Will milk go bad if left out overnight? (one Tip)

In this brief guide, we will answer the question,’ Will Milk go bad if left out overnight?’ We will discuss the factors that make milk a perishable food and the appearance of signs that indicate spoilage.

Will milk go bad if left out overnight?

Refrigeration is essential; when it comes to milk safety. Do not leave milk out overnight; otherwise, it will spoil. Food Spoilage bacteria multiply rapidly at room temperature and above.

How to store milk?

Milk must not be left out of the fridge and especially not overnight.

Aseptic Milk, packaged after processing at an Ultra High Temperature, will last for six months if the seal was left intact.

However, an opened tetra pack will spoil if left out of the fridge overnight. Pasteurized milk cannot be left at room temperature for more than 2 hours.

The pasteurization process kills bacteria by heating the milk at high temperatures. After potentially harmful microorganisms destroy, then it is aseptically sealed to ensure its longevity.  Once the seal has broken, pathogenic microorganisms cause the milk to spoil.

Traditional Milk, on the other hand, does not undergo any such a process. Therefore you can expect it to have a shorter life, even if it has an intact seal.

Pasteurization versus UHT Treatment 

Milk bought from a store is in either of the three forms. It could be either raw, pasteurized or UHT treated.

 Milk is sometimes, however, only pasteurized and not subjected to ultra-high Treatment. Pasteurized milk is not shelf-stable and needs to be refrigerated at all times. 

In pasteurization, the milk is first chilled and then heated. Milk is allowed to move between heated stainless steel plates at a temperature of 161 F. The heating occurs for 15 seconds only. Then the temperature is brought down again from 161 F to 39 F before it is packaged and distributed.

Ultra-High Treatment differs slightly, as milk, is heated at 136 to 145 Fahrenheits for only 2-4 seconds. 

As pasteurized milk is heated to a cooler temperature, it is more prone to spoilage. Some bacteria tend to survive during the process, thus require constant refrigeration. 

Pasteurized Milk will spoil if left out overnight. It has to be refrigerated at 2-6 C and, the shelf-life lasts for 2-5 days.

Milk treated to UHT lasts much longer than pasteurized milk and hence, is processed and sold at room temperature. 

After it has opened, a UHT Milk pack can go bad, if left out for a night. It may not show any sign of spoilage, but drinking or using UHT Milk that was left out overnight is not recommended. 

In contradiction, raw milk does not undergo any heat treatment and hence, has the shortest shelf-life. Raw Milk keeps for 7 to 10 days when it is properly refrigerated. The presence of bacteria such as lactobacilli makes the milk spoil quickly and, hence it can become unsuitable for consumption sooner than the heat-treated counterpart.

Other FAQs about Milk which you may be interested in.

How long does breast milk last outside the fridge?

Can you freeze soy milk?

How does milk spoil?

Milk is home to bacteria such as lactococci and lactobacilli. At temperatures above room temperature, they breed and reproduce. The probiotic bacteria change the lactose milk sugar into its compositional units; glucose and galactose.

The formation of lactic acid starts, as proved by the familiar tangy smell and taste. As the casein develops, the curdling process starts. 

As milk deteriorates further, the bacteria take over the fats, which causes the occurrence of molds.

Spoilt raw milk may become inedible as it curdles and gains its distinct sour taste and odor.  However, it does not need to be discarded, as curdled milk is used to make cheese, cream, and kefir.

On the other hand, pasteurized milk does not become sour due to the absence of bacteria that cause the milk to curdle. Instead, it rots and becomes unfit for consumption.

As milk spoils, it develops undesirable characteristics. Even if milk seems safe, you can expect bacterial growth to start at a temperature greater than 40 Fahrenheits. 

The US Food and Drug Administration advises against leaving your milk out of the fridge for more than 2 hours. Considering the USDA advice, milk let out of the refrigerator overnight becomes moot.

Even the 2-hour benchmark becomes 1 instead, in case the temperature exceeded 90 F.

As milk spoils, you can detect a rancid and sour smell. 

The texture and consistency change too and, the color becomes pale with a yellowish tinge. 

If you drink milk that was left out overnight, you risk falling sick. Even if it smells and looks fine, it must be tossed out. 

Spoiled Milk can cause food poisoning with symptoms such as vomiting and diarrhea. 

In this brief guide, we answered the question,’ Will milk go bad if left out overnight?’ We discussed the factors that make the milk a perishable food and the appearance of signs that indicate spoilage.








Hi, I am Charlotte, I love cooking and in my previous life, I was a chef. I bring some of my experience to the recipes on this hub and answer your food questions.