Milk is a perishable food that must be kept refrigerated to prevent bacterial growth. However, sometimes milk may be left out at room temperature for an extended period before being refrigerated. So how long can milk sit unrefrigerated before it goes bad?
The answer depends on several factors, including the temperature of the room, how long the milk sits out before being refrigerated again, and what type of milk it is. Here is a quick overview of how long milk can be left out at 50 degrees Fahrenheit:
- Whole milk – 5-7 hours
- Reduced-fat (2%) milk – 5-7 hours
- Low-fat (1%) milk – 5-7 hours
- Fat-free (skim) milk – 5-7 hours
At 50°F, most types of milk should be discarded after sitting out for more than 5-7 hours. The higher fat content in whole milk gives it a slightly longer shelf life compared to skim milk. But all milk types should be refrigerated again within 7 hours when left out at 50°F.
Factors That Affect How Long Milk Can Sit Out
Several key factors influence how long milk can safely sit at room temperature before spoiling:
Temperature
Temperature is one of the most important factors. The higher the temperature milk is left out at, the faster bacteria will grow and multiply. At colder temperatures close to 50°F, the bacteria grow more slowly, allowing milk to last longer unrefrigerated.
Here is how temperature impacts how long milk can be left out:
- Under 40°F – 24-48 hours
- 50°F – 5-7 hours
- 70°F – 2-3 hours
- 90°F – 1-2 hours
As you can see, milk left out at 50°F lasts significantly longer than milk left out at room temperature or warmer. The cooler the milk is kept, the longer it will last outside the fridge.
Duration Out of Refrigeration
The amount of time milk sits at room temperature also plays a major role. The longer milk sits unrefrigerated, the greater the bacterial growth. Even at cool 50°F temperatures, bacteria multiply over time.
Here are the general time limits for milk at 50°F:
- 0-2 hours – Still very safe
- 3-5 hours – Use quickly once refrigerated again
- 5-7 hours – Discard after this point
- Over 7 hours – Higher risk of spoilage
Keeping milk’s time out of refrigeration as short as possible is key. If milk sits for over 7 hours at 50°F, it is best to be cautious and discard it.
Type of Milk
The variety of milk also impacts its perishability. Here are the typical time limits by milk type:
- Whole milk – 5-7 hours at 50°F
- 2% reduced-fat milk – 5-7 hours at 50°F
- 1% low-fat milk – 5-7 hours at 50°F
- Skim milk – 5-7 hours at 50°F
- Ultra-pasteurized milk – Slightly longer than regular milk
Most milk types last about 5-7 hours at 50°F. Whole and full-fat milk may last slightly longer thanks to their higher fat content. Ultra-pasteurized milk also can sit out a bit longer due to the higher temperature pasteurization process used.
How Bacteria Grow in Milk
To understand why refrigeration is so important for milk, it helps to know how bacteria grow and multiply. Here’s an overview:
Milk naturally contains low levels of bacteria derived from the teat skin of dairy cows. Refrigeration slows the growth of these bacteria.
But when milk is left at room temperature for too long, the bacteria rapidly grow and divide. Two bacteria can become 4, then 8, then 16 and so on exponentially. In just a few hours, the bacteria population can explode, spoiling the milk.
At 50°F, bacterial growth is slower but still steadily increasing over time. Even at this cool temperature, bacteria will eventually multiply to the point of milk spoilage after about 5-7 hours.
Types of Bacteria in Milk
Here are some of the main types of bacteria found in milk:
- Lactic acid bacteria – Major contributor to spoilage
- Coliforms like E. coli
- Salmonella and Listeria
- Staphylococcus aureus
- Bacillus cereus
Most of these bacteria cause dairy products to spoil. But some like Salmonella, Listeria, E. coli, and Staph aureus can also cause foodborne illness if dairy products made from contaminated milk are consumed.
Toxin Production
One big danger of bacterial overgrowth in milk is that many bacteria produce heat-stable toxins as they grow. Even if contaminated milk is boiled, these toxins can survive.
Staphylococcus aureus in particular produces a highly heat-stable enterotoxin. If staphylococci grow to high levels, pre-formed toxins may persist even after cooking.
For this reason, milk should never be consumed if left out too long at room temperature. The toxins can cause nausea, vomiting, and diarrhea even if the milk is boiled or baked into other products.
How to Tell if Milk is Bad After Sitting Out
Here are some signs that indicate milk has spoiled and should be discarded after sitting out too long:
- Sour odor
- Clumpy texture
- Discolored or yellowish tinge
- Mold on surface
- Yeasty smell
- Expired sell-by date
Rancid odors, changed consistency, and visible mold are all red flags. Sourness and separation are two common types of spoilage. If in doubt, use your senses or give the milk a quick sniff test.
As a good rule of thumb, discard milk if more than 5-7 hours have passed since being left out at 50°F.
Taste Test
Never taste milk that you suspect has spoiled due to sitting out too long. Consuming spoiled milk, even just a sip, can cause food poisoning symptoms. This is especially true if Staph aureus or other pathogens have produced toxins in the milk.
Err on the side of caution and discard milk past the safe 5-7 hour time limit at 50°F. A foodborne illness is never worth the risk.
