What to do if I accidentally left the milk out overnight?

In this brief guide, we will answer the question “what to do if I accidentally left the milk out overnight?” with an in-depth analysis of what should be done if you accidentally left the milk out overnight?. Moreover, we will also discuss the methods to check whether the milk left outside is safe to use or should be discarded.

What to do if I accidentally left the milk out overnight?

If you accidentally left milk outside overnight, the most safe procedure is to  cook the milk. Even if the milk has not curdled naturally or does not present any sign of spoilage, it can be spoiled. 

By heating, an acidic (spoiled) milk will curdle, while a good milk will not (1). In addition, heat treatments can reduce the microbial contamination of milk and extend its shelf life (2).  

However, if you are in a hurry and have no time to cook the milk, it is still possible to consume the milk if you accidentally left it out overnight. 

If the milk does not have any sour odor and does not present a lumpy texture, it is highly recommended to place the milk in the refrigerator as soon as possible and to cook it prior to its use or its consumption. 

What happens if you accidentally leave milk out overnight?

If milk is left out of the fridge for an extended time or overnight, it can become a food safety issue. Bacteria start to grow and replicate at a greater rate when the temperature of the milk increases.

There are many bacteria in the pasteurized milk that are not eliminated through pasteurization. These bacteria can grow when milk is stored in refrigeration temperatures at a low rate and that is why milk goes bad after a long time of storage (2,3,4). 

In the case of shelf stable milk, which undergoes ultra-pasteurization processes, a more severe heat treatment, many spores of bacteria that survive in milk despite sterilization will start replicating as soon as the conditions are favorable, that is, as soon as temperature increases.  

What to do with the milk if you accidentally left the milk out overnight?

If you accidentally left milk outside overnight, you don’t have to discard the milk. You can cook the milk and consume it in cooked dishes, such as sauces and cakes.

Cooking is an efficient method to destroy a significant amount of bacteria that possibly might have grown in the milk while it was kept out (3).

A thermal treatment at 72-75°C during a time of 15-20 seconds can ensure a safe consumption of milk for at least 2 days, if the milk has still not spoiled (3). In the case of spoiled milk, it will curdle by heating, due to the low thermal stability of the acidified milk (1).

How long does it take for milk to spoil if left out?

The time necessary for the milk to spoil when left out is not possible to estimate. However, as a safety recommendation, no food product that requires refrigeration should be kept at temperatures above 40°F for more than 2 hours, according to the United States Department of Agriculture (6). 

However, a study reported that milk could be left out for one day before presenting signs of spoilage, if it has been stored properly in the refrigerator (4).

The time that takes for milk to spoil depends on many factors, such as (2,3,4,5): 

• The initial microbial load of the milk: the spoilage of milk is caused in most of the cases, by bacteria. With a higher initial population of bacteria in the milk, it will spoil faster
• The type of milk processing: Milk can be processed through pasteurization, sterilization or ultra high temperature sterilization. Each of these thermal treatments have different effects on extending the shelf life of milk
• The time of storage: If the milk has kept opened in the fridge for a long period before being left out, it is more likely that it will spoil than a milk that has been kept opened in the fridge since the day before before being left out
• The type of milk: Spoilage of whole milk differs from of spoilage of skim milk, as the milk bacteria produce enzymes that cleave milk fat in milk proteins and these nutrients are in different concentrations in these milks
• The thermal history of milk: if the milk has been left out previously or if has been frozen, its shelf life can be reduced compared to a fresh milk. Freeze-thawing of milk as well as frozen storage can alter the physicochemical properties of milk, by affecting its compounds, damaging fat globules and causing protein degradation, leading to the generation of off-flavors  

What are the risks of consuming milk that has been left out overnight?

The risks of consuming milk that has been left out overnight are of experiencing a foodborne illness. 

Pathogens such as Salmonella ssp., Staphylococcus aureus, Streptococcus agalactiae, Listeria monocytogenes, Yersinia enterocolitica, and Escherichia coli O157:H7 and coliforms are possible bacteria that grow in milk and are able to cause negative effects on health. 

If you accidentally drink spoiled milk, you may experience gastrointestinal symptoms, such as diarrhea, abdominal cramping, and other symptoms, such as fever, vomiting and headache (7).

How can you tell if the milk is spoiled?

Spoiled milk has a distinct sour odor, which is due to lactic acid produced by bacteria. Off-odors such as fruity, bitter, rancid, yeasty are used to describe spoiled milk.

Changes in the texture are possible, such as coagulation, gelation, increased viscosity and syneresis due to the increase of acidity, reduction of the pH and consequently precipitation of the milk proteins (2).

The alteration of flavor is also a possible sign. Spoiled milk can have off-flavor such as sour, bitter, soapy or rancid. 

If you notice one or more of these signs, avoid consuming the milk.

Conclusion:

In this brief guide, we have answered the question “what to do if I accidentally left the milk out overnight?” with an in-depth analysis of what should be done if you accidentally left the milk out overnight?. Moreover, we have also discussed the methods to check whether the milk left outside is safe to use or should be discarded.

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Citations:

1. O’Connell, J. E., and P. F. Fox. The two-stage coagulation of milk proteins in the minimum of the heat coagulation time-pH profile of milk: Effect of casein micelle size. J dairy sci, 2000, 83, 378-386.
2. Velázquez-Ordoñez, Valente, et al. Microbial contamination in milk quality and health risk of the consumers of raw milk and dairy products. Nutrition in Health and disease-our challenges Now and Forthcoming time. 2019.
3. Gedam, Kishor, et al. The study on UHT processing of milk: A versatile option for rural sector. World J Dairy Food Sci, 2007, 2, 49-53.
4. Barbano, D. Mistanbulis, Y. Ma, and Marcos Veiga dos Santos. Influence of raw milk quality on fluid milk shelf life. J dairy sci, 2006, 89, E15-E19.
5. Tribst, A. A. L., L. T. P. Falcade, and M. M. De Oliveira. Strategies for raw sheep milk storage in smallholdings: Effect of freezing or long-term refrigerated storage on microbial growth. J dairy sci, 2019, 102, 4960-4971.
6. Danger Zone. United States Department of Agriculture.
7. Food Poisoning Symptoms. Food Poisoning Symptoms | CDC