatures will facilitate the growth of bacteria in the fluid, and other changes may occur, such as the fat content turning rancid. 

As a general rule, milk that requires refrigeration should not stay out for more than two hours. This is because, at room temperature, Lactobacilli present in milk may being to grow unchecked, and cause the milk to curdle. 

Most types of milk, unless they’re ultrapasteurized and opened milk, require refrigeration. 

Leaving them at room temperature can greatly reduce their shelf-life, or altogether trigger spoilage.

We advise our readers to be mindful of how they store their food, paying special attention to their more perishable products, such as milk and other foods with high moisture content.  

How should milk be stored? 

How milk should be stored will depend on its presentation. Namely, fresh milk that is procured in cartons from the refrigerated food aisle should be stored accordingly. 

Ultrapasteurized milk, which is often packaged in aluminum-lined cartons, can be stored at room temperature until it is opened, at which point, it should be stored in refrigeration. 

Powdered milk can be stored in a cool, dry pantry, minding that moisture does not build up or seep into the container. 

Alternatively, some types can be frozen, which is a convenient option for our readers who purchase in bulk. However, it bears mentioning that freezing and then defrosting can alter the consistency of milk, as separation of fat content may occur. Therefore, freezing is not necessarily recommended for milk with low-fat content. 

Why is milk so perishable? 

Milk is more perishable than other types of food, due to its moisture content, combined with its sugar (lactose) and fat content. 

Its moisture content facilitates the growth of microbes such as molds and bacteria, which will break down its lactose content into its two components; glucose and galactose. 

Additionally, unchecked lactobacillus can convert lactose into lactic acid, spoiling the taste of milk and making it sour. 

Additionally, its fat content is susceptible to balmy temperatures and may break down into rancid fatty acids that promote oxidation. 

What are the signs of milk being spoiled? 

Signs of milk being spoiled include having discoloration –such as turning a shade of green or yellow, giving off foul or otherwise questionable odors, having a bitter taste, and being curdled. Its taste will be unpleasant, and it should not be used in cooking or any other applications, as it will have lost most of its nutritional properties. 

Curdling occurs when dairy is subjected to low pH levels, and casein clumps together and separates from whey, which remains in a fluid state. 

Spoiled milk can also show signs of growth such as molds, and bacterial colonies and the dairy fat may have surged to the top, at which point it will have begun to turn rancid. 

Milk with any of these signs should be promptly discarded by being poured down the drain, or if necessary, thrown in the trash within a tight-sealing bag. 

What are the risks of drinking spoiled milk? 

The risks of drinking spoiled milk include suffering symptoms of food poisoning, though these may resolve themselves shortly after consuming spoiled milk. 

However, some individuals may be at heightened risk when suffering symptoms of food poisoning, such as pregnant women, the elderly, small children, and patients with compromised immune function. 

Symptoms may include vomiting, diarrhea, indigestion, stomach aches, and in more severe cases, dehydration, muscle aches, and headaches. 

We urge our readers to be mindful of the state their food is in, and avoid unnecessary symptoms, eschew any foods that show signs of spoilage. 


In this brief guide, we’ve addressed the search query: “Will milk go bad if it is left out overnight?” Also, we’ve explored how milk should be stored, why milk is perishable, what the signs of milk being spoiled are, and what are the risks of drinking spoiled milk. 



Leave a Comment