How to Keep Milk Safe When the Power Goes Out
Power outages can create challenges for keeping milk and dairy products cold and safe. Here are some tips:
- Keep refrigerator door closed as much as possible.
- Place milk jugs or cartons in a cooler with ice packs.
- Freeze milk or small ice cubes in advance to help keep cold air trapped in fridge and freezer.
- Store milk on refrigerator shelves rather than the door to keep colder.
- Throw out milk and other perishables after 4 hours without power.
The key is limiting how long milk sits in the “danger zone” between 40-140°F. If the power is out for over 2-3 hours, it’s safest to discard any perishable milk and dairy items.
Does Sour Milk Always Mean Milk is Spoiled?
A sour odor and taste is one of the most common signs of spoiled milk. However, with raw milk or certain types of pasteurized milk, a level of natural sourness can sometimes occur without necessarily meaning the milk has fully spoiled and become unsafe.
Here’s a bit more on what a sour milk odor or flavor may indicate:
- Raw milk – Can naturally have some sourness due to lactic acid bacteria. But extreme sourness likely means spoilage.
- Organic pasteurized milk – More likely to sour quickly than conventional milk due to fewer stabilizers and additives. But strong sourness still indicates spoilage.
- Conventional pasteurized milk – Strong sourness is almost always a sign of spoilage from bacterial overgrowth.
In general, the stronger and more intense the sour flavor, the more likely the milk has spoiled due to excessive bacterial growth. Mild sourness may just indicate natural lactic acid development.
When in doubt, use your senses and immediately discard extremely sour smelling or tasting milk after sitting out. Don’t consume it.
Can Spoiled Milk Make You Sick?
Yes, consuming milk that has been left out too long can cause foodborne illness. Here’s what you need to know:
- Bacteria like E. coli, Salmonella, Listeria, and Staph can grow and produce toxins.
- Staph aureus is especially dangerous, producing a heat-stable toxin that survives pasteurization and boiling.
- Symptoms include nausea, vomiting, diarrhea, fever, and cramps.
- More vulnerable groups like children, pregnant women, elderly, and those with weak immune systems are at higher risk of illness.
- Food poisoning onset occurs within 30 minutes to 6 hours after ingesting spoiled dairy products.
The bacteria counts can rapidly climb into the millions per milliliter when milk is left out at room temperature. Many of these bacteria can lead to illness.
While pasteurization kills most bacteria in milk meant for retail sale, aging, recontamination, and breakdown allow bacteria to return and multiply, especially when milk is left out too long.
Treatment for Sickness
Most cases of food poisoning from spoiled milk require no treatment beyond resting, staying hydrated, and waiting for symptoms to resolve. But some cases may require hospitalization for IV fluids.
Seek medical care immediately if you experience any of the following after consuming spoiled milk:
- Bloody diarrhea
- Inability to keep liquids down
- Signs of dehydration such as excessive thirst, dry mouth, dizziness
- Severe abdominal pain and cramps
- Blurred vision
- Difficulty breathing
- High fever above 101.5°F
Notify your doctor promptly if you suspect a spoiled milk foodborne illness so they can provide appropriate supportive treatment and intervene if your condition deteriorates.
How Long Does Milk Last in the Fridge Once Opened?
Here is a look at how long milk will remain fresh in the refrigerator once the carton or jug has been opened:
- Non-ultra pasteurized milk: 5-7 days past printed sell-by or use-by date.
- Ultra-pasteurized milk: 7-10 days past printed date.
The higher heat pasteurization methods used for ultra-pasteurized milk give it a slightly longer fridge life. For whole milk, 2% milk, 1% milk, and skim milk that have not been ultra-pasteurized, plan on using within 5-7 days of opening.
How To Tell If Opened Milk Is Bad
Watch for these signs that opened milk has spoiled and needs to be discarded:
- Sour odor and taste
- Clumps or curdling
- Change in color or grayish tinge
- Mold around bottle opening or floating on surface
Milk doesn’t necessarily have to be past its expiration date if these indicators of spoilage arise. Particularly if milk has been left out of the fridge for periods, spoilage can occur well before the printed sell-by or use-by date.
Freezing Milk and Thawing
For longer term storage, milk can be frozen as follows:
- Freeze in air-tight container with as much air removed as possible.
- Label milk with freezing date.
- Use within 3-4 months for highest quality.
- Thaw slowly in fridge, gently mixing contents.
Frozen properly in an airtight container, milk can retain its freshness for up to 4 months in the freezer. Always thaw slowly and gently mix contents before consuming thawed frozen milk.
Takeaway Tips on Milk Freshness
Here are some key takeaways on milk freshness when sitting out at room temperature or once opened:
- When left out at 50°F, discard milk after 5-7 hours maximum
- Time and temperature are major factors impacting milk freshness
- Sour odors, changed texture, expiration dates all indicate potential spoilage
- Boiling or cooking spoiled milk does not make it safe for consumption
- Once opened, use refrigerated milk within 5-7 days and ultra-pasteurized within 7-10 days
Being aware of time and temperature limits, signs of spoilage, and proper fridge storage helps ensure milk stays fresh and safe to drink. When in doubt, remember the cardinal rule – if it smells funky, looks chunky, or you think it may be spoiled, toss it out!
